SDL3 Pen: Confusing pen proximity api

[deiflou]

(I tested what follows only in linux and x11).

I was going to use the pen capabilities that check for pen proximity and noticed that the concept is not what I expected (I'm used to qt pen api). I was expecting that SDL_EVENT_PEN_PROXIMITY_IN/OUT would be sent when the pen enters/leaves tablet proximity (when the pen is moved near or far from the tablet) but it seems that right now they are sent when the pen is connected/disconnected. No events are sent when the pen is moved near or far from the tablet, but it would be great that they were.

My proposal is to reuse SDL_EVENT_PEN_PROXIMITY_IN/OUT to notify that the pen went near/far away the tablet and add two new event types, SDL_EVENT_PEN_ADDED/REMOVED, and SDL_Event::pdevice (similar to other devices) to notify that the devide was (dis)connected.

I made a sample implementation for what I propose for x11: https://github.com/deiflou/SDL/tree/pen_proximity, I couldn't make it work using wayland though.
It is the first time I code any x11/xinput stuff and I don't know if that is the best way.

[slouken]

I'm pretty sure this is supposed to work the way you'd expect. Pinging @icculus to verify.

[icculus]

I think there was some weird limitation, like Wayland only reports pens on connect/disconnect and not on tradition proximity, but I could be totally wrong. I'll check tonight when I'm in from of a real computer!

[deiflou]

Just as an informative note: I based the code in qt's handling of the proximity: https://github.com/qt/qtbase/blob/731d9a13956ef40e3f563f5ff5ff5c250b8d6d94/src/plugins/platforms/xcb/qxcbconnection_xi2.cpp#L1515

[icculus]

Okay, I looked back over the subsystem, and we're not going to add new events. Several serious platforms don't report devices coming and going. Some only report proximity events when a pen moves, without any indication that the pen or tablet existed before. Some don't even send proximity events! You don't know the pen exists until it sends a touch event and then we synthesize a proximity event from there.

That being said, since we can apparently get better proximity events from X11, we should make them work as expected, instead of only sending proximity_out if the tablet is unplugged.