iOS how to detect programmatically when top view controller is popped?

Question

Suppose I have a nav controller stack with 2 view controllers: VC2 is on top and VC1 is underneath. Is there code I can include in VC1 that will detect that VC2 has just be

Answer 1

swift 3

override func viewWillAppear(_ animated: Bool) {
        super.viewWillAppear(animated)
        if self.isMovingToParentViewController {
            print("View is moving to ParentViewControll")
        }
}

Answer 2

Yes, in VC1 you can check whether VC2 is popped or not. UINavigationController there is one method viewControllers which will returns the array of pushed Controllers, which are in the stack (i.e. which have been pushed).

So you iterate through loop by comparing class. If VC2 is there, will have match, otherwise not.

Answer 3

isMovingTo/FromParentViewController won't work for pushing and popping into a navigation controller stack.

Here's a reliable way to do it (without using the delegate), but it's probably iOS 7+ only.

UIViewController *fromViewController = [[[self navigationController] transitionCoordinator] viewControllerForKey:UITransitionContextFromViewControllerKey];

if ([[self.navigationController viewControllers] containsObject:fromViewController])
{
    //we're being pushed onto the nav controller stack.  Make sure to fetch data.
} else {
    //Something is being popped and we are being revealed
}

In my case, using the delegate would mean having the view controllers' behavior be more tightly coupled with the delegate that owns the nav stack, and I wanted a more standalone solution. This works.

Answer 4

iOS 5 introduced two new methods to handle exactly this type of situation. What you're looking for is -[UIViewController isMovingToParentViewController]. From the docs:

isMovingToParentViewController

Returns a Boolean value that indicates that the view controller is in the process of being added to a parent.

- (BOOL)isMovingToParentViewController

Return Value
YES if the view controller is appearing because it was added as a child of a container view controller, otherwise NO.

Discussion
This method returns YES only when called from inside the following methods:

-viewWillAppear:
-viewDidAppear:

In your case you could implement -viewWillAppear: like so:

- (void)viewWillAppear:(BOOL)animated
{
    [super viewWillAppear:animated];

    if (self.isMovingToParentViewController == NO)
    {
        // we're already on the navigation stack
        // another controller must have been popped off
    }
}

EDIT: There's a subtle semantic difference to consider here—are you interested in the fact that VC2 in particular popped off the stack, or do you want to be notified each time VC1 is revealed as a result of any controller popping? In the former case, delegation is a better solution. A straight-up weak reference to VC1 could also work if you never intend on reusing VC2.

EDIT 2: I made the example more explicit by inverting the logic and not returning early.

Answer 5

What are you specifically trying to do?

If you're trying to detect that VC1 is about to be shown, this answer should help you. Use UINavigationControllerDelegate.

If you're trying to detect that VC2 is about to be hidden, I would just use the viewWillDisappear: of VC2.

Answer 6

I got the same situation but with slight more specific use case. In my case we wanted to determine if a VC1 is appeared/displayed when user tap on VC2's back button where VC2 is pushed on navigationController over VC1.

So I used help of snarshad's answer to customised as per my need. Here is the code in VC1's viewDidAppear in swift 3.

// VC1: ParentViewController
// VC2: ChildViewController

    override func viewDidAppear(_ animated: Bool) {
        super.viewDidAppear(animated)

        if let transitionCoordinator = navigationController?.transitionCoordinator,
            let fromVC = transitionCoordinator.viewController(forKey: UITransitionContextViewControllerKey.from),
            let toVC = transitionCoordinator.viewController(forKey: UITransitionContextViewControllerKey.to),
            fromVC is ChildViewController,
            toVC is ParentViewController {

            print("Back button pressed on ChildViewController, and as a result ParentViewController appeared")
        }
    }