Implement touch handling on web target(s)

[alvinhochun]

Adding touch event handling shouldn't be too hard, but it is tightly coupled with mouse handling due to the web using a unified Pointer Event model. Here are my thoughts:

• Grabbing touch input from PointerEvent is simple enough. However, I would be concerned about correctly mapping the pointerType :
  • "mouse" - obviously should be mapped to CursorMove, CursorEntered, CursorLeft and MouseInput. In addition, I believe that only pointers with isPrimary === true should be reported.
  • "touch" - obviously should be mapped to Touch.
  • "pen" - this is tricky, as winit does not have an event for pen/stylus. I suggest that for now we just map pen input the same way as mouse input until winit finally supports pen events properly.
  • Empty strings or unknown values - the same as mouse.
• Supporting browsers without PointerEvent - I suggest we do not add support for TouchEvent, because:
  • PointerEvent is getting widespread support.
  • Adding TouchEvent handling will likely make the code quite complicated.
  • If the user really care about supporting older browsers, they can use a JavaScript polyfill. Turns out the polyfill does not provide offsetX/offsetY which winit is currently using...
• Should touch input also synthesize mouse input?
  • My idea would be that, since some users may not want to go through the trouble of implementing explicit touch handling there should be an option to allow user code to disable touch support and automatically treat the primary touch input as mouse input. Otherwise, touch input will only generate touch events and not mouse events.
  • However, it seems that this is not an option for other platforms. Perhaps it makes more sense to just provide one behaviour, which is for touch input to only generate touch events and not mouse events.

[elliegoldstein]

I agree with your points on how to handle PointerEvent, and that we should avoid TouchEvent. I think there's an open meta issue somewhere for pen / styluses?

I think touch events from the browser should only generate touch events within winit. This is (I think?) consistent with other platforms, and allows the user to combine them if they want.

[alvinhochun]

I think there's an open meta issue somewhere for pen / styluses?

Both #336 and #99 are related. (Somehow the latter is unsearchable on GitHub.) I would hope to push for changing to a unified pointer event model and add pen support as part of it, but that would be outside the scope of this issue.

[alvinhochun]

It turns out the PEP polyfill doesn't work because it does not provide offsetX/offsetY which winit is currently using. (jquery-archive/PEP#217)

Still, I don't think it's really worth the effort to add TouchEvent support.

[ryo33]

Can anyone review #2188 ?

[ryo33]

Thanks for the review Liamolucko!
Then, when will this be merged? Does it need more review?