[kekekeks]

WPF:

• dispatcher op calls Shutdown, ShutdownStarted called frames are cancelled
• dispatcher yields back to WndProc<-DispatcheMessage<-PushFrameImpl
• PushFrame sees that frame is cancelled and exits
• once all frames exited -> all pending dispatcher ops are aborted
• ShutdownFinished called

Avalonia:

• dispatcher op calls Shutdown, ShutdownStarted called frames are cancelled
• dispatcher drains any jobs in the queue before yielding (we do this because on some platforms yielding on every op has prohibitive costs on some platforms)
• dispatcher yields back to WndProc<-DispatchMessage<-Win32DispatcherImpl::RunLoop
• Win32DispatcherImpl::RunLoop sees that frame is cancelled and exits
• once all frames exited -> all pending dispatcher ops are aborted
• ShutdownFinished called

This PR makes dispatcher to yield on every operation once shutdown has started.

[avaloniaui-bot]

You can test this PR using the following package version. 12.2.999-cibuild0069978-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[MrJul]

Note: it's possible for users to see some slight differences in behavior after this PR.

For example:

if (ApplicationLifetime is IClassicDesktopStyleApplicationLifetime desktopLifetime)
    desktopLifetime.Exit += (_, _) => Dispatcher.Post(() => Console.WriteLine("Saving"));

If the shutdown was initiated from a dispatcher job, e.g. Dispatcher.Post(() => desktopLifetime.Shutdown()), this printed "Saving" before, while this isn't the case anymore after this PR.

However, in the sample above, the operation's execution has never been guaranteed. That code doesn't print anything today on main for a shutdown initiated outside an operation, for example by simply closing the main window.

[MrJul]

LGTM!