public abstract class Gesture : EventController
`GtkGesture` is the base class for gesture recognition.
Although `GtkGesture` 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 [property@Gtk.Gesture:n-points] property, if a gesture is keeping track of less or more than that number of sequences, it won't check whether the gesture is recognized.
As soon as the gesture has the expected number of touches, it will check regularly if it 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.
GtkGesture:update], whenever an input event is processed.
GtkGesture:end] when the gesture is no longer recognized.
In order to receive events, a gesture needs to set a propagation phase through [method@Gtk.EventController.set_propagation_phase].
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.
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
Whenever input interaction happens, a single event may trigger a cascade of `GtkGesture`s, 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 `GdkEventSequence`s triggering those.
Within a widget, gestures can be grouped through [method@Gtk.Gesture.group]. Grouped gestures synchronize the state of sequences, so calling [ method@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.
- Emitting [signal@Gtk.Gesture:
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 [method@Gtk.Gesture.set_sequence_state] to know about the possible lifetimes of a `GdkEventSequence`.
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:
- If the gesture has gtk_phase_none, ensuring events of type gdk_touchpad_swipe and gdk_touchpad_pinch are handled by the `GtkGesture`