Gestures

TODO: Expand.

How are hit tests performed?

  • The RendererBinding is HitTestable, which implies the presence of a hitTest method. The default implementation defers to RenderView, which itself implements hitTest to visit the entire render tree. Each render object is given an opportunity to add itself to a shared HitTestResult.

  • GestureBinding.dispatchEvent (via HitTestDispatcher) uses the PointerRouter to pass the original event to all render objects that were hit (RenderObject implements HitTestTarget, and therefore provides handleEvent). If a GestureRecognizer is utilized, the event’s pointer is passed via GestureRecognizer.addPointer which registers with the PointerRouter to receive future events.

  • Related events are sent to all original HitTestTarget as well as any routes registered with the PointerRouter. PointerDownEvent will close the gesture area, barring additional entries, whereas PointerUpEvent will sweep it, resolving competing gestures and preventing indecisive gestures from locking up input.

How are gestures captured and propagated?

  • Window.onPointerDataPacket captures pointer updates from the engine and generates PointerEvents from the raw data. PointerEventConverter is utilized to map physical coordinates from the engine to logical coordinates, taking into account the device’s pixel ratio (via PointerEventConverter.expand).

What does a gesture recognizer do?

  • A pointer is added to the gesture recognizer by client code on PointerDownEvent. Gesture recognizers determine whether a pointer is allowed by overriding GestureRecognizer.isPointerAllowed. If so, the recognizer subscribes to future pointer events via the PointerRouter and adds the pointer to the gesture arena via GestureArenaManager.add.

  • The recognizer will process incoming events, outcomes from the gesture arena (GestureArenaMember.accept/rejectGesture), spontaneous decisions about the gesture (GestureArenaEntry.resolve), and other externalities. Typically, recognizers watch the stream of PointerEvents via HitTestTarget.handleEvent, looking for terminating events like PointerDown, or criteria that will cause acceptance / rejection. If the gesture is accepted, the recognizer will continue to process events to characterize the gesture, invoking user-provided callbacks at key moments.

  • The gesture recognizer must unsubscribe from the pointer when rejecting or done processing, removing itself from the PointerRouter (OneSequenceGestureRecognizer.stopTrackingPointer does this).

What does the gesture arena do?

  • The arena disambiguates multiple gestures in a way that allows single gestures to resolve immediately if there is no competition. A recognizer “wins” if it declares itself the winner or if it’s the last/sole survivor.

Can gesture recognizers be grouped together?

  • A GestureArenaTeam combines multiple recognizers together into a group.

  • Captained teams cause the captain recognizer to win when all unaffiliated recognizers reject or a constituent accepts.

  • A non-captained team causes the first added recognizer to win when all unaffiliated recognizers reject. However, if a constituent accepts, that recognizer still takes the win.

What auxiliary classes support gesture handling?

  • There are two major categories of gesture recognizers, multi-touch recognizers (i.e., MultiTapGestureRecognizer) that simultaneously process multiple pointers (i.e., tapping with two fingers will register twice), and single-touch recognizer (i.e., OneSequenceGestureRecognizer) that will only consider events from a single pointer (i.e., tapping with two fingers will register once).

  • There is a helper “Drag” object that is used to communicate drag-related updates to other parts of the framework (like DragScrollActivity)

  • There’s a VelocityTracker that generates fairly accurate estimates about drag velocity using curve fitting.

  • There are local and global PointerRoutes in PointerRouter. Local routes are used as described above; global routes are used to react to any interaction (i.e., to dismiss a tooltip).