iOS Restoration - why is another View displayed before the one I'm restoring?

Question

My storyboard has: NavigationController -> UIViewController 1 -> UIViewController 2 -> UIViewController 3 -> UITabBarController -> four UIViewControllers (a,b,c,d)

I'm adding restoration support to the app. I've added it to UIViewController #1 and #3 and am now adding it to UIViewController 'c' under the UITabBarController.

When I kill my app using Xcode when it's displaying UIViewController 'c' and then start the app again the viewcontroller's methods are called correctly and the content of its UITextFields are restored correctly.

But in placing breakpoints in 'c' view controller I can see that UIViewController 3 is temporarily displayed. It's viewDidLoad and viewWillAppear: methods are called.

What would typically be the cause of UIViewController #3 being displayed?

I've pressed on implementing restoration for my UIViewControllers and there doesn't appear to be any negative effect from an end-users point of view - without the breakpoints the display doesn't get a chance to update to show the intermediate view.

Answer 1

That's what view controller restoration is. The app launches from scratch; to reach the view controller that was being displayed at termination, the app walks the view controller creation sequence down the chain from the root view controller to the view controller that is to be displayed to the user. It couldn't possibly happen any other way. (After all, if VC2's Back button is to lead to VC1, then VC2 must be above VC1 in the nav controller stack.) And you wouldn't want it not to happen, because you might have important work to do as the sequence is reconstructed. By setting breakpoints in the middle of the restoration process, you have revealed "the man behind the curtain". But the user doesn't see that process (it is covered by the launch image), so there's no problem.

In particular it is very important that viewDidLoad is called in correct sequence right down the chain of creation, and that this happens for all view controllers before they are sent decodeRestorableStateWithCoder:, also in order down the chain.

(However, you should not count on anything having to do with viewWillAppear: and its cousins. These may or may not be called and you can't be sure just when. I regard this inconsistency / unpredictability with regard to the timing of viewWillAppear: as a bug, but Apple, unfortunately, does not.)