A unique identifier for a “interaction” of touch events in an event stream.
Touch events are observed as a succession of interactions, as fingers engage
and disengage with the display.
The parameters of the associated view and viewport, sufficient to correctly
interpret the position and scale of pointer events dispatched to this view.
The possible states of a pointer event. These phases of events in a stream
follow a state machine that starts with the ADD phase, followed by zero or
more CHANGE phases, and finally terminates with REMOVE or CANCEL
phase.