Touch events

[Chase Douglas]

On 02/21/2012 09:15 PM, Bill Spitzak wrote:
> Daniel Stone wrote:
>> Hi,
>>
>> On 22 February 2012 00:13, Bill Spitzak <spitzak at gmail.com> wrote:
>>> It seems like it would be better if clients got the touch events first, and
>>> the compositor only did things if the client said it was uninterested in the
>>> events. For a lot of touch events this can be decided immediately on the
>>> touch-down, since the client knows the user is pushing nothing.
>>
>> No, this just isn't true.  You want global gestures (e.g. three finger
>> swipe to change desktop) to take precedence over local shortcuts.  And
>> you cannot -- cannot -- know at touch-down time who's going to be
>> ultimately interested in it.  Because for a three-finger swipe, there
>> will be a time when there's only one finger down, or two.  At which
>> point the client will say, 'ah yes, I'm interested in this!'.  And
>> then the third finger lands and you regret a terrible design decision
>> you made.
> 
> I would think the client could say "not interested" when it sees the 
> third finger.

The client won't see the third finger if it touches outside its window.
In the wayland case, only the WM has all the info needed to determine if
a touch is part of a global gesture. The WM needs to make the decision,
not the client.

> If the clients can look at things first, this would allow the compositor 
> to do things like "one finger can be used to change desktops if the 
> underlying program does not use it".

That would be bad UI design because then global gestures would fire only
sometimes. Further, it would break global gestures if touches occur over
a broken application.

> Solutions like "three fingers are needed" are just like the solutions 
> for shortcuts where you have to hold Alt and Ctrl and the right-hand 
> Shift and push the key, in an attempt to not collide with keystrokes the 
> program wants.

Yes, system and application gestures may collide. There's not much one
can do about it other than provide guidelines for application
developers. Unity's guidelines are: three and four finger gestures are
reserved for global uses. Everything else is fair game.

I think we can look at other OSes as case studies. iOS and OS X employ
effective global gestures imo, and they take precedence over the
application receiving touch or gesture events.

-- Chase