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Created attachment 25810 [details]
It's impossible to use `NSOpenGLContext` from any threads except the Main UI thread.
NSOpenGLContext.FlushBuffers(), NSOpenGLContext.CGLContext.Lock() and some other methods throw an exception:
"AppKit Consistency error: you are calling a method that can only be invoked from the UI thread."
But this is not true.
According to the Apple documentations (and samples) we are allowed and we are forced to invoke this methods from other threads (e.g. due to the DisplayLink callback)
Assembly Browser shows the cause of that exception in some CompilerGenerated methods:
It seems that this guard is unnecessary and moreover, harmful.
## Steps to reproduce
1. Open attached TestGL demo project in Visual Studio.
2. In order to see an exception from background threads make some settings in Visual Studio for Mac (Run -> New Exception Catchpoint, choose "When an exception is thrown" for 'System.Exception' and set 'Include Subclasses' checkbox, press Create)
3. Start Debugging
4. Observe thrown exception as a result of invoking _mainContext.CGLContext.Lock() (_mainContext.CGLContext.FlushBuffer and so on)
Visual Studio Community 2017 for Mac
Version 7.2.2 (build 11)
Mono 220.127.116.11 (2017-06/1f4613aa1ac) (64-bit)
GTK+ 2.24.23 (Raleigh theme)
Package version: 504010006
NuGet Version: 18.104.22.16845
Runtime: Not installed
SDK: Not installed
MSBuild SDKs: /Library/Frameworks/Mono.framework/Versions/5.4.1/lib/mono/msbuild/15.0/bin/Sdks
Apple Developer Tools
Xcode 9.1 (13532)
Version: 22.214.171.124 (Visual Studio Community)
Mac OS X 10.12.6
Darwin 16.7.0 Darwin Kernel Version 16.7.0
Thanks for the information!
The threading checks are debug helpers, i.e. they are removed from release builds (by the managed linker), unless extra arguments are given to mtouch, as they have a performance impact on applications (e.g. when used in tight loops).
We acknowledge this enhancement request, therefore marked it as CONFIRMED.
However, we need to do a full API review on NSOpenGLContext to not dismiss cases
where the UI thread check is valid.
You can workaround this by disabling the check before the NSOpenGLContext.FlushBuffers() call (and re-enabling it afterward). E.g. something like:
NSApplication.CheckForIllegalCrossThreadCalls = false;
NSApplication.CheckForIllegalCrossThreadCalls = true;
Hope this helps!
OpenGL context are restricted to a single thread - but it does not have to be the main thread.
The current check is excessive.
Merged in master https://github.com/xamarin/xamarin-macios/commit/3355fdfc6e06d56b9b6c741335efeb9d47f971f9