path: root/specs/XI2proto.txt

The X Input Extension 2.x
=========================
:toc:
:numbered:

.This is a work in progress!
*****************************************************************************
While XI 2.0 is final and widely deployed, XI 2.1 is still a work in progress;
the protocol is not final and is subject to change at any time. While testing
and feedback is strongly encouraged and very much welcome, please be aware that
any part of the XI 2.1 additions may change from underneath you. We strongly
recommend against deploying it in production for this reason.

Developing against XI 2.1 requires passing the --enable-unstable-protocol
argument when building inputproto, and additionally defining
XINPUT2_1_USE_UNSTABLE_PROTOCOL when building your applications. I'm sure you
get the point by now, so I promise not to mention it again.
*****************************************************************************

Authors:

- Peter Hutterer (Red Hat) <peter.hutterer@redhat.com>
- Daniel Stone (Collabora Ltd.) <daniel@fooishbar.org>
- Chase Douglas (Canonical, Ltd.) <chase.douglas@canonical.com>

[[history]]
History 
-------

- v2.2, ??: Multitouch support added
- v2.0, October 2009: Initial release of XI2 protocol

[[intro-xi20]]
Introduction
------------

The X Input Extension version 2.0 (XI2) is the second major release of the X
Input Extension.

XI2 provides a number of enhancements over version 1.5, including:

- use of XGE and GenericEvents. GenericEvents are of flexible length with a
  minimum length of 32 bytes.
- explicit device hierarchy of master and slave devices. See Section 4.
- use of multiple independent master devices (Multi-Poiner X or MPX).
- the ability for devices to change capabilities at runtime.
- raw device events

XI2's intent is to replace both core input processing and prior versions of
the X Input Extension. Historically, the majority of applications employed the
core protocol requests and events to handle user input. The core protocol does
not provide information about which device generated the event. The X Input
Extension version up to 1.5 requires the differentiation between core and
extended devices. Extended devices may not be core devices and thus cannot be
used on applications employing the core protocol. XI2 addresses both of these
issues by enabling devices to be both extended and core devices and providing
device information in each event (with the exception of core events).

2.1 Changes
-----------
Changes introduced by version 2.1

- RawEvents are sent regardless of the grab state.

2.2 Changes
-----------
Changes introduced by version 2.2

XI 2.2 introduces support for multi-touch devices. The traditional
pointer/keyboard approach enforced by XI 2.0 with the master/slave device
hierarchy is not always suitable for multi-touch devices that can provide a
dynamic number of touchpoints per physical device; it is not known without
client-specific interpretation whether the touchpoints must be considered
separately or grouped together.

The additions in XI 2.2 aim to:

- support a dynamic number of simultaneous touch points,
- support devices that are both multi-touch and traditional pointer devices,
- allow touchpoints to be either grouped together or handled separately,
- while supporting pre-XI 2.2 clients through emulation of XI 2.x/XI 1.x and core
- be backwards-compatible to pre-XI 2.2 clients through emulation of XI 2.x/XI 1.x and core
  pointer events.

XI 2.2 caters for two modes of touch input devices:

- 'Direct' multi-touch input devices such as touchscreens. These devices
  provide independent touchpoints that can occur anywhere on the screen;
  "direct" here refers to the user manipulating objects at their screen
  location, e.g. touching an object and physically moving it.
- 'Dependent' touch input devices such as multi-touch trackpads and mice with
  additional touch surfaces. These devices provide independent touchpoints that
  often need to be interpreted relative to the current position of the cursor
  on that same device. Such interactions are usually the result of a gesture
  performed on the device, rather than direct manipulation.

Touch events are only available to clients supporting version 2.2 or later of
the X Input Extension. Clients must use the XIQueryVersion request to announce
support for this version. Touch devices may generate emulated pointer events
alongside XI 2.2 touch events to support older clients; see Section
<<multitouch-processing,Touch event delivery>>.

//                            ❧❧❧❧❧❧❧❧❧❧❧

2. Notations used in this document
----------------------------------

Notation for requests:

    ┌───
        Name of request
            name of request field:       type of request field
            name of request field:       type of request field
            ▶
            name of reply field:         type of reply field
    └───

Notation for events:

    ┌───
        Name of event
            name of field:               type of field
            name of field:               type of field
    └───

Complex fields are specified in the following notation:

          name of field:                  COMPLEXFIELDTYPE

or, if multiple of these fields exist:

          name of field:                  LISTofCOMPLEXFIELDTYPE

    COMPLEXFIELDTYPE:  { name of subfield:   type of subfield,
                         name of subfield:   type of subfield }

//                            ❧❧❧❧❧❧❧❧❧❧❧

3. Interoperability between version 1.x and 2.0
-----------------------------------------------

There is little interaction between 1.x and 2.x versions of the X Input
Extension. Clients are requested to avoid mixing XI1.x and XI2 code as much as
possible. Several direct incompatibilities are observable:

[[interop-xi1-limitations]]
Limitations resulting from different variable ranges
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

XI2 provides a larger range for some fields than XI1. As a result, XI1 clients
may not receive data an XI2 client receives.
These fields include:

- devices with a deviceid of greater than 127 are invisible to XI1 clients.
- key events and key grabs featuring larger than 255 can only be sent to XI2
  clients.
- no subpixel information is available to XI1 clients. If motion events are in
  a subpixel range only, the server may omit these events and an XI 1.x client
  will not receive events until the pixel boundary is crossed.


[[interop-xi1-grabs]]
Blocking of grabs
~~~~~~~~~~~~~~~~~

XI1 grabs are different to XI2 grab and a device may not be grabbed through an
XI2 grab if an XI1 grab is currently active on this device or vice versa.
Likewise, a keycode or button already grabbed by an XI 1.x or XI2 client may
not be grabbed with the same modifier combination by an XI2 or XI 1.x client,
respectively.

[[interop-xi1-device-list]]
Invisibility of Master Devices
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

XI 1.x was not designed with support for multiple master devices. As a
result, only the first master pointer and master keyboard are visible to XI
1.x clients; all other master devices are invisible and cannot be accessed
from XI 1.x calls.

3.4 Smooth scrolling
~~~~~~~~~~~~~~~~~~~~

Historically, X implemented scrolling events by using button press events:
button 4 was one “click” of the scroll wheel upwards, button 5 was downwards,
button 6 was one unit of scrolling left, and button 7 was one unit of scrolling
right.  This was sufficient for scroll wheel mice, but not for touchpads which
are able to provide scrolling events through multi-finger drag gestures, or
simply dragging your finger along a designated strip along the side of the
touchpad.

Newer X servers may provide scrolling information through valuators to
provide scroll events with more precision than the button events. Valuators
for axes sending scrolling information must have one ScrollClass for each
scrolling axis.

If scrolling valuators are present on a device, the server must provide
two-way emulation between these valuators and the legacy button events for
each delta unit of scrolling.

One unit of scrolling in either direction is considered to be equivalent to
one button event, e.g. for a unit size of 1.0, -2.0 on an valuator type
Vertical sends two button press/release events for button 4. Likewise, a
button press event for button 7 generates an event on the Horizontal
valuator with a value of +1.0. The server may accumulate deltas of less than
one unit of scrolling.

Any server providing this behaviour marks emulated button or valuator events
with the XIPointerEmulated flag for DeviceEvents, and the XIRawEmulated flag
for raw events, to hint at applications which event is a hardware event.

If more than one scroll valuator of the same type is present on a device,
the valuator marked with Preferred is used to convert legacy button events
into scroll valuator events. If no valuator is marked Preferred or more than
one valuator is marked with Preferred, this should be considered a driver
bug and the behaviour is implementation-dependent.

[[hierachy]]
The Master/Slave device hierarchy
---------------------------------

XI2 introduces a device hierarchy split up into so-called Master Devices (MD)
and Slave Devices (SD).

[[hierachy-master]]
Master devices
~~~~~~~~~~~~~~
An MD is a virtual device created and managed by the server. MDs may send core
events and XI events. However, an MD does not represent a physical device and
relies on SDs for event generation. MDs come in two forms: as master pointers
or as master keyboards. A master pointer is represented by a visible cursor on
the screen. A master keyboard is represented by a keyboard focus.

Each master pointer is paired with the respective master keyboard and vice
versa, and this pairing is constant for the lifetime of both input devices.
Clients can use this pairing behaviour to implement input paradigms that
require pointer and keyboard interation (e.g. SHIFT + Click).

[[hierachy-slave]]
Slave devices
~~~~~~~~~~~~~
An SD is usually a physical device configured in the server. SDs are not
represented by a cursor or keyboard focus and may be attached to a master
pointer or master keyboard. SDs can only be attached to any master of the same
type (e.g. a physical pointer device can be attached to any master pointer).

If an event is generated by an SD

- if the SD is attached to a master pointer, it changes the position and/or
  button state of the master pointer.
- if the SD is attached to a master keyboard, it sends events to this
  keyboard's focus window (if applicable) and/or changes the modifier state of
  this keyboard.
- if the SD is not attached to an MD ("floating"), it does not change
  any master device. The SD has its own (invisible) sprite and its own focus.
  Both the sprite and the focus must be managed explicitly by the client
  program.

[[hierachy-dcce]]
Event processing for attached slave devices
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Whenever an SD changes its logical state,

- the event is delivered as an XI event to any interested clients. If the
  device is floating, event processing stops.
  Otherwise, if the device is attached,
- the master device changes its classes to reflect the SD's capabilities. All
  interested clients are notified of this device change.
- then, the event is delivered as an XI event from the MD to any interested
  clients. If the event has been delivered, event processing stops.
  Otherwise,
- the event is delivered as a core event to any interested clients.

Given that W is the event window, and P the parent window of W, event delivery
to P is only attempted if neither the XI event, nor the core event has been
delivered on W. Once an event has been delivered as either XI or core event,
event processing stops.

[[clientpointer]]
The ClientPointer principle
~~~~~~~~~~~~~~~~~~~~~~~~~~~

Many core protocol and some extension requests are ambiguous when multiple
master devices are available (e.g. QueryPointer does not specfy which pointer).
The X server does not have the knowledge to chose the contextually correct
master device. For each client, one master pointer is designated as this
clients's "ClientPointer". Whenever a client sends an ambiguous request (e.g.
QueryPointer), the ClientPointer or the keyboard paired with the ClientPointer
is chosen to provide the data for this request.

This ClientPointer may be explicitly assigned to a client with the
SetClientPointer call. If no ClientPointer is set when a client issues an
ambiguous request, the server choses one device as the ClientPointer. The
method of chosing a ClientPointer from the available master pointers is
implementation-specific.

If the master pointer currently set as ClientPointer for one or more clients is
removed, the server may either unset the ClientPointer setting or change the
ClientPointer to a different master pointer.

[[multitouch]]
Touch device support
--------------------

Touch event processing differs from normal event processing in a few ways.
The most notable differences are that touch events are processed partially
out-of-band from pointer and keyboard events, and that touch events may be
sent to multiple clients simultaneously. For more details see Section
<<multitouch-processing, Touch event delivery>>.

[[multitouch-lifecycle]]
Touch event sequences
~~~~~~~~~~~~~~~~~~~~~

Touch input follows a three-stage cycle:

        begin - update - update - ... - end

i.e. “begin” the sequence by touching the device, “update” the current
touch location or properties any number of times, and finally “end” the
sequence by ceasing to touch the device.  Within this document, the term
"touch sequence" is used to describe the above sequence of events.
In the protocol, the three stages are represented with the event
types TouchBegin, TouchUpdate, and TouchEnd, respectively. A touch sequence
always generates TouchBegin and TouchEnd events, and may also generate
TouchUpdate events.  Clients must select for all three of these events
simultaneously.

When a touch starts, clients are sent a TouchBegin event
detailing the position of the touchpoint, as well as the
initial properties of the touchpoint.  Note that the logical state of the
device (as seen through the input protocol) may lag the physical state if event
processing is affected by grabs.  Multiple touchpoints may be active on the
same device at any time, potentially owned by and/or delivered to a different
set of clients.

Whenever the touch position or any other property of the touchpoint changes,
a TouchUpdate event is sent to all clients listening
to events for that touchpoint with the updated information.

When the touch has physically ended, or a client will otherwise not receive
any more events for a given touchpoint, a TouchEnd event will be sent to
that client.

Passive touch grabs are similar to standard input event grabs in that they
take precedence over event selections and are searched from the root window
to the child window (as opposed to selections, which start their search at the
child window and continue up to the root window).  When a touch grab activates,
the client whose grab activates becomes the “owner” of this touch sequence,
and must decide what to do with it, as per Section
<<multitouch-ownership,Ownership of touch sequences>>.  See the
<<requests-passivegrabdevice,XIPassiveGrabDevice>> request
documentation for more information on passive grab activation.

Only one client may select for touch events from a given device on a window.

[[multitouch-ownership]]
Ownership of touch sequences
^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Once a grabbing client becomes the owner of a touch, it must either “accept” or
"reject" the touch sequence using the XIAllowEvents request. If a touch sequence
is rejected, a TouchEnd event is sent to the rejecting client, and it will not
receive any more events for this touch.  The server then looks to the next
window in the stack for another passive grab, and attempts to pass ownership
on to the next candidate for a passive grab (i.e. the next window towards
the final child window with a matching grab), or to the first applicable
event selection if there are no more grabs.

If a touch sequence is accepted by its owner, all other clients receive
TouchEnd events, and the touch sequence is exclusively delivered to the
owner from that point on.

If the touch sequence physically ends while the owner of the touch sequence
has not yet accepted or rejected ownership, all clients receive a
TouchUpdate event with the TouchPendingEnd flag set. The owner must still
accept or reject the sequence nonetheless. If the owner rejects the touch
sequence, the server will still attempt to exhaust all other passive grabs
and/or event selections looking for a final owner.

Clients may opt for touch events to be delivered before they become the
owner of the touch sequence. In this case, the logical state of the device (as
seen by means of the protocol) always matches the physical state of the device.
Clients must use caution if they opt for this feature; any action taken must be
undone if the touch sequence ends without the client becoming the owner.

To select for touch events regardless of ownership, a client must set the
TouchOwnership event mask in addition to the
TouchBegin, TouchUpdate and TouchEnd mask. When selected, a client will receive
touch events as they occur on the device without delay. If and when the client
becomes the owner of a touch sequence, a TouchOwnership event is sent to the
client. If the client is the initial owner of the sequence, the TouchBegin is
immediately followed by the TouchOwnership event. Otherwise, TouchUpdate events
may preceed a TouchOwnership event. A client is not guaranteed to become the
owner of any given touch sequence.

The server delivers touch events to all clients that have selected for
TouchOwnership and to the current owner of the sequence in parallel.

If a client has selected for TouchOwnership and is not the current owner of
the sequence and the current owner accepts the sequence, the client receives
a TouchEnd event and no further events from this sequence are sent to this
client.

If a client has selected for TouchOwnership and the physical touch ends
before the current owner has accepted or rejected the sequence, the client
receives a TouchUpdate event with the TouchPendingEnd flag set. No further
TouchUpdate events will be sent for this sequence. If the current owner
accepts the sequence, the client receives a TouchEnd event. Otherwise, if
the current owner rejects the sequence, the client may become 
the owner of the touch sequence and receive a TouchOwnership event and a
TouchEnd event.

[[multitouch-device-modes]]
Touch device modes
~~~~~~~~~~~~~~~~~~

Touch devices come in many different forms with varying capabilities. The
following device modes are defined for this protocol:

'DirectTouch':
    These devices map their input region to a subset of the screen region. Touch
    events are delivered to window at the location of the touch. An example
    of a DirectTouch device is a touchscreen.

'DependentTouch':
    These devices do not have a direct correlation between a touch location and
    a position on the screen. Touch events are delivered according to the
    location of the device's cursor. An Example of a DependentTouch device is a
    trackpad.

A device is identified as only one of the device modes above at any time, and
the touch mode may change at any time. If a device's touch mode changes, an
XIDeviceChangedEvent is generated.

[[multitouch-processing]]
Touch event delivery
~~~~~~~~~~~~~~~~~~~~

For direct touch devices, the window set for event propagation is the set of
windows from the root window to the topmost window lying at the co-ordinates
of the touch.

For dependent devices, the window set for event propagation is the set of
windows from the root window to the window that contains the device's
pointer. A dependent device may only have one window set at a time, for all
touches. Any future touch sequence will use the same window set. The window set
is cleared when all touch sequences on the device end.

A window set is calculated on TouchBegin and remains constant until the end
of the sequence. Modifications to the window hierarchy, new grabs or changed
event selection do not affect the window set.


[[multitouch-emulation]]
Pointer emulation from multitouch events
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Touch sequences from direct touch devices may emulate pointer events. Only one
touch sequence from a device may emulate pointer events at a time; which touch
sequence emulates pointer events is implementation dependent.

Pointer events are emulated as follows:

- A TouchBegin event generates a pointer motion event to the location of the
  touch with the same axis values of the touch event, followed by a button press
  event for button 1.
- A TouchUpdate event generates a pointer motion event to the location of the
  touch and/or to update axis values of the pointer device.
- A TouchEnd event generates a pointer motion event to the location of the touch
  and/or to update the axis values if either have changed, followed by a button
  release event for button 1.

If a touch sequence emulates pointer events and an emulated pointer event
triggers the activation of a passive grab, the grabbing client becomes the
owner of the touch sequence.

The touch sequence is considered to have been accepted if

- the grab mode is asynchronous, or
- the grab mode is synchronous and the device is thawed as a result of
  AllowEvents with AsyncPointer or AsyncDevice

Otherwise, if the button press is replayed by the client, the touch sequence
is considered to be rejected.

Touch event delivery precedes pointer event delivery. A touch event emulating
pointer events is delivered:

- as a touch event to the top-most window of the current window set if a
  client has a touch grab on this window,
- otherwise, as a pointer event to the top-most window of the current window
  set if a client has a pointer grab on this window,
- otherwise, to the next child window in the window set until a grab has been
  found.

If no touch or pointer grab on any window is active and the last window in the
window set has been reached, the event is delivered:

- as a touch event to the window if a client has selected for touch events
  on this window
- otherwise, as a pointer event to the window if a client has selected for
  pointer events.
- otherwise, to the next parent window in the window set until a selection has
  been found.

Emulated pointer events will have the PointerEmulated flag set.

[[glossary-notations]]
Notations used in this document
-------------------------------

Notation for requests:

    ┌───
        Name of request
            name of request field:       type of request field
            name of request field:       type of request field
            ▶
            name of reply field:         type of reply field
    └───

Notation for events:

    ┌───
        Name of event
            name of field:               type of field
            name of field:               type of field
    └───

Complex fields are specified in the following notation:

          name of field:                  COMPLEXFIELDTYPE

or, if multiple of these fields exist:

          name of field:                  LISTofCOMPLEXFIELDTYPE

    COMPLEXFIELDTYPE:  { name of subfield:   type of subfield,
                         name of subfield:   type of subfield }


[[glossary-datatypes]]
Data types
----------

    BUTTONMASK
            A binary mask defined as (1 << button number).
            A SETofBUTTONMASK is a binary OR of zero or more BUTTONMASK.

    DEVICE { DEVICEID, AllDevices, AllMasterDevices }
            A DEVICE specifies either a DEVICEID or AllDevices or
            AllMasterDevices.

    DEVICEID { CARD16 }
            A DEVICEID is a numerical ID for a device currently available in the
            server. The server may re-use a device ID after a device's removal.
            The device IDs 0 and 1 are reserved.
            AllDevices ........ 0
            AllMasterDevices .. 1

    DEVICEUSE { MasterPointer, MasterKeyboard, SlavePointer,
                SlaveKeyboard, FloatingSlave }
            A DEVICEUSE field specifies the current use of a device in the MD/SD
            device hierarchy. See Section 4 for more information.

    EVENTMASK
            An EVENTMASK is a binary mask defined as (1 << event type).
            A SETofEVENTMASK is a binary OR of zero or more EVENTMASK.

    FP1616
            Fixed point decimal in 16.16 format as one INT16 and one CARD16.
            The INT16 contains the integral part, the CARD32 the decimal fraction
            shifted by 16.

    FP3232
            Fixed point decimal in 32.32 format as one INT32 and one CARD32.
            The INT32 contains the integral part, the CARD32 the decimal fraction
            shifted by 32.

    VALUATORMASK
            A binary mask defined as (1 << valuator number).
            A SETofVALUATORMASK is a binary OR of zero or more VALUATORMASK.


[[errors]]
Errors
------

Errors are sent using core X error reports.

    Device
            A value for a DEVICE argument does not specify a valid DEVICE.


[[requests]]
Requests:
---------

The server does not guarantee that the length of a reply remains constant in
future revisions of XI2. A client must always retrieve the exact length of the
protocol reply from the connection, even if the reply is longer than defined
for the XI2 version supported by the client.
Additional bytes in a request may include data supported in later versions of
XI2. Clients should ignore this data. Padding bytes in XI2 protocol requests
are required to be 0.

[[requests-xi20]]
Requests introduced in version 2.0
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

[[requests-queryversion]]
    ┌───
        XIQueryVersion
        major_version:          CARD16
        minor_version:          CARD16
        ▶
        major_version:          CARD16
        minor_version:          CARD16
    └───

The client sends the highest supported version to the server and the
server sends the highest version it supports, but no higher than the
requested version. Major versions changes can introduce incompatibilities
in existing functionality, minor version changes introduce only backward
compatible changes.  It is the client's responsibility to ensure that the
server supports a version which is compatible with its expectations.

    major_version
        Major XI2 version.
    minor_version
        Minor XI2 version.

If major_version is less than 2, a BadValue error occurs.

[[requests-querydevice]]
    ┌───
        XIQueryDevice
        DEVICE                  deviceid
        ▶
        num_devices:            CARD16
        deviceinfo:             LISTofDEVICEINFO
    └───

    DEVICEINFO { deviceid:              DEVICEID
                 use:                   DEVICEUSE
                 attachment:            DEVICEID
                 enabled:               BOOL
                 num_classes:           CARD16
                 name_len:              CARD16
                 name:                  LISTofCHAR8
                 classes:               LISTofCLASS }

    CLASS { BUTTONCLASS, KEYCLASS, AXISCLASS, SCROLLCLASS, TOUCHCLASS }

    BUTTONCLASS { type:                 ButtonClass
                  length:               CARD16
                  sourceid:             CARD16
                  buttons_len:          CARD16
                  state:                SETofBUTTONMASK
                  labels:               LISTofATOM }

    KEYCLASS    { type:                 KeyClass
                  length:               CARD16
                  sourceid:             CARD16
                  num_keys:             CARD16
                  keys:                 LISTofCARD32 }

    AXISCLASS   { type:                 AxisClass
                  length:               CARD16
                  sourceid:             CARD16
                  axisnumber:           CARD16
                  label:                ATOM
                  min:                  FP3232
                  max:                  FP3232
                  value:                FP3232
                  resolution:           CARD32
                  mode:                 CARD8 }

    SCROLLCLASS* {type:                 ScrollClass
                  length:               CARD16
                  sourceid:             CARD16
                  axisnumber:           CARD16
                  scroll_type:          SCROLLTYPE
                  flags:                SETofSCROLLFLAGS
                  increment:            FP3232 }

    SCROLLTYPE { Vertical, Horizontal }

    SCROLLFLAGS { NoEmulation, Preferred }

    TOUCHCLASS† { type:                 TouchClass
                  length:               CARD16
                  sourceid:             CARD16
                  mode:                 TOUCHMODE
                  num_touches:          CARD16
                  num_props:            CARD16
                  props:                LISTofATOM }

    TOUCHMODE { DirectTouch, DependentTouch }

    * since XI 2.1
    † since XI 2.2

XIQueryDevice details information about the requested input devices.

    devices
        The device to list. If devices is AllDevices, all enabled and
        disabled devices are listed. If devices is AllMasterDevices, all
        enabled and disabled master devices are listed. If devices is a
        valid DEVICE, only this DEVICE is listed and num_devices is 1.
    num_devices
        The number of deviceinfos returned.

Each deviceinfo is detailed as follows:

    deviceid
        The unique ID of the device. Device IDs may get re-used when a device
        is removed.
    use
        If the device is a master pointer, use is MasterPointer.
        If the device is a master keyboard, use is MasterKeyboard.
        If the device is a slave pointer, use is SlavePointer.
        If the device is a slave keyboard, use is SlaveKeyboard.
        If the device is a floating slave, use is FloatingSlave.
    attachment
        If the device is a master pointer or a master keyboard, attachment
        specifies the paired master keyboard, or the paired master pointer,
        respectively.  If the device is a non-floating slave device
        attachment specifies the master device this device is attached to.
        If the device is a floating slave, attachment is undefined.
    enabled
        Zero if the device is disabled, non-zero otherwise.
    num_classes
        Number of classes provided.
    name_len
        Length of the name in bytes not including padding.
    classes
        Details the available classes provided by the device in an undefined
        order.
    name
        The device's name. padded to a multiple of 4 bytes.

For all classes, type specifies the device class. Clients are required
to ignore unknown device classes. The length field specifies the length
of the class in 4 byte units.
The following classes may occur only once: ButtonClass, KeyClass

    ButtonClass:
    type
        Always ButtonClass.
    length
        Length in 4 byte units.
    sourceid
        The device this class originates from.
    num_buttons
        Number of buttons provided by the device.
    labels
        List of Atoms specifying the label for each button. An Atom of None
        specifies an unlabeled button. Buttons are listed in the device-native
        order regardless of the current button mapping.
    state
        The current button mask for this device after button mapping is
        applied. Each bit representing a button is 1 if this button is
        logically down, or 0 otherwise. State is a multiple of 4-byte units
        and always contains at least num_buttons bits.

    KeyClass:
    type
        Always KeyClass.
    length
        Length in 4 byte units.
    sourceid
        The device this class originates from.
    num_keys
        Number of keycodes provided by the device.
    keys
        List of keycodes provided.

    AxisClass:
    type
        Always AxisClass.
    length
        Length in 4 byte units.
    sourceid
        The device this class originates from.
    axisnumber
        Axis number of this axis. The axis number is in device-native
        order and potential axis mappings are ignored.
    label
        Atom specifying the axis name. An Atom of None specifies an unlabeled
        axis.
    min
        Minimum value.
    max
        Minimum value.
    resolution
        Resolution in counts/meter.
    mode
        Relative or Absolute.
    value
        Last published axis value (if mode is absolute).

An axis in Relative mode may specify min and max as a hint to the
client. If no min and max information is available, both must be 0.

    ScrollClass:
    type
        Always ScrollClass.
    axisnumber
        Axis number that is referred to. This axis number must be listed in
        the ValuatorClassInfo.
    scroll_type:
        Vertical for a vertical scrolling axis, Horizontal for a horizontal
        scrolling axis.
    flags:
        A set of flags that apply to this scroll axis.
        NoEmulation: no legacy scroll button events are generated for events
                     on this scrolling axis.
        Preferred: This axis is the preferred axis for emulating valuator
                   events from legacy scroll button events.
    increment:
        The valuator delta equivalent to one positive unit of scrolling.

A ScrollClass may only exist if the device has at least one ValuatorClass
and each axisnumber listed in any ScrollClass. Only one ScrollClass may
exist per ValuatorClass.

    TouchClass:
    type
        Always TouchClass.
    length
        Length in 4 byte units.
    sourceid
        The device this class originates from.
    mode
        The device type of the touch device.  This mode may change at runtime.
    num_touches
        The maximum number of simultaneous touchpoints the device may send.
        If num_touches is 0, the number of supported touches is unknown or
        unlimited.
    num_props:
        The number of elements in props.
    props
        A list of properties to denote extra information about the device.

Devices with a TouchClass emit touch events with the same axes as pointer
events.

[[requests-selectevents]]
    ┌───
        XISelectEvents
            window:         Window
            num_masks:      CARD16
            masks:          LISTofEVENTMASK

    └───

    EVENTMASK { deviceid:          DEVICE,
                mask_len:          CARD16,
                mask:              SETofEVENTMASK

    window
        The window to select the events on.
    num_masks
        Number of items in masks.
    deviceid
        Numerical deviceid, or AllDevices, or AllMasterDevices.
    mask_len
        Length of mask in 4 byte units.
    mask
        Event mask. An event mask for an event type T is defined as (1 << T).

XISelectEvents selects for XI2 events on window.

If num_masks is 0, a BadValue error occurs.

Each mask sets the (and overwrites a previous) event mask for the DEVICE
specified through deviceid. The device AllDevices or
AllMasterDevices is treated as a separate device by server. A client's
event mask is the union of AllDevices, AllMasterDevices and the
per-device event mask.
The removal of device from the server unsets the event masks for the
device. If an event mask is set for AllDevices or AllMasterDevices, the
event mask is not cleared on device removal and affects all future
devices.

If mask_len is 0, the event mask for the given device is cleared.

The mask for XIHierarchyEvents may only be selected for XIAllDevices.
Setting it for any other device results in a BadValue error.

A client selecting for any of XI_TouchBegin, XI_TouchUpdate, or XI_TouchEnd
must select for all three events at the same time, else a BadValue error
will be generated. A client selecting for XI_TouchOwnership must select for
all three of the other touch events. If the selection for these touch events
overlaps a current selection by another client (e.g. selecting for a
specific device when another client has a selection for XIAllDevices), a
BadAccess error occurs.

[[requests-getselectedevents]]
    ┌───
        XIGetSelectedEvents
            window:         Window
            ▶
            num_masks:      CARD16
            masks:          LISTofEVENTMASK
    └───

    window
        The window to select the events on.
    num_masks
        Number of items in masks.
    masks
        Selected event masks by this client.

Masks are returned on a per-device basis, with masks for AllDevices and
AllMasterDevices returned separately. A client can calculate the
effective mask for a device with a bitwise OR of the AllDevices, the
AllMasterDevices and the device-specific mask.

If num_masks is 0, no events have been selected by this client on the
given window.

[[requests-querypointer]]
    ┌───
        XIQueryPointer
            window:         Window
            deviceid:       DEVICEID
            ▶
            root:           Window
            child:          Window
            root_x:         FP1616
            root_y:         FP1616
            win_x:          FP1616
            win_y:          FP1616
            same_screen:    BOOL
            mods:           MODIFIERINFO
            group:          GROUPINFO
            buttons_len:    CARD16
            buttons:        SETofBUTTONMASK
    └───

Query a master pointer device for its current position.

    root
        The root window the pointer is logically on.
    child
        The child window of window that contains the pointer or None.
    root_x
    root_y
        Pointer position relative to the root window's origin.
    win_x
    win_y
        Pointer position relative to window or 0 if same_screen is false.
    same_screen
        True if window is on the same screen as the pointer.
    mods
        XKB modifier state on the paired device.
    group
        XKB group state on the paired device.
    buttons_len
        The length of buttons in 4 byte units.
    buttons
        Button state.

If the device is not a master pointer device or not a floating slave
pointer, a BadDevice error results.

[[requests-warppointer]]
    ┌───
        XIWarpPointer
            src_win:         Window
            dst_win:         Window
            src_x:           FP1616
            src_y:           FP1616
            src_width:       INT16
            src_height:      INT16
            dst_x:           FP1616
            dst_y:           FP1616
            deviceid:        DEVICEID
    └───

WarpPointer moves the pointer of deviceid as if the user had moved
the pointer. WarpPointer can only be called for MasterPointer and
FloatingSlave devices.

    src_win
       If src_window is not None, the move only takes place if src_window
       contains the pointer and the pointer is contained in the specified
       rectangle of src_window.
    dst_win
       If dst_win is None, this request moves the pointer by offsets
       dst_x/dst_y relative to the current position of the pointer. If
        dst_window is a window, this request moves the pointer to
       dst_x/dst_y relative to dst_win's origin.
    src_x
    src_y
    src_width
    src_height
       Specifies the source window rectangle.
    dst_x
    dst_y
        The relative coordinates to move the pointer if dst_win is None, or
        the absolute coordinates if dst_win is a window.
    deviceid
        The device to warp.

This request cannot be used to move the pointer outside the confine-to
window of an active pointer grab. An attempt will only move the pointer as
far as the closest edge of the confine-to window.

This request will generate events just as if the user had instantaneously
moved the pointer.

[[requests-changecursor]]
    ┌───
        XIChangeCursor
            win:             Window
            cursor:          Cursor
            deviceid:        DEVICEID
    └───

Change a master pointer's cursor on the specified window.

    window
        The window.
    cursor
        The new cursor or None.
    deviceid
        The master pointer device.

Whenever device enters a window W, the cursor shape is selected in the
following order:

- if the current window has a device cursor C(d) defined for device,
  display this cursor C(d).
- otherwise, if the current window has a cursor C(w) defined in the core
  protocol's window attributes, display cursor C(w).
- repeat on parent window until a cursor has been found.

The device cursor for a given window is reset once the window is destroyed
or the device is removed, whichever comes earlier.

If deviceid does not specify a master pointer, a BadDevice error
is returned.

[[requests-changehierachy]]
    ┌───
        XIChangeHierarchy
            num_changes:     CARD8
            changes:         LISTofHIERARCHYCHANGES
    └───

    HIERARCHYCHANGE { ADDMASTER, REMOVEMASTER, ATTACHSLAVE, DETACHSLAVE }

    HIERARCHYCHANGETYPE { AddMaster, RemoveMaster, AttachSlave, DetachSlave }

    CHANGEMODE { Float, Attach }

    ADDMASTER { type:        HIERARCHYCHANGETYPE
                length:      CARD16
                name_len:    CARD16
                send_core:   BOOL
                enable:      BOOL
                name:        LISTofCHAR8 }

    REMOVEMASTER { type:            HIERARCHYCHANGETYPE
                   length:          CARD16
                   deviceid:        DEVICEID
                   return_mode:     CHANGEMODE
                   return_pointer:  DEVICEID
                   return_keyboard: DEVICEID }

    ATTACHSLAVE   { type:        HIERARCHYCHANGETYPE
                    length:      CARD16
                    deviceid:    DEVICEID
                    master:      DEVICEID }

    DETACHSLAVE { type:       HIERARCHYCHANGETYPE
                  length:     CARD16
                  deviceid:   DEVICEID }

XIChangeHierarchy allows a client to modify the
<<hierachy,Master/Slave device hierarchy>>.

    num_changes
        The number of changes to apply to the current hierarchy.
    changes
        The list of changes.

The server processes the changes in the order received from the client and
applies each requested change immediately. If an error occurs, processing
stops at the current change and returns the number of successfully applied
changes in the error.

    ADDMASTER creates a pair of master devices.
    type
        Always AddMaster.
    length
        Length in 4 byte units.
    name_len
        Length of name in bytes.
    send_core
        True if the device should send core events.
    enable
        True if the device is to be enabled immediately.
    name
        The name for the new master devices. The master pointer's name is
        automatically appended with " pointer", the master keyboard's name is
        automatically appended with " keyboard".

    REMOVEMASTER removes an existing master device.
    type
        Always RemoveMaster.
    length
        Length in 4 byte units.
    deviceid
        The device to remove.
    return_mode
        Return mode for attached slave devices.
        If return_mode is Float, all slave devices are set to floating.
        If return_mode is Attach, slave pointers are attached to
        return_pointer and slave keyboards are attached to
        return_keyboard.
    return_pointer
    return_keyboard
        The master pointer and master keyboard to attach slave devices to, if
        return_mode is Attach. If return_mode is Float, return_pointer
        and return_keyboard are undefined.

Removing a master pointer removes the paired master keyboard and vice
versa.

    ATTACHSLAVE attaches a slave device to a given master device.
    type
        Always ChangeAttachment.
    length
        Length in 4 byte units.
    deviceid
        Deviceid of the slave device.
    master
        The new master device to attach this slave device to.

If any clients are selecting for touch events from the slave device, their
selection will be canceled.

    DETACHSLAVE detaches a slave device from its current master device.
    type
        Always ChangeAttachment.
    length
        Length in 4 byte units.
    deviceid
        Deviceid of the slave device.

[[requests-setclientpointer]]
    ┌───
        XISetClientPointer
            win:             Window
            deviceid:        DEVICEID
    └───

Set the ClientPointer for the client owning win to the given device.

    win
         Window or client ID.
    deviceid
         The master pointer or master keyboard that acts as ClientPointer.

Some protocol requests are ambiguous and the server has to choose a device
to provide data for a request or a reply. By default, the server will
choose a client's ClientPointer device to provide the data, unless the
client currently has a grab on another device. See section 4.4 for more
details.

If win is None, the ClientPointer for this client is set to the given
device. Otherwise, if win is a valid window, the ClientPointer for the
client owning this window is set to the given device. Otherwise, if win is
not a valid window but a client with the client mask equal to win exists,
this client's ClientPointer is set to the given device.

If deviceid does not specify a master pointer or master keyboard, a
BadDevice error is returned.

If window does not specify a valid window or client ID and is not None, a
BadWindow error is returned.

[[requests-getclientpointer]]
    ┌───
        XIGetClientPointer
            win:             Window
            ▶
            set:             BOOL
            deviceid:        DEVICEID
    └───

Query the ClientPointer for the client owning win.

    win
        The window or client ID.
    set
        True if the client has a ClientPointer set.
    deviceid
        The master pointer that acts as a ClientPointer if set is True.

No difference is made between a ClientPointer set explicitly through
XISetClientPointer and a ClientPointer implicitly assigned by the server
in response to an ambiguous request.

[[requests-setfocus]]
    ┌───
        XISetFocus
            focus:           Window
            deviceid:        DEVICEID
            time:            Time
    └───

Set the focus for the given device to the given window. Future key events
from this device are sent to this window.
This request generates FocusIn and FocusOut events.

    focus
        A viewable window or None.
    deviceid
        The device to modify the focus window for.
    time
        Specifies the time to change the focus or CurrentTime.

If focus is None, key events from this device are discarded until a new
focus window is set. If focus is a viewable window, key events from this
device are sent to this window. If the window becomes unviewable, the
window's first viewable ancestor automatically becomes the focus window
and FocusIn and FocusOut events are sent as if a client had changed the
focus window.
This is equivalent to RevertToParent in the core XSetInputFocus window.

This request has no effect if the specified time is earlier than the
current last-focus-change time or is later than the current X server time.
Otherwise, the last-focus-change time is set to the specified time.

[[requests-getfocus]]
    ┌───
        XIGetFocus
            deviceid:        DEVICEID
            ▶
            focus:           Window
    └───

Return the current focus window for the given device.

[[requests-grabdevice]]
    ┌───
        XIGrabDevice
            deviceid:        DEVICEID
            grab_window:     Window
            owner_events:    BOOL
            grab_mode:       { Synchronous, Asynchronous }
            paired_device_mode: { Synchronous, Asynchronous }
            time:            TIMESTAMP or CurrentTime
            cursor:          Cursor
            mask_len:        CARD16
            masks:           SETofEVENTMASK
            ▶
            status:          Success, AlreadyGrabbed, Frozen, InvalidTime, NotViewable
    └───

This request actively grabs control of the specified input device. Further
input events from this device are reported only to the grabbing client.
This request overides any previous active grab by this client for this
device.  This request does not, however, affect the processing of XI 2.2
touch events.

    deviceid
        The device to grab.
    grab_window
        Events are reported relative to the grab window.
    owner_events
        Specifies whether event will be reported normally or relative to the
        grab window.
    grab_mode
        Specifies if this device will be frozen as a result of the grab.
    paired_device_mode
        Specifies if the master device paired with this device will be frozen
        as a result of the grab.
    time
        A valid server time or CurrentTime.
    cursor
        The cursor to display for the duration of the grab or None.
    mask_len
        Length of mask in 4 byte units.
    mask
        Event mask. An event mask for an event type T is defined as (1 << T).
    status
        Success or the reason why the grab could not be established.

The masks parameter specifies which events the client wishes to receive
while the device is grabbed.

If owner-events is False, input events generated from this device are
reported with respect to grab-window, and are only reported if selected by
being included in the event-list.  If owner-events is True, then if a
generated event would normally be reported to this client, it is reported
normally, otherwise the event is reported with respect to the grab-window,
and is only reported if selected by being included in the event-list. For
either value of owner-events, unreported events are discarded.

If grab-mode is Asynchronous, device event processing continues normally.
If the device is currently frozen by this client, then processing of
device events is resumed. If grab-mode is Synchronous, the state of the
grabbed device (as seen by means of the protocol) appears to freeze,
and no further device events are generated by the server until the
grabbing client issues a releasing XIAllowEvents request or until the
device grab is released. Actual device input events are not lost while the
device is frozen; they are simply queued for later processing.

If the device is a slave device, the paired-device-mode is ignored.
Otherwise, if this device is a master device and paired-device-mode is
Asynchronous, event processing is unaffected by activation of the grab. If
this device is a master device and paired-device-mode is Synchronous, the
state of the master device paired with this device (as seen by means of the
protocol) appears to freeze, and no further events are generated by the
server until the grabbing client issues a releasing XIAllowEvents request
or until the device grab is released. Actual events are not lost while the
devices are frozen; they are simply queued for later processing.

If the cursor is not None and the device is a master pointer device, the
cursor will be displayed until the device is ungrabbed.

This request fails and returns:

    AlreadyGrabbed: If the device is actively grabbed by some other client.
    NotViewable: If grab-window is not viewable.
    InvalidTime: If the specified time is earlier than the last-grab-time for
                 the specified device or later than the current X server time.
                 Otherwise, the last-grab-time for the specified device is set
                 to the specified time and CurrentTime is replaced by the
                 current X server time.
    Frozen: If the device is frozen by an active grab of another client.

To release a grab of a device, use XIUngrabDevice.

[[requests-ungrabdevice]]
    ┌───
        XIUngrabDevice
            deviceid:        DEVICEID
            time:            TIMESTAMP or CurrentTime
    └───

This request releases the device if this client has it actively grabbed
(from either XIGrabDevice or  XIPassiveGrabDevice) and
releases any queued events. If any devices were frozen by the grab,
XIUngrabDevice thaws them.

    deviceid
        The device to grab.
    time
        A valid server time or CurrentTime.

The request has no effect if the specified time is earlier than the
last-device-grab time or is later than the current server time.
This request generates FocusIn and FocusOut events.
An XIUngrabDevice is performed automatically if the event window for an
active device grab becomes not viewable.

[[requests-allowevents]]
    ┌───
        XIAllowEvents:
            deviceid:        DEVICEID
            time:            TIMESTAMP or CurrentTime
            event_mode:      { AsyncDevice, SyncDevice,
                               AsyncPairedDevice, SyncPairedDevice,
                               ReplayDevice, AsyncPair, SyncPair, AcceptTouch*,
                               RejectTouch* }
            touchid*:        CARD32
            grab_window*:    Window
    └───

* since XI 2.2

The XIAllowEvents request releases some queued events if the client
has caused a device to freeze. It also is used to handle touch grab and
ownership processing.

    deviceid
        The device to grab.
    time
        A valid server time or CurrentTime.
    event_mode
        Specifies whether a device is to be thawed and events are to be
        replayed, or how to handle a grabbed touch sequence.
    touchid
        The ID of the touch sequence to accept or reject. The value is undefined
        for event modes other than AcceptTouch and RejectTouch.
    grab_window
        The window on which to accept or reject a touch sequence grab. The value
        is undefined for event modes other than AcceptTouch and RejectTouch.

The request has no effect if the specified time is earlier than the last-grab
time of the most recent active grab for the client, or if the specified time is
later than the current X server time. The time parameter must be CurrentTime for
requests with event modes of AcceptTouch and RejectTouch.

When event-mode is AcceptTouch, a BadValue error occurs if the touch ID is
invalid. A BadAccess error occurs if this client is not the current or potential
owner of the specified touch ID.

The following describes the processing that occurs depending on what constant
you pass to the event-mode argument:

    AsyncDevice:
        If the specified device is frozen by the client, event processing for that
        device continues as usual. If the device is frozen multiple times  by the
        client on behalf of multiple separate grabs, AsyncDevice thaws for
        all.
        AsyncDevice has no effect if the specified device is not frozen by the
        client, but the device need not be grabbed by the client.
    SyncDevice:
        If the specified device is frozen and actively grabbed by the client,
        event processing for that device continues normally until the next
        event is reported to the client. At this time, the specified device
        again appears to freeze. However, if the reported event causes the
        grab to be released, the specified device does not freeze.
        SyncDevice has no effect if the specified device is not frozen by the
        client or is not grabbed by the client.
     ReplayDevice:
        If the specified device is actively grabbed by the client and is frozen
        as the result of an event having been sent to the client (either from
        the activation of a XIGrabButton or from a previous XIAllowEvents with
        mode SyncDevice, but not from a Grab), the grab is released and
        that event is completely reprocessed.  This time, however, the request
        ignores any passive grabs at or above (towards the root) the
        grab-window of the grab just released.
        The request has no effect if the specified device is not grabbed by
        the client or if it is not frozen as the result of an event.
     AsyncPairedDevice
        If the paired master device is frozen by the client, event processing
        for it continues as usual. If the paired device is frozen multiple
        times by the client on behalf of multiple separate grabs,
        AsyncPairedDevice thaws for all.
        AsyncPairedDevice has no effect if the device is not frozen by the
        client, but those devices need not be grabbed by the client.
        AsyncPairedDevice has no effect if deviceid specifies a slave device.
     SyncPairedDevice
        If the paired master device is frozen by the client, event processing (for
        the paired master device) continues normally until the next button or key
        event is reported to the client for the grabbed device (button event for
        the grabbed device, key or motion event for the device), at which time
        the device again appears to freeze. However, if the reported event causes
        the grab to be released, then the device does not freeze.
        SyncPairedDevice has no effect if the specified device is not grabbed
        by the client or if it is no frozen as the result of an event.
        SyncPairedDevice has no effect if deviceid specifies a slave device.
     SyncPair
        If both the device and the paired master device are frozen by the
        client, event processing (for both devices) continues normally until
        the next XIButtonPress, XIButtonRelease, XIKeyPress, or XIKeyRelease
        event is reported to the client for a grabbed device (button event for
        a pointer, key event for a keyboard), at which time the devices again
        appear to freeze. However, if the reported event causes the grab to be
        released, then the devices do not freeze (but if the other device is
        still grabbed, then a subsequent event for it will still cause both
        devices to freeze).
        SyncPair has no effect unless both the device and the paired master
        device are frozen by the client. If the device or paired master device
        is frozen twice by the client on behalf of two separate grabs,
        SyncPair thaws for both (but a subsequent freeze for SyncPair will
        only freeze each device once).
        SyncPair has no effect if deviceid specifies a slave device.
     AsyncPair
        If the device and the paired master device are frozen by the client,
        event processing for both devices continues normally. If a device is
        frozen twice by the client on behalf of two separate grabs, AsyncBoth
        thaws for both. AsyncPair has no effect unless both the device and the
        paired master device frozen by the client.
        AsyncPair has no effect if deviceid specifies a slave device.
     AcceptTouch
        The client is deemed to have taken control of the touch sequence once it
        owns the sequence. TouchEnd events will be sent to all clients listening
        to the touch sequence that have either grabbed the touch sequence on a
        child window of the grab_window or have received events for the touch
        sequence through event selection. These clients will no longer receive
        any TouchUpdate events.
     RejectTouch
        The client is no longer interested in the touch sequence, and will
        receive a TouchEnd event. If the client is the current owner of the
        sequence, ownership will be passed on to the next listener.

[[requests-passivegrabdevice]]
    ┌───
        XIPassiveGrabDevice
            deviceid:        DEVICE
            detail:          CARD32
            grab_type:       GRABTYPE
            grab_window:     Window
            cursor:          Cursor
            owner_events:    Bool
            grab_mode:       { Synchronous, Asynchronous, Touch* }
            paired_device_mode: { Synchronous, Asynchronous }
            num_modifiers:   INT16
            mask_len:        CARD16
            masks:           SETofEVENTMASK
            modifiers:       CARD32 or GrabAnyModifier
            ▶
            num_modifiers_return:    INT16
            modifiers_return:        GRABMODIFIERINFO
    └───

        GRABTYPE         { GrabtypeButton, GrabtypeKeycode, GrabtypeEnter,
                           GrabtypeFocusIn, GrabtypeTouchBegin }

        GRABMODIFIERINFO {   status:    Access
                             modifiers: CARD32 }

* since XI 2.2

Establish an explicit passive grab for a button or keycode
on the specified input device.

        cursor
            The cursor to display for the duration of the grab. If grab_type
            is not GrabtypeButton, this argument is ignored.
        deviceid
            The device to establish the passive grab on or AllDevices or
            AllMasterDevices.
        detail
            The button number, or key symbol to grab for.
            Must be 0 for GrabtypeEnter, GrabtypeFocusIn, and
            GrabtypeTouchBegin.
        grab_type
            The type of grab to establish.
        grab_window
            Events are reported relative to the grab window.
        grab_mode
            If grab-mode is Asynchronous, device event processing continues
            normally.  If the device is currently frozen by this client, then
            processing of device events is resumed. If grab-mode is
            Synchronous, the state of the grabbed device (as seen by means of
            the protocol) appears to freeze, and no further device events are
            generated by the server until the grabbing client issues a
            releasing XIAllowEvents request or until the device grab is
            released. Actual device input events are not lost while the device
            is frozen; they are simply queued for later processing. If grab_type
            is GrabtypeTouchBegin, grab_mode must be set to Touch.
        mask_len
            Length of mask in 4 byte units.
        mask
            Event mask. An event mask for an event type T is defined as (1 << T).
        modifiers
            XKB modifier state to activate this passive grab.
        num_modifiers
            Number of elements in modifiers.
        owner_events
            Specifies whether event will be reported normally or relative to the
            grab window.
        num_modifiers_return
            Number of elements in modifiers_return
        modifiers_return
            XKB modifier state that could not be grabbed.

If owner-events is False, input events generated from this device are
reported with respect to grab-window, and are only reported if
selected by being included in the event-list.  If owner-events is
True, then if a generated event would normally be reported to this
client, it is reported normally, otherwise the event is reported
with respect to the grab-window, and is only reported if selected
by being included in the event-list. For either value of
owner-events, unreported events are discarded.

If deviceid specifies a master pointer, the modifiers of the paired
master keyboard are used. If deviceid specifies a slave pointer
the modifiers of the master keyboard paired with the attached master
pointers are used. If deviceid specifies a slave keyboard, the
modifiers of the attached master keyboard are used. Note that
activating a grab on a slave device detaches the device from its
master. In this case, the modifiers after activation of the grab are
from the slave device only and may be different to the modifier state
when the grab was triggered.

In the future, if grab_type is GrabtypeButton or GrabtypeKeyboard, the
device is actively grabbed if:

        - the device is not grabbed, and
        - the specified modifier keys are down, and
        - the grab_type is GrabtypeButton and the button specified in detail
          is logically pressed or the grab_type is GrabtypeKeycode and the
          keycode specified in detail is logically pressed, and
        - the grab_window contains the pointer, and
        - a passive grab on the same button/keycode + modifier
          combination does not exist on an ancestor of grab_window.

Otherwise, if grab_type is GrabtypeEnter or GrabtypeFocusIn, the
device is actively grabbed if:

        - the device is not actively grabbed, and
        - the specified modifier keys are down, and
        - the grab_type is GrabtypeEnter and the device's pointer has moved
          into grab_window or a descendant of grab_window, or the grab_type is
          GrabtypeFocusIn and the device's focus has been set to the
          grab_window or a descendant of grab_window, and
        - a passive grab of the same grab_type + modifier combination does not
          does not exist on an ancestor of grab_window.

Otherwise, if grab_type is GrabtypeTouchBegin, a touch grab begins if:

        - the device is not actively grabbed, and
        - the specified modifier keys are down
        - a touch begins in grab_window or a descendant of grab_window, and
        - a passive grab of the same grab_type + modifier combination does not
          does not exist on an ancestor of grab_window.

Ownership of the touch sequence is granted to the grabbing client if:

        - a TouchBegin or pointer grab for an emulated touch sequence of a
          direct touch device with the same modifier set does not exist on
          an ancestor of grab_window, or all applicable grabs have released
          ownership.

A modifier of GrabAnyModifier is equivalent to issuing the request for
all possible modifier combinations (including no modifiers). A client
may request a grab for GrabAnyModifier and explicit modifier
combinations in the same request.

A GrabtypeButton or GrabtypeKeyboard grab is released when all buttons
or keycode are released, independent of the state of modifier keys.
A GrabtypeEnter or GrabtypeFocusIn grab is released when the
pointer or focus leaves the window and all of its descendants,
independent of the state of modifier keys.
A GrabtypeTouchBegin grab is released when the touch sequence ends or
the client uses XIAllowEvents with mode RejectTouch.
Note that the logical state of a device (as seen by means of the
protocol) may lag the physical state if device event processing is
frozen.

This request overrides all previous passive grabs by the same
client on the same button/key/enter/focus in + modifier combinations
on the same window.

If some other client already has issued a XIPassiveGrabDevice request
with the same button or keycode and modifier combination, the
failed modifier combinations is returned in modifiers_return. If some
other client already has issued an XIPassiveGrabDevice request of
grab_type XIGrabtypeEnter, XIGrabtypeFocusIn, XIGrabtypeTouchBegin, or
XIGrabtypeTouchBeginInert with the same grab_window and the same
modifier combination, the failed modifier combinations are returned
in modifiers_return. If num_modifiers_return is zero, all passive
grabs have been successful.

If a button grab or enter grab activates, EnterNotify and LeaveNotify
events with mode Grab are generated as if the pointer were to suddenly
warp from its current position some position in the grab_window.
However, the pointer does not warp, and the pointer position is used
as both the initial and final positions for the events.

If a keycode grab or focus grab activates, FocusIn and FocusOut events
with mode Grab are generated as if the focus were to change from the
current window to the grab_window.

If an enter or focus in grab activates, additional EnterNotify events
with mode XIPassiveGrabNotify are generated as if the pointer or focus
were to suddenly warp from its current position to some position in
the grab window.  These events are sent to the grabbing client only
and only if the grab event mask has selected for it. If such a passive
grab deactivates, addional LeaveNotify events with mode
XIPassiveUngrabNotify are generated and sent to the grabbing client
before the grab deactivates.

For GrabtypeTouchBegin, grab_mode must be Touch or a BadValue error
is generated.

See section 4.4 for additional notes on touch grabs, as they do not
behave like traditional grabs: in particular, they do not freeze the
device, and delivery of touch events continues even if the device is
frozen due to a grab by another client.

[[requests-passiveungrabdevice]]
    ┌───
        XIPassiveUngrabDevice
            deviceid:        DEVICEID
            detail:          CARD32
            grab_type:       GRABTYPE
            grab_window:     Window
            num_modifiers:   INT16
            modifiers:       MODIFIERINFO
    └───

Release an explicit passive grab on the specified input device.

        deviceid
            The device to establish the passive grab on.
        detail
            The button number or key symbol to ungrab.
            Must be 0 for GrabtypeEnter, GrabtypeFocusIn, and
            GrabtypeTouchBegin.
        grab_type
            The type of grab to establish.
        grab_window
            Events are reported relative to the grab window.
        modifiers
            XKB modifier state to activate this passive grab.
        num_modifiers
            Number of elements in modifiers.

This request has no effect if the client does not have a passive grab
of the same type, same button or keycode (if applicable) and modifier
combination on the grab_window.

[[requests-listproperties]]
    ┌───
        XIListProperties
            deviceid:        DEVICEID
            ▶
            num_properties:  INT16
            properties:      LISTofATOM
    └───

List the properties associated with the given device.

        deviceid
            The device to list the properties for.
        num_atoms
            Number of atoms in the reply
        atoms
            All properties on the device.

[[requests-changeproperty]]
    ┌───
        XIChangeProperty
            deviceid:        DEVICEID
            property:        ATOM
            type:            ATOM
            format:          { 8, 16, 32 }
            mode:            { Append, Prepend, Replace }
            num_items:       CARD32
            data:            LISTofINT8, or LISTofINT16, or LISTofINT32
    └───

Change the given property on the given device.

        deviceid
            The device to change the property on.
        property
            The property to modify.
        type
            The property's type.
        mode
            One of Append, Prepend, or Replace
        num_items
            Number of items following this request.
        data
            Property data (nitems * format/8 bytes)

The type is uninterpreted by the server. The format specifies whether
the data should be viewed as a list of 8-bit, 16-bit, or 32-bit
quantities so that the server can correctly byte-swap as necessary.

If the mode is Replace, the previous propert y value is discarded.  If
the mode is Prepend or Append, then the type and format must match the
existing property value (or a Match error results). If the property is
undefined, it is treated as defined with the correct type and format
with zero-length data. For Prepend, the data is tacked on to the
beginning of the existing data, and for Append, it is tacked on to the
end of the existing data.

The lifetime of a property is not tied to the storing client. Properties
remain until explicitly deleted, until the device is removed, or
until server reset.

A property cannot be deleted by setting nitems to zero. To delete a
property, use XIDeleteProperty.

This request generates an XIPropertyEvent.

[[requests-deleteproperty]]
    ┌───
        XIDeleteProperty
            deviceid:        DEVICEID
            property:        ATOM
    └───

Deletes the given property on the given device.

        deviceid
            The device to delete the property on.
        property
            The property to delete.

If the property is deleted, an XIPropertyEvent is generated on the device.
If the property does not exist, this request does nothing.

[[requests-getproperty]]
    ┌───
        XIGetProperty
            deviceid:        DEVICEID
            property:        ATOM
            type:            Atom or AnyPropertyType
            offset:          CARD32
            len:             CARD32
            delete:          BOOL
            ▶
            type:            Atom
            bytes_after:     CARD32
            num_items:       CARD32
            format:          { 8, 16, 32 }
            data:            LISTofINT8, or LISTofINT16, or LISTofINT32
    └───

Get the data for the given property on the given device.

        deviceid
            The device to retrieve the property data from.
        property
            The property to retrieve the data from..
        type
            The property type to retrieve or AnyPropertyType
        offset
            The offset in 4-byte units.
        len
            Number of bytes to receive in 4-byte units.
        delete
            Delete the property after retrieving the data.
        bytes_after
            Number of unread bytes in the stored property
        num_items
            Number of items in data
        format
            8, 16, or 32
        data
            Property data (nitems * format/8 bytes)

If the specified property does not exist for the specified device, then
the return type is None, the format and bytes-after are zero, and the value is
empty. The delete argument is ignored in this case. If the specified property
exists but its type does not match the specified type, then the return
type is the actual type of the property, the format is the actual format of the
property (never zero), the bytes-after is the length of the property in bytes
(even if the format is 16 or 32), and the value is empty. The delete
argument is ignored in this case. If the specified property exists and
either AnyPropertyType is specified or the specified type matches the actual
type of the property, then the return type is the actual type of the property,
the format is the actual format of the property
(never zero), and the bytes-after and value are as follows, given:
         N = actual length of the stored property in bytes
            (even if the format is 16 or 32)
         I = 4 * long-offset
         T = N−I
         L = MINIMUM(T, 4 * long-length)
         A = N − (I + L)
The returned value starts at byte index I in the property (indexing
from 0), and its length in bytes is L. However, it is a Value error if
offset is given such that L is negative. The value of bytes_after is A,
giving the number of trailing unread bytes in the stored property. If
delete is True and the bytes_after is zero, the property is also
deleted from the device, and a XIPropertyNotify event is generated on
the device.  
     
[[events]]
Events
------

An event specifies its length in 4-byte units after the initial 32 bytes.
Future versions of the protocol may provide additional information
in the same event, thus increasing the event size. Clients are required to
always read the number of bytes specified by the event, not the size of the
event they may have been compiled against.


The following event types are available in XI2.

Version 2.0:

        - HierarchyChanged
        - DeviceChanged
        - KeyPress
        - KeyRelease
        - ButtonPress
        - ButtonRelease
        - Motion
        - RawKeyPress
        - RawKeyRelease
        - RawButtonPress
        - RawButtonRelease
        - RawMotion
        - Enter
        - Leave
        - FocusIn
        - FocusOut
        - PropertyEvent

Version 2.2:

        - TouchBegin
        - TouchUpdate
        - TouchOwnership
        - TouchEnd
        - RawTouchBegin
        - RawTouchUpdate
        - RawTouchEnd

All events have a set of common fields specified as EVENTHEADER.


    EVENTHEADER { type:                       BYTE
                  extension:                  BYTE
                  sequenceNumber:             CARD16
                  length:                     CARD32
                  evtype:                     CARD16
                  deviceid:                   DEVICEID
                  time:                       Time }

    type
        Always GenericEvent.
    extension
        Always the X Input extension offset.
    sequenceNumber
        Sequence number of last request processed by the server.
    length
        Length in 4-byte units after the initial 32 bytes.
    evtype
        XI-specific event type.
    deviceid
        Numerical device id for a device.
    time
        Time in ms when the event occurred.


[[events-xi20]]
Events introduced in version 2.0
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

[[events-hierachyevent]]
    ┌───
        HierarchyEvent:
            EVENTHEADER
            flags:                      SETofHIERARCHYMASK
            num_info:                   CARD16
            info:                       LISTofHIERARCHYINFO
    └───


    HIERARCHYMASK { MasterAdded, MasterRemoved, SlaveAttached, SlaveDetached,
                    SlaveAdded, SlaveRemoved, DeviceEnabled, DeviceDisabled }

    HIERARCHYINFO { deviceid:           DEVICEID,
                    attachment:         DEVICEID,
                    type:               DEVICEUSE
                    enabled:            BOOL
                    flags:              SETofHIERARCHYMASK}

    flags
        Set of the changes that have occured, causing this event.
    num_info
        The number of device info structs following the request.
    info:
        The current hierarchy information.

An XIHierarchyEvent is sent whenever the device hierarchy been
changed. The flags specify all types of hierarchy modifiations that have
occured.
For all devices, info details the hierarchy information after the
modification of the hierarchy has occured. For each device specified with
deviceid:

- if type is MasterPointer or MasterKeyboard, attachment decribes the
  pairing of this device.
- if type is SlavePointer or SlaveKeyboard, attachment describes the
  master device this device is attached to.
- if type is FloatingSlave device, attachment is undefined.

    enabled
         True if the device is enabled and can send events. A disabled master
         device will not forward events from an attached, enabled slave
         device.

Note: Multiple devices may be affected in one hierarchy change,
deviceid in an XIHierarchyEvent is always the first affected
device. Clients should ignore deviceid and instead use the devices list.

[[events-devicechangedevent]]
    ┌───
        DeviceChangedEvent:
            EVENTHEADER
            reason:                CHANGEREASON
            source:                DEVICEID
            num_classes:           CARD16
            classes:               LISTofCLASS
    └───

    CHANGEREASON { SlaveSwitch, DeviceChange }

A DeviceChangeEvent is sent whenever a device changes it's capabilities.
This can happen either by a new slave device sending events through a
master device, or by a physical device changing capabilities at runtime.

    reason
        The reason for generating this event.
        If reason is SlaveSwitch, the slave device sending events through
        this device has changed and source specifies the new slave device.
        A SlaveSwitch reason can only occur on a master device.
        If reason is DeviceChange, the device itself has changed through
        other means (e.g. a physical device change) and source is
        the device itself.
    source
        The source of the new classes.
    num_classes
        Number of classes provided.
    classes
        Details the available classes provided by the device.  The order the
        classes are provided in is undefined.

For a detailed description of classes, see the XIQueryDevice request.

[[events-deviceevent]]
    ┌───
        DeviceEvent:
            EVENTHEADER
            detail:                     CARD32
            root:                       Window
            event:                      Window
            child:                      Window
            root_x:                     FP1616
            root_y:                     FP1616
            event_x:                    FP1616
            event_y:                    FP1616
            buttons_len:                CARD16
            valuators_len:              CARD16
            sourceid:                   DEVICEID
            mods:                       MODIFIERINFO
            group:                      GROUPINFO
            flags:                      DEVICEEEVENTFLAGS
            buttons:                    SETofBUTTONMASK
            valuators:                  SETofVALUATORMASK
            axisvalues:                 LISTofFP3232
    └───

    BUTTONBIT { (1 << Button1), (1 << Button2), ... , (1 << ButtonN) }
    VALUATORBIT { (1 << 1), ( 1 << 2), ... ( 1 << n) }

    MODIFIERINFO  { base_mods:           CARD32,
                    latched_mods:        CARD32,
                    locked_mods:         CARD32,
                    effective_mods:      CARD32}
    GROUPINFO     { base_group:          CARD8,
                    latched_group:       CARD8,
                    locked_group:        CARD8,
                    effective_group:     CARD8}

    DEVICEEVENTFLAGS (all events): none
    DEVICEEVENTFLAGS (key events only): { KeyRepeat }
    DEVICEEVENTFLAGS (pointer and touch events only): { PointerEmulated }
    DEVICEEVENTFLAGS (touch events only): { TouchPendingEnd }

An XIDeviceEvent is generated whenever the logical state of a device
changes in response to a button press, a button release, a motion, a key
press or a key release. The event type may be one of KeyPress,
KeyRelease, ButtonPress, ButtonRelease, Motion.

    XI 2.2: The event type may also be TouchBegin, TouchUpdate, or TouchEnd.

    detail
        The button number, key code, touch ID, or 0.
    root
    event
    child
        The root window, event window or subwindow, respectively. See core
        protocol specification for more detail.
    root_x
    root_y
        The position of the pointer in screen coordinates (16.16 fixed point).
    event_x
    event_y
        The position of the pointer in screen coordinates relative to the
        event window (16.16 fixed point).

    buttons_len
        The length of buttons in 4 byte units.
    valuators_len
        The length of valuators in 4 byte units.
    sourceid
        The source device that originally generated the event.
    mods
        XKB modifier state before the event occured.
    group
        XKB group state before the event.
    buttons
        Button state before the event.
    valuators
        Bitmask of valuators provided in axisvalues.
    axisvalues
        Valuator data in device-native resolution.
    flags
        Miscellaneous information about this event; the union of the
        common flag set and either the key or pointer flag set,
        depending on the event type.
        KeyRepeat means that this event is for repeating purposes, and
        the physical state of the key has not changed.  This is only
        valid for KeyPress events.
        PointerEmulated signals that the event has been emulated from another
        XI 2.x event for legacy client support, and that this event should
        be ignored if the client listens for these events.  This flag is
        set on scroll ButtonPress and RawButtonPress events (buttons 4, 5, 6
        and 7) if a smooth-scrolling event on the Rel Vert Scroll or
        Rel Horiz Scroll axes was also generated. It is also set on Motion,
        ButtonPress, and ButtonRelease events generated by direct touch devices.
        TouchAccepted (for TouchEnd events only) means that the current owner
        of the touch stream has “accepted” it, and this client will not receive
        any further events from that touch sequence.
        TouchPendingEnd (for touch events only) means that the touch
        has physically ended, however another client still holds a grab, so the
        touch should be considered alive until all grabbing clients have
        accepted or passed on ownership.  The touch will not generate any
        further TouchUpdate events once an event with TouchPendingEnd has been
        received.

Modifier state in mods is detailed as follows:

    base_mods
        XKB base modifier state.
    latched_mods
        XKB latched modifier state.
    locked_mods
        XKB locked modifier state.

    Group state in group is detailed as follows:
    base_group
        XKB base group state.
    latched_group
        XKB latched group state.
    locked_group
        XKB locked group state.

    XI 2.2:

A TouchBegin event is generated whenever a new touch sequence initializes
A TouchEnd event is generated whenever a touch sequence ceases. A
TouchUpdate event is generated whenever a valuator value changes, or a flag
flag (e.g. pending end) has changed for that touch sequence; this may result
in a TouchUpdate event being sent with zero valuators. A TouchOwnership event
is sent when a client becomes the owner of a touch.

The average finger size is significantly larger than one pixel. The
selection of the hotspot of a touchpoint is implementation dependent and
may not be the logical center of the touch.

Touch tracking IDs are provided in the detail field of touch events. Its
value is always provided in every touch event. Tracking IDs are
represented as unsigned 32-bit values and increase strictly monotonically in
value for each new touch, wrapping back to 0 upon reaching the numerical limit
of IDs. The increment between two touch IDs is indeterminate. Clients may not
assume that any future touches will have specific touch IDs. IDs are globally
unique.

Touch events do not generate enter/leave events.

[[events-rawevent]]
    ┌───
        RawEvent
            EVENTHEADER
            detail:                    CARD32
            sourceid*:                 DEVICEID
            flags:                     DEVICEEVENTFLAGS
            valuators_len:             CARD16
            valuators:                 SETofVALUATORMASK
            axisvalues:                LISTofFP3232
            axisvalues_raw:            LISTofFP3232
    └───

    * since XI 2.1

A RawEvent provides the information provided by the driver to the
client. RawEvent provides both the raw data as supplied by the driver and
transformed data as used in the server. Transformations include, but are
not limited to, axis clipping and acceleration.
Transformed valuator data may be equivalent to raw data. In this case,
both raw and transformed valuator data is provided.
RawEvents are sent exclusively to all root windows.
Clients supporting XI 2.0 receive raw events when the device is not grabbed,
or when the device is grabbed by the client but not when the device is
grabbed by another client.
Clients supporting XI 2.1 or later receive raw events at all times, even
when the device is grabbed by another client.


    eventtype
        The type of event that occured on the device.
    detail
        The button number, keycode or touch ID*.
    sourceid
        The source device that originally generated the event. The sourceid
        is undefined for clients not supporting XI 2.1.
    flags
        Flags as described in DeviceEvent.
    valuators_len
        The length of valuators in 4 byte units.
    valuators
        Bitmask of valuators provided in axisvalues and axisvalues_raw.
    axisvalues
        Valuator data in device-native resolution.
    axisvalues_raw
        Untransformed valuator data in device-native resolution.

* since XI 2.2

[[events-enterleave]]
    ┌───
        Enter or Leave or FocusIn or FocusOut
            EVENTHEADER
            root:               Window
            event:              Window
            child:              Window
            sourceid:           DEVICEID
            root_x:             FP1616
            root_y:             FP1616
            event_x             FP1616
            event_y:            FP1616
            mode:               NOTIFYMODE
            detail:             NOTIFYDETAIL
            same_screen:        BOOL
            focus:              BOOL
            mods:               MODIFIERINFO
            group:              GROUPINFO
            buttons_len:        CARD16
            buttons:            SETofBUTTONMASK
    └───

    NOTIFYMODE { Normal, Grab, Ungrab }
    NOTIFYDETAIL { Ancestor, Virtual, Inferior, Nonlinear, NonlinearVirtual,
                   Pointer, PointerRoot, None }

Enter or Leave events are sent whenever a device's pointer enters or
leaves a window.
FocusIn or FocusOut events are sent whenever a device's focus is set to or
away from a window.
The enter/leave and focus in/out model is described in the core protocol
specification, Section 11. (EnterNotify, LeaveNotify events).

For enter and leave events, the modifier and group state is the state of
the paired master device if the device is a master device, or the state of
the attached master keyboard if the device is an attached slave device, or
zero if the device is a floating slave device.

For focus in and out events, the button state is the state of the paired
master device if the device is a master device, or the state of the
attached master keyboard if the device is an attached slave device, or
zero if the device is a floating slave device.

    root
    event
    child
        The root window, event window, and child window, respectively. See the
        core protocol specification for more detail.
    sourceid
        The device that caused the pointer to move.
    root_x
    root_y
        The pointer coordinates relative to the root window.
    event_x
    event_y
        The pointer coordinates relative to the event window.
    mode
        Normal pointer motion events have mode Normal. Pseudo-motion events
        when a grab activates have mode Grab, and pseudo-motion events when a
        grab deactivates have mode Ungrab. Pseudo-motion events caused by the
        activation or deactivation of a passive enter or focus in grab have mode
        XIPassiveGrabNotify or XIPassiveUngrabNotify.
    detail
        Specifies the relation of the event window to the window the pointer
        entered or left. See the core protocol spec for details.
    same_screen
        True if the event window is on the same screen as the pointer's root
        window.
    focus
        If the event window is the focus window or an inferior of the focus
        window, then focus is True. Otherwise, focus is False. This field is
        unspecified for focus in/out events.
    mods
        XKB modifier state before the event occured.
    group
        XKB group state before the event.
    buttons_len
        The length of buttons in 4 byte units.
    buttons
        Button state before the event.

[[events-propertyevent]]
    ┌───
        XIPropertyEvent
            EVENTHEADER
            property:           ATOM
            what:               { PropertyCreated, PropertyDeleted, PropertyModified }
    └───

XIPropertyEvents are sent whenever a device property is created, deleted or
modified by a client.

    property
        The property that has been created, deleted, or modified
    what
        Specifies what has been changed.
     
[[events-xi22]]
Events introduced in version 2.2
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

[[events-touchownershipevent]]
    ┌───
        TouchOwnershipEvent (since XI 2.2):
            EVENTHEADER
            touchid:                    CARD32
            root:                       Window
            event:                      Window
            child:                      Window
            sourceid:                   DEVICEID
            flags:                      SETofTOUCHOWNERSHIPFLAGS
    └───

    TOUCHOWNERSHIPFLAGS:    (none currently defined)

A TouchOwnershipEvent indicates that ownership has changed, and the client
is now the owner of the touch sequence specified by touchid.

    touchid
        The identifier of the touch sequence.
    root
    event
    child
        The root window, event window, and child window, respectively. See the
        core protocol specification for more detail.
    sourceid
        The source device that originally generated the event.
    flags
        A bitmask of flags for this event.


// FIXME: why do we get '11. Appendix A:' rather than just 'Appendix A:'?!
[[xi22-usecases]]
[appendix]
XI 2.2 Use-cases
----------------

All use-cases that include the receiving and processing of touch events
require the client to announce XI 2.2 support in the XIQueryVersion request.

* Client C wants to process touch events from a device D on window W.
** C calls XISelectEvent for XI_Touch{Begin|Update|End} from D on W.
** C receives TouchBegin whenever a touch sequence starts within W's borders.
** C receives TouchUpdate events whenever an axis valuator value changes for a
   touch sequence it received a TouchBegin event for.
** C receives TouchEnd whenever a touch it received a TouchBegin event for
   ceases.

* Client C wants to pre-process touch events from a device D on window W, while
  client I wants to pre-process touch events from device D on the parent window
  of W.
** C calls XISelectEvent for XI_Touch{Begin|Update|Ownership|End} from D on W.
** I calls XIPassiveGrab for XI_Touch{Begin|Update|Ownership|End} from D on a
   parent window of W.
** I receives TouchBegin whenever a touch begins within window W, as well as a
   TouchOwnership event indicating that it currently owns the touch sequence.
   C receives a TouchBegin event as well, but without TouchOwnership.
** When an axis valuator changes in this touch sequence, both I and C receive a
   TouchUpdate event.  I may process the event to determine if it is going to
   accept or reject the touch, whereas C may perform reversible processing.
** If I decides it is going to claim the touch sequence for its exclusive
   processing, it calls XIAllowEvents with an event mode of XIAcceptTouch; at
   this point, C receives a TouchEnd event, and undoes any processing it has
   already performed due to the touch sequence.  Further TouchUpdate events are
   delivered only to I.
** Alternatively, if I decides it does not want to receive further events
   from this touch sequence, it calls XIAllowEvents with an event mode of
   XIRejectTouch; at this point, I receives a TouchEnd event confirming that it
   has rejected the touch.  C receives a TouchOwnership event confirming that it
   is now the new owner of the touch, and further TouchUpdate events are
   delivered only to C.  As C now owns the touch, it is free to perform
   irreversible processing of the sequence.
** When the touch physically ceases, a TouchEnd event is sent to C.

* Client C wants to pre-process touch events from a direct touch device D on
  window W, while client I wants to process pointer events on window W's parent,
  window Y.
** I calls XIPassiveGrab for XI_{ButtonPress,MotionNotify,ButtonRelease} to
   create a synchronous pointer grab from D on Y.
** C calls XISelectEvent for XI_Touch{Begin|Update|Ownership|End} from D on W.
** I receives a ButtonPress event whenever a touch begins within W, and is
   considered the owner of the event.  C receives a TouchBegin event, but does
   not receive a TouchOwnership event.
** When the touchpoint moves, C will receive a TouchUpdate event.  Event
   delivery to I is be subject to the synchronous delivery mechanism. The
   emulated motion notify event is queued in the server while the device is
   frozen.
** I may assert ownership by calling XIAllowEvents on Y with any mode other
   than ReplayDevice, which will cause all further events to be sent only to I,
   with a TouchEnd event being sent to C.
** Alternatively, I may reject the touch sequence by calling XIAllowEvents on
   Y with mode ReplayDevice, which will cause no further events from that touch
   to be sent to I, and a TouchOwnership event to be sent to C, with subsequent
   motion events being sent as TouchUpdate events.

*  Driver DRV provides touch support from tracked device D:
** DRV initializes a TouchClass for the device.
** DRV parses D's device protocol and selects one touch sequence to be emulated
   as pointer event.
** DRV calls the respective input driver API with the touch sequence data. The
   touch sequence emulating a pointer has the respective flag set. DRV does not
   submit pointer data for any touchpoint.