gtk:event-controller, gobject:object, common-lisp:standard-object, common-lisp:t

gtk:gesture-rotate, gtk:gesture-single, gtk:gesture-zoom

The gtk:gesture object 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.

The number of touches that a gtk:gesture object need to be recognized is controlled by the n-points property, if a gesture is keeping track of less or more than that number of event sequences, it will not 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 gtk:gesture subclasses.

A recognized gesture will then emit the following signals:

• The "begin" signal when the gesture is recognized.
• A number of "update" signals, whenever an input event is processed.
• The "end" signal when the gesture is no longer recognized.

Event propagation

In order to receive events, a gesture needs to set a propagation phase through the gtk:event-controller-propagation-phase function.

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 an event sequence

Whenever input interaction happens, a single event may trigger a cascade of gestures, 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 event sequences triggering those.

Within a widget, gestures can be grouped through the gtk:gesture-group function, grouped gestures synchronize the state of event sequences, so calling the gtk:gesture-set-state function on one will effectively propagate the state throughout the group.

By default, all event sequences start out in the :none state, event sequences in this state trigger the gesture event handler, but event propagation will continue unstopped by gestures.

If an event sequence enters into the :denied state, the gesture group will effectively ignore the event sequence, letting events go unstopped through the gesture, but the "slot" will still remain occupied while the touch is active.

If an event sequence enters in the :claimed state, the gesture group will grab all interaction on the event sequence, by:

• Setting the same event sequence to :denied on every other gesture group within the widget, and every gesture on parent widgets in the propagation chain.
• Calling the "cancel" signal on every gesture in widgets underneath in the propagation chain.
• Stopping event propagation after the gesture group handles the event.

Note: If an event sequence is set early to :claimed on :touch-begin or :button-press events, so those events are captured before reaching the event widget, this implies :phase-capture, one similar event will emulated if the event sequence changes to :denied. This way event coherence is preserved before event propagation is unstopped again.

Event sequence states can not be changed freely, see the gtk:gesture-set-state function to know about the possible lifetimes of a gdk:event-sequence instance.

Touchpad gestures

On the platforms that support it, the gtk:gesture object will handle transparently touchpad gesture events. The only precautions users of the gtk:gesture object should do to enable this support are:

• If the gesture has the :none value of the gtk:propagation-phase enumeration, ensuring events of type :touchpad-swipe and :touchpad-pinch are handled by the gtk:gesture object.