[RFC libinput] Add an API for touchpad gesture events

[Jonas Ådahl]

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.


Jonas

> 
> Cheers,
>    Peter