Running Timer with CoreDispatcherScheduler in UWP app could result in freezing

[sowenzhang]

Hello and thank you for using dotnet/reactive. Please select a category and detail your issue by answering the questions there:

Bug

Which library version?

5.0

What are the platform(s), environment(s) and related component version(s)?

Windows 10

What is the use case or problem?

Creating a timer, which runs some background tasks; however, I accidentally create the timer using the CoreDispachScheduler instead of any other scheduler.

The timer will perform the job; however, when leaving the computer unattended, which results the app in the suspended state. The observable freezes the app when trying to resume it.

What is the expected outcome?

the UWP app can resume.

What is the actual outcome?

the UWP app cannot resume, just freezes, can maximize/minimize, but the rendering hangs, no control can be interacted

What is the stacktrace of the exception(s) if any?
N/A

Do you have a code snippet or project that reproduces the problem?

For testing, you can just try to do this:

// declare the scheduler in the  class level 
private readonly IScheduler _scheduler = new CoreDispatcherScheduler(CoreApplication.MainView.CoreWindow.Dispatcher);

// add a button, and wire the event handler 
       private void Button1Clicked(object sender, RoutedEventArgs e)
        {
            var timer = Observable
                .Timer(TimeSpan.Zero, TimeSpan.FromSeconds(30), _scheduler)
                .Subscribe(l =>
            {
                Debug.WriteLine("Timer running");
                this.ResponseText.Text = $"{l} - {DateTime.Now} - Ticking";
            });
        }

The above code updates a TextBlock name ResponseText. Now minimize this window, leaves it for a few minutes.
Next, bring it back from task bar. You will see the app frozen and you can't interact with any other ui control.

If I use the TaskPoolScheduler.Default, it will not have such a problem. In theory, I agree, for a background task, I should not use the DispacheScheduler. But even I do, when the Dispatcher is in a suspended state, the timer should be paused automatically instead of freezing the app. It seems the suspended scheduler break the observable and cause the app fail to receive WM_ACTIVATE. Is this a bug in the framework?