[PATCH libinput 01/11] Add an API for touchpad gesture events

[Bill Spitzak]

On 02/18/2015 04:26 AM, Hans de Goede wrote:
> For touchscreens we always send raw touch events to the compositor, and the
> compositor or application toolkits do gesture recognition. This makes sense
> because on a touchscreen which window / widget the touches are over is
> important context to know to interpret gestures.
>
> On touchpads however we never send raw events since a touchpad is an absolute
> device which primary function is to send pointer motion delta-s, so we always
> need to do processing (and a lot of it) on the raw events.

I don't quite understand this distinction. I can certainly imagine 
touchpad gestures that depend on where the cursor is currently.

Conversely I can imagine users wanting full-screen gestures on their 
touchscreen, for instance a fast swipe should not be sent to the widget 
the down event is at, but to where the cursor was before the swipe was 
started.

I think what is needed is for gesture recognition to be in libinput but 
also send the raw events. Then a program can ignore the gesture if it 
wants, and this also allows new gestures to be added without making 
older programs freeze when you attempt them. (for an implementation I 
would add a field to events to indicate if they are the last in a set).

The only effect of gesture recognition that is unavoidable is that the 
focus may not match the location of the cursor, in that the raw events 
are sent to the same target that the gesture is being sent to. I think 
this is a necessary ability to make the gestures consistent.