Split gestures into recognition and application
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Gesture detection is not an exact science, but the algorithms
inevitably involve decisions influenced by what gestures are available
to choose from. Therefore, the recognition should be considered an
inherent property of the device itself, regardless of user interaction
context. Just imagine having a keyboard that resized slightly
everytime you switched from chrome to gimp.


Tag events as being part of a gesture
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One of the problems faced when replacing one set of events with
another is where and how this filtering should take place. One window
might listen for spiral gestures, whereas another window is a paint
program where you actually want to draw a spiral. By introducing a
relation between a gesture and an event, filtering becomes something
simple that can be performed close to the window. The relation can be
implemented as a bitmask representing what gestures the event is part
of.


Help the client to keep track of active gestures
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A gesture starts, continues, and either completes or aborts.
A client that listens to a subset of gestures will have to keep track.
Compatible to Qt approach.


Allow clients to listen to different gestures
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Clients listening on certain gestures will get those gesture events that
apply to the client, which makes it possible for different clients to
listen to different types of gestures. For instance, having an MT drawing
program next to a web browser, a two finger stroke in the drawing program
will result in two lines, whereas in the web browser, it will scroll the
text.


Propagate gesture events based on the focus point
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Like pointer events, a gesture applies to a window or a window manager,
based on where it is performed on the screen. In order to route the gesture
events to the right window, we propagate them through the window hierarchy
based on the gesture focus points. Based on the target windows, their
respective gesture type masks, and the active set of gestures, final
gesture events are emitted to the right windows.


Allow Window Manager to grab system-wide gestures
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Even if clients are not listening to global window-manager gestures, it
could well be that no other gestures should be displayed while the global
gesture is taking place.