Crash when calling ChangeCanExecute() from non-UI thread

[Mikilll94]

Description

App crashes when running async method which triggers ChangeCanExecute(). I think this should the responsibility of the renderers to trigger any visual updates on the UI thread. The workaround for this problem is to wrap all ChangeCanExecute() in Device.BeginInvokeOnMainThread()

Steps to Reproduce

Run async methods which triggers ChangeCanExecute(). You can i.e. create a button with async Command which changes one of the binding property and this binding property raise ChangeCanExecute() in the setter.

Expected Behavior

App should not crash.

Actual Behavior

App crashes when running async method which triggers ChangeCanExecute(). The problem exists both on Android and iOS.

Basic Information

• Version with issue: Xamarin Forms 3.6.0.264807
• Last known good version:
• IDE: Visual Studio 2017 15.9.4
• Platform Target Frameworks:
  • iOS: 8.0
  • Android: 8.1

Reproduction Project

Here is a project with a repro:

App1.zip

[kingces95]

Well, throws something but my debugger doesn't quite let me dig the message out...

[Mikilll94]

@kingces95

You have to click "Press to see the error" button. This will fire "AnotherCommand" in the view model. In view model I have wrapped the command in try catch block. Here is the code:

        private Task AnotherCommandExecute()
        {
            return Task.Run(() =>
            {
                try
                {
                    ButtonActivated = !ButtonActivated;
                }
                catch(Exception e)
                {
                    Debug.WriteLine(e);

                    throw;
                }
            });
        }

As you can see I caught the exception and wrote it to the output using Debug.WriteLine. If you want you can set a breakpoint on Debug.WriteLine and look and the "e" variable. The error message says that you can only make a change in UI on the main thread. Thus ChangeCanExecute() cannot be executed by an async method because the will crash the app. And I think that should not work like that. Renderers should be responsible for executing UI changes on the main thread.