25 Input Devices

Functions for handling extended input devices

25.1 Overview

In addition to the normal keyboard and mouse input devices, GTK+ also contains support for extended input devices. In particular, this support is targeted at graphics tablets. Graphics tablets typically return sub-pixel positioning information and possibly information about the pressure and tilt of the stylus. Under X, the support for extended devices is done through the XInput extension.

Because handling extended input devices may involve considerable overhead, they need to be turned on for each <gdk-window> individually using gdk-input-set-extension-events. (Or, more typically, for GtkWidgets, using gtk-widget-set-extension-events). As an additional complication, depending on the support from the windowing system, its possible that a normal mouse cursor will not be displayed for a particular extension device. If an application does not want to deal with displaying a cursor itself, it can ask only to get extension events from devices that will display a cursor, by passing the ‘GDK_EXTENSION_EVENTS_CURSOR’ value to gdk-input-set-extension-events. Otherwise, the application must retrieve the device information using gdk-devices-list, check the field, and, if it is ‘#f’, draw a cursor itself when it receives motion events.

Each pointing device is assigned a unique integer ID; events from a particular device can be identified by the field in the event structure. The events generated by pointer devices have also been extended to contain , and fields which contain the extended information reported as additional valuators from the device. The field is a a double value ranging from 0.0 to 1.0, while the tilt fields are double values ranging from -1.0 to 1.0. (With -1.0 representing the maximum tilt to the left or up, and 1.0 representing the maximum tilt to the right or down.)

One additional field in each event is the field, which contains an enumeration value describing the type of device; this currently can be one of ‘GDK_SOURCE_MOUSE’, ‘GDK_SOURCE_PEN’, ‘GDK_SOURCE_ERASER’, or ‘GDK_SOURCE_CURSOR’. This field is present to allow simple applications to (for instance) delete when they detect eraser devices without having to keep track of complicated per-device settings.

Various aspects of each device may be configured. The easiest way of creating a GUI to allow the user to configure such a device is to use the <gtk-input-dialog> widget in GTK+. However, even when using this widget, application writers will need to directly query and set the configuration parameters in order to save the state between invocations of the application. The configuration of devices is queried using gdk-devices-list. Each device must be activated using gdk-device-set-mode, which also controls whether the device's range is mapped to the entire screen or to a single window. The mapping of the valuators of the device onto the predefined valuator types is set using gdk-device-set-axis-use. And the source type for each device can be set with gdk-device-set-source.

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.

The interfaces in this section will most likely be considerably modified in the future to accomodate devices that may have different sets of additional valuators than the pressure and .

25.2 Usage