beginAppearanceTransition(_:animated:)
Tells a child controller its appearance is about to change.
Declaration
func beginAppearanceTransition(_ isAppearing: Bool, animated: Bool)Parameters
Discussion
If you are implementing a custom container controller, use this method to tell the child that its views are about to appear or disappear. Do not invoke viewWillAppear(_:), viewWillDisappear(_:), viewDidAppear(_:), or viewDidDisappear(_:) directly.