GtkGesture is the base object for gesture recognition, although this object is quite generalized to serve as a base for multi-touch gestures, it is suitable to implement single-touch and pointer-based gestures (using the special NULL GdkEventSequence value for these).

The number of touches that a GtkGesture need to be recognized is controlled by the GtkGesture:n-points property, if a gesture is keeping track of less or more than that number of sequences, it won’t check wether the gesture is recognized.

As soon as the gesture has the expected number of touches, the gesture will run the GtkGesture::check signal regularly on input events until the gesture is recognized, the criteria to consider a gesture as “recognized” is left to GtkGesture subclasses.

A recognized gesture will then emit the following signals: - GtkGesture::begin when the gesture is recognized. - A number of GtkGesture::update, whenever an input event is processed. - GtkGesture::end when the gesture is no longer recognized.

Event propagation

In order to receive events, a gesture needs to either set a propagation phase through gtk_event_controller_set_propagation_phase(), or feed those manually through gtk_event_controller_handle_event().

In the capture phase, events are propagated from the toplevel down to the target widget, and gestures that are attached to containers above the widget get a chance to interact with the event before it reaches the target.

After the capture phase, GTK+ emits the traditional GtkWidget::button-press-event, GtkWidget::button-release-event, GtkWidget::touch-event, etc signals. Gestures with the GTK_PHASE_TARGET phase are fed events from the default GtkWidget::event handlers.

In the bubble phase, events are propagated up from the target widget to the toplevel, and gestures that are attached to containers above the widget get a chance to interact with events that have not been handled yet.

States of a sequence # {#touch-sequence-states}

Whenever input interaction happens, a single event may trigger a cascade of GtkGestures, both across the parents of the widget receiving the event and in parallel within an individual widget. It is a responsibility of the widgets using those gestures to set the state of touch sequences accordingly in order to enable cooperation of gestures around the GdkEventSequences triggering those.

Within a widget, gestures can be grouped through gtk_gesture_group(), grouped gestures synchronize the state of sequences, so calling gtk_gesture_set_sequence_state() on one will effectively propagate the state throughout the group.

By default, all sequences start out in the #GTK_EVENT_SEQUENCE_NONE state, sequences in this state trigger the gesture event handler, but event propagation will continue unstopped by gestures.

If a sequence enters into the #GTK_EVENT_SEQUENCE_DENIED state, the gesture group will effectively ignore the sequence, letting events go unstopped through the gesture, but the “slot” will still remain occupied while the touch is active.

If a sequence enters in the #GTK_EVENT_SEQUENCE_CLAIMED state, the gesture group will grab all interaction on the sequence, by: - Setting the same sequence to #GTK_EVENT_SEQUENCE_DENIED on every other gesture group within the widget, and every gesture on parent widgets in the propagation chain. - calling GtkGesture::cancel on every gesture in widgets underneath in the propagation chain. - Stopping event propagation after the gesture group handles the event.

Note: if a sequence is set early to #GTK_EVENT_SEQUENCE_CLAIMED on

GDK_TOUCH_BEGIN/#GDK_BUTTON_PRESS (so those events are captured before

reaching the event widget, this implies #GTK_PHASE_CAPTURE), one similar event will emulated if the sequence changes to #GTK_EVENT_SEQUENCE_DENIED. This way event coherence is preserved before event propagation is unstopped again.

Sequence states can’t be changed freely, see gtk_gesture_set_sequence_state() to know about the possible lifetimes of a GdkEventSequence.

Touchpad gestures

On the platforms that support it, GtkGesture will handle transparently touchpad gesture events. The only precautions users of GtkGesture should do to enable this support are: - Enabling GDK_TOUCHPAD_GESTURE_MASK on their GdkWindows - If the gesture has GTK_PHASE_NONE, ensuring events of type GDK_TOUCHPAD_SWIPE and GDK_TOUCHPAD_PINCH are handled by the GtkGesture.