[RFC libinput] Add an API for touchpad gesture events

[Peter Hutterer]

On Tue, Jan 27, 2015 at 10:55:50AM +0800, Jonas Ådahl wrote:
> On Tue, Jan 27, 2015 at 12:15:49PM +1000, Peter Hutterer wrote:
> > On Fri, Jan 23, 2015 at 12:20:55PM +0100, Hans de Goede wrote:
> > > Hi,
> > > 
> > > On 23-01-15 09:51, Jonas Ådahl wrote:
> > > >On Fri, Jan 23, 2015 at 09:38:06AM +0100, Hans de Goede wrote:
> > > >>Hi,
> > > >>
> > > >>On 23-01-15 07:18, Jonas Ådahl wrote:
> > > 
> > > <snip>
> > > 
> > > >>>>As for 2 finger swipe at least: isn't that covered by scrolling? with the
> > > >>>>caller deciding when a scroll event event is a swipe and when it is a scroll
> > > >>>>event. It's the odd one out of course, but scrolling is a special case and
> > > >>>>will be the odd one out anyway.
> > > >>>
> > > >>>Scrolling doesn't completely cover it because a user can disable
> > > >>>2-finger scroll for the whole session, and the application won't even
> > > >>>see the scroll events. If an application wants to handle the 2-finger
> > > >>>swipe no matter what configuration the user has chosen, then it'd
> > > >>>probably have to come via something like a gesture API.
> > > >>
> > > >>AFAIK there is no such thing as a 2fg swipe on other platforms, why would
> > > >>we make things difficult to try and support it in the first place ?
> > > >
> > > >Well, don't we already have it fully implemented except we call it
> > > >scroll and only enable it if it (although by default) configured so? I
> > > >agree though that its not very nice to send out duplicate events for the
> > > >same gesture, but still, it feels some how wrong to not let applications
> > > >depend on the most simple swipe (deliberately ignoring 1 finger swipe
> > > >here) gesture.
> > > 
> > > Yes it is the most simple swipe gesture, but it is already taken for
> > > scrolling and sending 2 events for the user doing a single thing just seems
> > > very very wrong to me, and I really believe that that is something which
> > > libinput should never ever do.
> > > 
> > > > Also in click-to-scroll configurations, don't we disable 2 finger scroll?
> > > 
> > > We don't have click-to-scroll for touchpads, we've edge scrolling or
> > > 2 finger scrolling.
> > 
> > my first thought here was: should we drop 2-finger scrolling and move it to
> > swipe events, then let the caller decide when to interpret swipe as scroll.
> > doable, with a couple of benefits and a couple of drawbacks.
> > 
> > I think what it comes down to though is that scrolling and swiping have two
> > different usages: scrolling is almost exclusively vertical, with the odd
> > horizontal component. it's also high-precision with slow finger movement.
> > swiping is almost exclusively horizontal and usually not very precise, it's
> > more of a flick than anything.
> > 
> > so I think we could enable 2fg swipe on touchpads and then let libinput
> > internally decide when to trigger a scroll and when to trigger a swipe. Plus
> > the subtle hint in the documentation that vertical 2fg swiping is not a good
> > gesture to rely on in a UI :)
> 
> I think that is dangerous. I for one tend to "swipe-scroll" when I want
> to get to the bottom of a long page, and I don't expect applications to
> implement support for gesture support just for scrolling fast.
> 
> I don't think we can assume swiping is generally horizontal, and
> scrolling vertical. The use case for swiping I'd use would be switching
> virtual workspaces in gnome which would probably be a vertical swipe as
> workspaces are placed in a column.

I agree with the swipe-scroll to the bottom of the page, the other use-case
I'm not so sure. unless you have only two workspaces anything that would try
to switch between the workspaces would have to be more precise than a swipe
fall into the scroll category (well, except for the top/bottom one in the
column...).

anyway, the other escape hatch we have is: we don't _have_ to support all
gestures. 2fg scrolling and swiping are incompatible, so short of changing
all scrolling to swiping and letting the caller deal with the differences I
think it'd be acceptable in libinput to say: vertical 2fg swipe doesn't
exist, horizontal 2fg swipe is speed dependent.

or, even easier: 2fg swipe doesn't exist, use scrolling instead.

Cheers,
   Peter