[UIKit] UIGestureRecognizer's custon OnDispose method needs to queue the actual operation on the main thread

[migueldeicaza]

Proposed fixed for:

#5388

[spouliot]

Calling RemoveTarget one the main thread (delayed) fix the reported issue.

Another way (I was trying) would be to remove the thread-check on RemoveTarget since it seem (at least Apple Main Thread Checker allows it) to be a thread safe call.

[monojenkins]

Build failure
✅ Build succeeded
✅ API Diff (from stable)
ℹ️ API Diff (from PR only) (please review changes)
✅ Generator Diff (no change)
🔥 Test run failed 🔥

Test results

1 tests failed, 0 tests skipped, 80 tests passed.

Failed tests

• monotouch-test/iOS Unified 32-bits - simulator/Debug (static registrar): Failed

[migueldeicaza]

The problem is that by the time it hits the main thread, the handle will have been cleared.

The Dispose() implementation just injects the result of the Dispose attribute, before calling Dispose(disposing). So there would be a potential race where the finalizer queue would run first, and release the handle, so our handle would be pointing to garbage.

That is why I took the ref, and then released it manually.

[rolfbjarne]

Do we really need to call RemoveTarget when Dispose is called from the GC? Won't things be freed anyway when the object is freed on the native side?

[migueldeicaza]

The hook is executed for all Dispose() kinds of invocations. I do not think it harms to keep it.

[VincentDondain]

I think this PR broke this test:

GestureRecognizerTest
	[FAIL] GestureRecognizerTest.NoStrongCycles :   Any finalized
  Expected: True
  But was:  False

		  at MonoTouchFixtures.UIKit.GestureRecognizerTest.NoStrongCycles () [0x001ce] in /Users/builder/jenkins/workspace/xamarin-macios-pr-builder/tests/monotouch-test/UIKit/GestureRecognizerTest.cs:81 
		  at (wrapper managed-to-native) System.Reflection.MonoMethod.InternalInvoke(System.Reflection.MonoMethod,object,object[],System.Exception&)
		  at System.Reflection.MonoMethod.Invoke (System.Object obj, System.Reflection.BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) [0x0003b] in /Library/Frameworks/Xamarin.iOS.framework/Versions/12.7.0.62/src/Xamarin.iOS/mcs/class/corlib/System.Reflection/MonoMethod.cs:305