Bug 58196 - CVDisplayLink Stop hangs the app
Summary: CVDisplayLink Stop hangs the app
Alias: None
Product: Xamarin.Mac
Classification: Desktop
Component: Base Class Libraries ()
Version: 3.4.0 (15.2)
Hardware: PC Mac OS
: --- normal
Target Milestone: ---
Assignee: Bugzilla
Depends on:
Reported: 2017-07-17 18:12 UTC by Alvaro Rivoir
Modified: 2017-07-21 00:22 UTC (History)
4 users (show)

Is this bug a regression?: ---
Last known good build:

Repro sample (347.65 KB, application/zip)
2017-07-17 18:12 UTC, Alvaro Rivoir

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Description Alvaro Rivoir 2017-07-17 18:12:13 UTC
Created attachment 23604 [details]
Repro sample

Calling method Stop of CVDisplayLink hangs the app, anything is executed, and the app remains unresponsive until killed.

        public override void ViewDidLoad()

            // Do any additional setup after loading the view.
            View.WantsLayer = true;
            View.Layer.BackgroundColor = NSColor.Red.CGColor;

        private async void Test()
            await Task.Delay(1000);
            var before = DateTime.Now;
            var displayLink = new CVDisplayLink();
            displayLink.SetOutputCallback(new CVDisplayLink.DisplayLinkOutputCallback((CVDisplayLink dl, ref CVTimeStamp inNow, ref CVTimeStamp inOutputTime, CVOptionFlags flagsIn, ref CVOptionFlags flagsOut) =>
                    NSRunLoop.Main.InvokeOnMainThread(() =>
                        if (dl.IsRunning)
                            if (DateTime.Now - before > TimeSpan.FromSeconds(2))
                                displayLink.Stop();//After this line anything is executed
                                View.Layer.BackgroundColor = NSColor.Red.CGColor;
                                var bytes = new byte[3];
                                var color = NSColor.FromRgb(bytes[0], bytes[1], bytes[2]);
                                View.Layer.BackgroundColor = color.CGColor;
                    return CVReturn.Success;
Comment 1 Alex Soto [MSFT] 2017-07-21 00:22:31 UTC
Hello, this is more a question of how to use an API than an issue in Xamarin.Mac, if I move the displayLink to stop after a couple of seconds instead of Stoping and disposing the object while executing the delegate it won't freeze or crash.

if (DateTime.Now - before > TimeSpan.FromSeconds(2))
	Stop (displayLink);
	View.Layer.BackgroundColor = NSColor.Red.CGColor;

void Stop (CVDisplayLink displayLink)
	var task = Task.Factory.StartNew (() => {
		System.Threading.Thread.Sleep (2000);
		displayLink.Stop ();
		displayLink.Dispose ();
		Console.WriteLine ("Done");

The broader developer community would be the best and quickest place for additional troubleshooting help on this issue.
Posting a question on Stack Overflow [0] or the Xamarin Forums [1] would be the best next step.

[0] http://stackoverflow.com/questions/tagged/xamarin
[1] https://forums.xamarin.com/

(Investigation by the Xamarin team in the bug tracker for an issue like this would usually require that the reporter include additional background info to hint that Xamarin is behaving differently compared to Xcode.)

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