[BUG] SKTouchAction.Cancelled not issued on iOS when SkCanvasView is in a ScrollView

[ghost]

Description

[On iOS] When a SkCanvasView is in a ScrollView, and the EnableTouchEvents property is set to 'true', the touch event is not cancelled (i.e.: SkTouchAction.Cancelled is not invoked) if the ScrollView is scrolled while touching the SkCanvasView. Neither is the SkTouchAction.Released event. Everything works fine on Android.

Code

See attached sample project.

Expected Behavior

SKTouchAction.Cancelled event invoked when the container is scrolled while touching the SkCanvasView.

Actual Behavior

Event is not invoked on iOS. Works fine on Android. Not tested on other platforms.

Basic Information

• Version with issue: 1.68.1
• Last known good version: unknown
• IDE: VS2019 Mac/Windows
• Platform Target Frameworks:
  • Xamarin.Forms

Reproduction Link

TestSkiaSharp.zip

[ghost]

Well, after further investigations this behavior seems to be related to iOS and not to SkiaSharp itself. In this particular scenario, the underlying scrollview, as soon as the scroll begins, invokes the method 'touchesCancelled' of the view and not the GestureRecognizer's. This is why the SKCanvasView does not receive it. But the native view does. I guess something can be made at that level.

[daltonks]

Hey @MaurizioPanzica, I am currently running into this problem. I need a ListView that contains SKCanvasViews with touch handling. Have you been able to find a workaround?

[mpanzica]

@daltonks Unfortunately not. Since then the project has been put on hold because of Covid and I've been working on another one. I guess it will be resumed in the near future. Rest assured that I will post the workaround, should I find one.

Cheers, M.

[daltonks]

@mpanzica Thanks for the reply :) I'm kind of led to believe that Xamarin is mucking up the touches somehow, because as far as I can tell, TouchesCancelled should be propagated to gesture recognizers when a scroll view sends the event.

[mpanzica]

Nope, if I recall well Xamarin was not guilty at all. The "cancelled" event never reaches the gesture recognizer handler... hence it is not propagated any further. I will be able to resume the project as soon as I'll be allowed to go back to office. I can't access the client's repository from home. :-)

[daltonks]

Thanks for the info @mpanzica ! I am currently thinking that this might be because SKTouchHandler doesn't change its UIGestureRecognizer.State.

In your custom subclass, implement whatever methods you need to process events. For example, if your gesture consists of touch events, implement the touchesBegan(:with:), touchesMoved(:with:), touchesEnded(:with:), and touchesCancelled(:with:) methods. Use incoming events to update the state property of your gesture recognizer. UIKit uses the gesture recognizer states to coordinate interactions with other objects in your interface.

https://developer.apple.com/documentation/uikit/touches_presses_and_gestures/implementing_a_custom_gesture_recognizer https://github.com/mono/SkiaSharp/blob/master/source/SkiaSharp.Views.Forms/SkiaSharp.Views.Forms.iOS/SKTouchHandler.cs

[daltonks]

Nope, that idea led nowhere 🙃. Setting the State stops the scroll view from receiving the touches.

[daltonks]

@mpanzica I found a way that seems to work! If you keep track of the touches with HashSet<UITouch> _touches, you can manually cancel the ones in the gesture recognizer's Reset method.

https://gist.github.com/daltonks/f01cf125b580ca8b7b9213413f9299a5

[mpanzica]

Thanks for sharing, Dalton! It's night over here... I will give your solution a try tomorrow morning before anything else! :-)

Il lun 29 giu 2020, 22:24 Dalton Spillman [email protected] ha scritto:

@mpanzica https://github.com/mpanzica I found a way that seems to work! If you keep track of the touches with HashSet<UITouch> _touches, you can manually cancel the ones in the gesture recognizer's Reset method.

https://gist.github.com/daltonks/f01cf125b580ca8b7b9213413f9299a5

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[daltonks]

Cool! I made a repo based off of your reproduction .zip: https://github.com/daltonks/FixedSKTouchHandler

Tabs/spaces got all screwy, and I did strange things to use my own SKTouchHandler, but you can see the majority of the changes here: https://github.com/daltonks/FixedSKTouchHandler/blob/master/TestSkiaSharp.iOS/Renderers/TestSKCanvasViewRenderer.cs

The key part is keeping track of the touches and overriding UIGestureRecognizer.Reset.

[mattleibow]

Thanks for all the back and forth on this. I'll try to get this into the next release.

Has anyone been using this? Do all the touches that are started get the opposite finish/cancel event? Is there a potential for a memory leak or persistent touch objects?

[daltonks]

I've been using this successfully in production for a bit over 3 months now. I haven't seen any touch issues, but it would be good to look at how it behaves in a ScrollView as well as when iOS gestures take over (like minimizing the app, and maybe some other ones).

[mattleibow]

OK, thanks for the comment. I'll try a few things.

[DuncWatts]

I fixed this issue a while ago on a UIGestureRecognizer by returning false to the ShouldRecognizeSimultaneously delegate, and also triggering the Cancelled at the same time.

I'm trying to switch over to use the SkiaSharp OnTouch and now hitting this issue. This may be a simpler fix than tracking the touches.

[daltonks]

@DuncWatts Would returning false for ShouldRecognizeSimultaneously completely stop scrolling if the canvas was in a scroll view and you pressed on the SkiaSharp view? That is not what I typically need, but I could see a use case for that.

[DuncWatts]

No, it allows the scroll gesture on the scroll view to take priority and cancels the touch event on the canvas, which is the functionality I was after.

[daltonks]

@DuncWatts I tried ShouldRecognizeSimultaneously = (recognizer, gestureRecognizer) => false; in my example project https://github.com/daltonks/FixedSKTouchHandler, but it didn't call Cancelled on scrolling. I could be doing it wrong though.

https://github.com/daltonks/FixedSKTouchHandler/blob/526c035459193d1d4d600e500653446ac510f646/TestSkiaSharp.iOS/Renderers/TestSKCanvasViewRenderer.cs#L47