GTK+ Wayland tablet support is merged

As for excuses go for blowing the lint off my blog, this is a pretty good one :). Wayland tablet support is something that got really close to being merged in 3.20, but the timing didn’t pan out in the end, wayland-protocols 1.3 included a tablet v1 protocol that went unused, till now. Now that we’re in 3.22, those bits are at last being merged. First came gtk+, which you can see working in these videos:

This improved tablet support brings some goodies (also on X11), there is a new GdkDeviceTool object that can used to track specific physical tools and provides the necessary info to do so in a persistent way (eg. across runs/reboots).

A note to all GTK+ app/widget developers

TL;DR: If you’re keeping track of motion events somewhere in your app, you want to check gdk_event_get_device(), see the bullet points below and check the best strategy for you.

One particularity in wayland tablet management is that whenever you have a tool in proximity to a tablet you’ll be driving a standalone onscreen pointer cursor, as opposed to the default behavior in X11 consisting of driving the same pointer than your mouse does. This difference is not moot, it doesn’t take too much thinking to realize that the user of a drawing tablet has a spare hand which could be using the mouse, so there’s in practical terms two pointers to react to. And there’s also setups where more than one drawing tablet is likely (the simplest one being a laptop with an integrated tablet plus a external one).

GTK+ is itself quite well prepared for these situations, as are all its widgets, it can’t account for widgets implementations elsewhere and signal callbacks in apps though.

But I’m not doing a drawing app, should I care?

Yes you should. Although tablets are mostly thought out for drawing, design, etc., ideally they should be able to drive the whole desktop without oddities, just as your mouse does. This can only drive to a pleasant experience with your app regardless of the input devices of choice.

If there’s somewhere in your app where you’re tracking pointer events over time (drawing is obviously an example, but so is rubberband selection, any kind of drag-to-move, etc…), there’s a chance that it could get confused by events with different gdk_event_get_device()s coming to the widget in question. There’s several things you might do:

  • First and foremost, look into GtkGesture and its several subclasses, it’s handled transparently there, and there’s a good chance that one of those (or a group) will fit your case
  • Maybe it’s sufficient a “first to come gets served” basis for the specific purposes of your operation? You can use gtk_device_grab_add/remove with that device, or check gdk_event_get_device() yourself on all events.
  • Think it’d be cool to handle the simultaneous pointers and want to go the extra mile? Again, track gdk_event_get_device(), and keep per-device state.

…And there’s more to come

There’s big plans for mutter in this cycle so tablet support will not be immediately merged, but be assured it will happen in due time. Gears are also grinding in order to have a version 2 of the tablet protocol, it will mostly focus on bridging pads (the sets of buttons and sliders along the sides of some tablets) to wayland clients, something which I expect to be an outstanding improvement compared to X11, so… stay tuned!

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