Checking if a UIViewController is about to get Popped from a navigation stack?

Override the viewWillDisappear method in the presented VC, then check the isMovingFromParentViewController flag within the override and do specific logic. In my case I'm hiding the navigation controllers toolbar. Still requires that your presented VC understand that it was pushed though so not perfect.

I don't think there is an explicit message for this, but you could subclass the UINavigationController and override - popViewControllerAnimated (although I haven't tried this before myself).

Alternatively, if there are no other references to the view controller, could you add to its - dealloc?

This is working for me.

- (void)viewDidDisappear:(BOOL)animated
{
    if (self.parentViewController == nil) {
        NSLog(@"viewDidDisappear doesn't have parent so it's been popped");
        //release stuff here
    } else {
        NSLog(@"PersonViewController view just hidden");
    }
}

You can catch it here.

- (void)navigationController:(UINavigationController *)navigationController willShowViewController:(UIViewController *)viewController animated:(BOOL)animated {

    if (viewController == YourAboutToAppearController) {
            // do something
    }
}

This will fire just before the display of the new View. Nobody's moved yet. I use all the time to do magic in front of the asinine NavigationController. You can set titles and button titles and do whatever there.

I needed to also prevent from popping sometimes so the best answer for me was written by Orkhan Alikhanov. But it did not work because the delegate was not set, so I made the final version:

import UIKit

class CustomActionsNavigationController: UINavigationController, 
                                         UIGestureRecognizerDelegate {
    override func viewDidLoad() {
        super.viewDidLoad()
        interactivePopGestureRecognizer?.delegate = self
    }

    override func popViewController(animated: Bool) -> UIViewController? {
        if let delegate = topViewController as? CustomActionsNavigationControllerDelegate {
            guard delegate.shouldPop() else { return nil }
        }
        return super.popViewController(animated: animated)
    }

    // important to prevent UI thread from freezing
    //
    // if popViewController is called by gesture recognizer and prevented by returning nil
    // UI will freeze after calling super.popViewController
    // so that, in order to solve the problem we should not return nil from popViewController
    // we interrupt the call made by gesture recognizer to popViewController through
    // returning false on gestureRecognizerShouldBegin
    func gestureRecognizerShouldBegin(_ gestureRecognizer: UIGestureRecognizer) -> Bool {
        if let delegate = topViewController as? CustomActionsNavigationControllerDelegate {
            if !delegate.shouldPop() {
                return false
            }
        }

        // This if statement prevents navigation controller to pop when there is only one view controller
        if viewControllers.count == 1 {
            return false
        }

        return true
    }
}

protocol CustomActionsNavigationControllerDelegate {
    func shouldPop() -> Bool
}

UPDATE

I have added viewControllers.count == 1 case, because if there is one controller in the stack and user makes the gesture, it will freeze the UI of your application.

You can observe the notification:

- (void)viewDidLoad{
    [super viewDidLoad];
    [NSNotificationCenter.defaultCenter addObserver:self selector:@selector(navigationControllerWillShowViewController:) name:@"UINavigationControllerWillShowViewControllerNotification" object:nil];
}

- (void)navigationControllerDidShowViewController:(NSNotification *)notification{
    UIViewController *lastVisible = notification.userInfo[@"UINavigationControllerLastVisibleViewController"];
    if(lastVisible == self){
        // we are being popped
    }
}

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

    const BOOL removingFromParent = ![self.navigationController.viewControllers containsObject:self.parentViewController];
    if ( removingFromParent ) {
        // cleanup
    }
}

Maybe you could use UINavigationBarDelegate's navigationBar:shouldPopItem protocol method.