In addition to a single pointer and keyboard for user interface input, GDK contains support for a variety of input devices, including graphics tablets, touchscreens and multiple pointers/keyboards interacting simultaneously with the user interface. Such input devices often have additional features, such as sub-pixel positioning information and additional device-dependent information.

In order to query the device hierarchy and be aware of changes in the device hierarchy (such as virtual devices being created or removed, or physical devices being plugged or unplugged), GDK provides GdkDeviceManager.

By default, and if the platform supports it, GDK is aware of multiple keyboard/pointer pairs and multitouch devices. This behavior can be changed by calling gdk_disable_multidevice() before gdk_display_open(). There should rarely be a need to do that though, since GDK defaults to a compatibility mode in which it will emit just one enter/leave event pair for all devices on a window. To enable per-device enter/leave events and other multi-pointer interaction features, gdk_window_set_support_multidevice() must be called on GdkWindows (or gtk_widget_set_support_multidevice() on widgets). window. See the gdk_window_set_support_multidevice() documentation for more information.

On X11, multi-device support is implemented through XInput 2. Unless gdk_disable_multidevice() is called, the XInput 2 GdkDeviceManager implementation will be used as the input source. Otherwise either the core or XInput 1 implementations will be used.

For simple applications that don’t have any special interest in input devices, the so-called “client pointer” provides a reasonable approximation to a simple setup with a single pointer and keyboard. The device that has been set as the client pointer can be accessed via gdk_device_manager_get_client_pointer().

Conceptually, in multidevice mode there are 2 device types. Virtual devices (or master devices) are represented by the pointer cursors and keyboard foci that are seen on the screen. Physical devices (or slave devices) represent the hardware that is controlling the virtual devices, and thus have no visible cursor on the screen.

Virtual devices are always paired, so there is a keyboard device for every pointer device. Associations between devices may be inspected through gdk_device_get_associated_device().

There may be several virtual devices, and several physical devices could be controlling each of these virtual devices. Physical devices may also be “floating”, which means they are not attached to any virtual device.

Master and slave devices

carlos@sacarino:~$ xinput list
⎡ Virtual core pointer                          id=2    [master pointer  (3)]
⎜   ↳ Virtual core XTEST pointer                id=4    [slave  pointer  (2)]
⎜   ↳ Wacom ISDv4 E6 Pen stylus                 id=10   [slave  pointer  (2)]
⎜   ↳ Wacom ISDv4 E6 Finger touch               id=11   [slave  pointer  (2)]
⎜   ↳ SynPS/2 Synaptics TouchPad                id=13   [slave  pointer  (2)]
⎜   ↳ TPPS/2 IBM TrackPoint                     id=14   [slave  pointer  (2)]
⎜   ↳ Wacom ISDv4 E6 Pen eraser                 id=16   [slave  pointer  (2)]
⎣ Virtual core keyboard                         id=3    [master keyboard (2)]
    ↳ Virtual core XTEST keyboard               id=5    [slave  keyboard (3)]
    ↳ Power Button                              id=6    [slave  keyboard (3)]
    ↳ Video Bus                                 id=7    [slave  keyboard (3)]
    ↳ Sleep Button                              id=8    [slave  keyboard (3)]
    ↳ Integrated Camera                         id=9    [slave  keyboard (3)]
    ↳ AT Translated Set 2 keyboard              id=12   [slave  keyboard (3)]
    ↳ ThinkPad Extra Buttons                    id=15   [slave  keyboard (3)]

By default, GDK will automatically listen for events coming from all master devices, setting the GdkDevice for all events coming from input devices. Events containing device information are #GDK_MOTION_NOTIFY,

GDK_BUTTON_PRESS, #GDK_2BUTTON_PRESS, #GDK_3BUTTON_PRESS,

GDK_BUTTON_RELEASE, #GDK_SCROLL, #GDK_KEY_PRESS, #GDK_KEY_RELEASE,

GDK_ENTER_NOTIFY, #GDK_LEAVE_NOTIFY, #GDK_FOCUS_CHANGE,

GDK_PROXIMITY_IN, #GDK_PROXIMITY_OUT, #GDK_DRAG_ENTER, #GDK_DRAG_LEAVE,

GDK_DRAG_MOTION, #GDK_DRAG_STATUS, #GDK_DROP_START, #GDK_DROP_FINISHED

and #GDK_GRAB_BROKEN. When dealing with an event on a master device, it is possible to get the source (slave) device that the event originated from via gdk_event_get_source_device().

On a standard session, all physical devices are connected by default to the “Virtual Core Pointer/Keyboard” master devices, hence routing all events through these. This behavior is only modified by device grabs, where the slave device is temporarily detached for as long as the grab is held, and more permanently by user modifications to the device hierarchy.

On certain application specific setups, it may make sense to detach a physical device from its master pointer, and mapping it to an specific window. This can be achieved by the combination of gdk_device_grab() and gdk_device_set_mode().

In order to listen for events coming from devices other than a virtual device, gdk_window_set_device_events() must be called. Generally, this function can be used to modify the event mask for any given device.

Input devices may also provide additional information besides X/Y. For example, graphics tablets may also provide pressure and X/Y tilt information. This information is device-dependent, and may be queried through gdk_device_get_axis(). In multidevice mode, virtual devices will change axes in order to always represent the physical device that is routing events through it. Whenever the physical device changes, the GdkDevice:n-axes property will be notified, and gdk_device_list_axes() will return the new device axes.

Devices may also have associated “keys” or macro buttons. Such keys can be globally set to map into normal X keyboard events. The mapping is set using gdk_device_set_key().

In GTK+ 3.20, a new GdkSeat object has been introduced that supersedes GdkDeviceManager and should be preferred in newly written code.