Question

I’m using viewWillAppear, viewDidAppear, viewWillDisappear, viewDidDisappear of UIViewController.
And I use UINavigationController to navigate view controllers.

For example, I create NSTimer or register notifications in viewWillAppear or viewDidAppear. And I invalidate the timer or remove notifications in viewWillDisappear or viewDidDisappear.

But if those 4 methods (viewWillAppear, viewDidAppear, viewWillDisappear, viewDidDisappear) are not called, the program will crash or retain cycle will happen.

I couldn’t find the documentation saying that it is guaranteed that viewWillAppear, viewDidAppear, viewWillDisappear, viewDidDisappear are always called.

Till now, they seem to be always called and my program works as I expected.

But are there cases that the 4 methods are not called?
Or is it possible the 4 methods are usually called but sometimes not called randomly with no reason?

Answer 1

Yes, they are always called. Just make sure to call the superclass implementation if you subclass one of your view controllers.