Quite often there are situations where multiple gestures try to
recognize, keeping track of the same set of points (for example an edge
drag gesture on the stage and a click gesture somewhere in the
scenegraph). Usually what's wanted here is that the first gesture to
move to RECOGNIZING wins over all other active gestures and "claims" the
point for itself.
We implement this by introducing a concept called "influencing". It
works by making all gestures operating on a shared set of points aware
of each other using ClutterAction->register_sequence().
ClutterGesture uses this vfunc to keep track of all other
ClutterGestures that are potentially conflicting, and keeps a list
(priv->cancel_on_recognizing) of those. As soon as the move to
RECOGNIZING happens, all gestures inside this list get moved to
CANCELLED.
To allow fine-grained control over this behavior, two APIs are
introduced:
1) on the implementation level (should_influence() and
should_be_influenced_by()): This is a vfunc that gets called as soon as
a potential conflict is detected. It's helpful when a specific gesture
always behaves the same towards another gesture, for example to make
sure a LongPress gesture never cancels a DragGesture.
2) on the gesture user level, clutter_gesture_can_not_cancel() is
introduced: This allows control for the user of a gesture to specify
that a specific instance of a gesture won't cancel another gesture.
Calling this twice so that both gestures can't cancel each other allows
for things like simultaneous recognition of a pinch-to-zoom and rotate
gesture.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2389>
Introduce ClutterGesture, a new ClutterAction subclass and the successor
of ClutterGestureAction that brings the necessary tools to handle
sequences of events and abstract touch and mouse gestures from those.
The big difference compared to ClutterGestureAction is that ClutterGesture
provides the implementation with point_added/moved/ended and
sequences_cancelled events and expects the implementation to move the
ClutterGesture through the ClutterGestureState state machine. This state
machine is then used internally by ClutterGesture to coordinate with other
gestures.
With the next commits, ClutterGesture will handle relationships between
conflicting gestures completely by itself, allowing the implementation or
the user to specify the details of the relationship between two gestures.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2389>
