[ios] fix memory leaks in GestureRecognizers

[jonathanpeppers]

Context: #20023 (comment)

One of the underlying reasons RadioButton appears to leak, is that it happens to use a TapGestureRecognizer internally. When testing, @tj-devel709 and I found we could add pretty much any recognizer to a simple control like BoxView, and it would cause the BoxView to live forever.

I could reproduce this in a new unit test that I setup for each type of gesture recognizer.

GesturePlatformManager.iOS.cs had two "cycles":

• *Handler -> ... -> GesturePlatformManager -> *Handler

• *GestureRecognizer -> GesturePlatformManager -> *GestureRecognizer

To fix these, we can make GesturePlatformManager not have strong references pointing back to its parent classes:

• Make _handler a WeakReference<IPlatformViewHandler>

• Make _gestureRecognizers a ConditionalWeakTable

With these changes the tests pass, except for PanGestureRecognizer, which had a "closure" in using the panRecognizer variable.

I fixed this one with:

• ((PanGestureRecognizer)panGestureRecognizer) as panGestureRecognizer was backed by a WeakReference.

[jonathanpeppers]

This one we probably need to go manually test all the recognizers. Is there a sample I can try that?

[PureWeen]

This one we probably need to go manually test all the recognizers. Is there a sample I can try that?

I don't think so, Device Tests don't really support gestures so we haven't really used them very heavily other than basic logical tests.

[jonathanpeppers]

Ok, it turns out in practice that GestureRecognizers do not leak. Because when controls' Window is lost, GesturePlatformManager.Dispose() is called. The HandlerDoesNotLeak test for RadioButton does not have a Window at all to unset.

Tested with dotnet/maui/main:

• jonathanpeppers/MauiGCTest@0489b5a

Something like this works in the sample:

new BoxView {
  GestureRecognizers = {
    new PanGestureRecognizer(),
    new DragGestureRecognizer(),
    new DropGestureRecognizer(),
    new PanGestureRecognizer(),
    new PinchGestureRecognizer(),
    new PointerGestureRecognizer(),
    new SwipeGestureRecognizer(),
    new TapGestureRecognizer(),
}

2024-03-11 16:33:49.851 GCTest[23861:336330] BoxView Page
2024-03-11 16:33:52.080 GCTest[23861:336340] ~BoxView Page

I will re-think this one.

There is still an unsolved issue with RadioButton in the above sample:

2024-03-11 16:38:49.099 GCTest[24155:340813] RadioButton Page

Note there is no ~RadioButton Page message.