multi-touch-dev team mailing list archive

[Chase Douglas <chase.douglas@xxxxxxxxxxxxx>]

On Wed, 2010-10-13 at 20:43 +0200, Henrik Rydberg wrote:
> On 10/13/2010 06:03 PM, Chase Douglas wrote:
> >> Good point. Would you mind going through, step-by-step, the case where we start
> >> with two fingers inside a "window", scroll a bit, then add another finger in a
> >> different window, and perform a three-finger pinch?
> > 
> > There's a few context questions we need to ask first (I'm trying to get
> > at what your asking, maybe I'm going off in the wrong direction though):
> > 
> > 1. Is a three finger pinch a system-wide gesture? I'll assume yes. Are
> > two finger gestures system-wide gestures? I'll assume no.
> > 
> > 2. Should a two-to-three finger movement that is not immediate, by which
> > I mean this isn't a three finger movement from the start where the first
> > couple of device touch frames caught only two fingers, trigger a three
> > finger gesture? I don't believe we've explicitly answered this anywhere
> > yet, but I think not. A three finger gesture must be initiated with
> > three fingers within a short period of time on the order of 10s of
> > milliseconds (I'm leaving aside the case of gesture continuation).
> 
> 
> I tend to agree, although that is all about policy. I was intending this example
> to trigger a global gesture, since it makes the example more difficult.
> 
> > Given these assumptions, I think the expected result is that the
> > scrollable region continues to scroll according to the two fingers over
> > it, and the single finger in the other window does whatever it should in
> > that window.
> 
> 
> Right, that is the easy path. So what about the one which triggers a global gesture?

I'm not sure we've defined a mechanism that would work in that case.
What you would need is the ability to peek at all touch events
individually and pick and choose which ones you want to keep and which
you want to replay. The reason this gets dicey is what does the client
do? Imagine the following:

1. Touch begins
2. WM replays touch events one at a time
3. Application gets touch begin, touch motion events
4. WM consumes some events
5. WM replays more events
6. WM consumes events
7. Touch ends

Issues:

* Does application get touch begin events when touch physically begins
if WM starts consuming all events? Or does it only receive a begin event
once the first replayed event is sent?
* Is application notified when WM starts consuming events again?
* What happens when events are replayed in step 5? It may seem that the
touch moved from one are of the screen to another instantaneously when
in reality the touches in between were consumed by the WM.
* If the WM consumes the touch end, does the application then receive
the touch end?

I think this use case is opposite to good UI design, and I think the
plumbing to make it possible opens up a big can of worms. X doesn't
support any replay mechanism like this for even legacy pointer events.
We seem to all agree that the original example bore the correct outcome,
so I'm hesitant to continue down this path until we find a scenario that
would require it.

There's also nothing that prevents us from adding support for this in
the future if we find we need it.

-- Chase