TouchEventCoalescingKeyHelper

Utility for determining coalescing keys for TouchEvents. To preserve proper ordering of events, move events should only be coalesced if there has been no up/down event between them (this basically only applies to multitouch since for single touches an up would signal the end of the gesture). To illustrate to kind of coalescing we want, imagine we are coalescing the following touch stream:

(U = finger up, D = finger down, M = move) D MMMMM D MMMMMMMMMMMMMM U MMMMM D MMMMMM U U

We want to make sure to coalesce this as

D M D M U M D U U

and not

D D U M D U U

To accomplish this, this class provides a way to initialize a coalescing key for a gesture and then increment it for every pointer up/down that occurs during that single gesture.

We identify a single gesture based on android.view.MotionEvent.getDownTime which will stay constant for a given set of related touches on a single view.

NB: even though down time is a long, we cast as an int using the least significant bits as the identifier. In practice, we will not be coalescing over a time range where the most significant bits of that time range matter. This would require a gesture that lasts Integer.MAX_VALUE * 2 ms, or ~48 days.

NB: we assume two gestures cannot begin at the same time.

NB: this class should only be used from the UI thread.