UIKit: Prevent redundant dismissal of already-dismissing view controller

[juliensagot]

This fixes an issue where presenting a view controller just after dismissing one results in dismissing the presenting view controller.

When a view controller was already in the process of being dismissed (isBeingDismissed = true) or moving from its parent (isMovingFromParent = true), the presentation logic would still attempt to call dismiss() on it again, causing unintended dismissals in the presentation hierarchy.

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Without the fix:

Pressing the save button dismisses the currently presented view controller and immediately presents the timer view controller. However, due to a bug, the presenting view controller is also dismissed.

Bug.mov

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With the fix:

Pressing the save button correctly dismisses the currently presented view controller and then presents the timer view controller without affecting the presenting view controller.

No_bug.mov

[stephencelis]

@juliensagot Thanks for this! We're definitely down to merge but in the interest of preventing regressions in the future do you think it's possible to get a test written that would fail with the existing behavior?

We have a suite here that will hopefully be easy to extend:

https://github.com/pointfreeco/swift-navigation/blob/main/Examples/CaseStudiesTests/PresentationTests.swift

Let us know if you have any questions!

[juliensagot]

While writing the test, I realized the fix isn’t needed after all—it works as expected. In my project, the issue is 100% reproducible, but only the first time I present the initial view controller. There’s definitely something odd going on, and I need to spend more time investigating.