Event controllers are standalone objects that can perform specific actions upon received GdkEvents. These are tied to a GtkWidget, and can be told of the event propagation phase at which they will manage the events.
Gestures are a set of specific controllers that are prepared to handle pointer and/or touch events, each gesture implementation attempts to recognize specific actions out the received events, notifying of the state/progress accordingly to let the widget react to those. On multi-touch gestures, every interacting touch sequence will be tracked independently.
Since gestures are “simple” units, it is not uncommon to tie several together to perform higher level actions, grouped gestures handle the same event sequences simultaneously, and those sequences share a same state across all grouped gestures. Some examples of grouping may be:
A “drag” and a “swipe” gestures may want grouping. The former will report events as the dragging happens, the latter will tell the swipe X/Y velocities only after recognition has finished.
Grouping a “drag” gesture with a “pan” gesture will only effectively allow dragging in the panning orientation, as both gestures share state.
If “press” and “long press” are wanted simultaneously, those would need grouping.
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