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I use this to clear the canvas.

mainCanvas().Children().Clear();

The UI will surely change after the event, but I would like to take a screenshot just after the canvas being cleared. I've tried various methods but always get a screenshot of uncleared canvas. At last, I separated the actions into different events to achieve it, but I still wonder how to do this in a single event.

This is very much the same situation as for WPF. Clearing canvas has a delay

ACGMN
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  • Do you have a minimal full reproducible code https://stackoverflow.com/help/minimal-reproducible-example – Simon Mourier May 30 '23 at 06:24
  • Have you tried to run `Dispatcher.RunIdleAsync(s => { do screenshot });` from some object (like the window) – Simon Mourier May 30 '23 at 14:57
  • Sadly, the DispatcherQueue in WinUI3 seems to remove all asynchronous APIs. There should be some extensions in CommunityToolkit, but it only supports C# and has not been updated for quite some time. – ACGMN May 31 '23 at 10:17
  • Have you tried `DispatcherQueue.TryEnqueue(DispatcherQueuePriority.Low, () => { do screenshot});` ? – Simon Mourier May 31 '23 at 10:42

1 Answers1

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In both WPF and WinUI the UI is rendered in another thread, not the "UI" thread of your application.

One approach would be to subscribe to CompositionTarget.Rendered event just before you call mainCanvas().Children().Clear();, then save the image and immediately unsubscribe from the event handler.

Nick
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  • Thanks. I tried to register a delegate with winrt::auto_revoke, but it seemed to be revoked too early. I will try more then. – ACGMN May 30 '23 at 07:56
  • Since rendering is done in two separate threads, it will always be a game with no clean winner. Changes in the rendering can come from many sources, including animations or transition effects. Therefore, there is never a reliable way to tell when exactly your canvas is empty. You may try with setting and monitoring a flag (set it after "Clear" and reset it after taking a screenshot), but this is still kinda flaky. – Nick May 30 '23 at 08:39
  • I agree with you that it's difficult to know whether the canvas is exactly empty on screen. – ACGMN May 31 '23 at 10:13