viewWillDisappear: Determine whether view controller is being popped or is showing a sub-view controller

StackOverflow https://stackoverflow.com/questions/1816614

Question

I'm struggling to find a good solution to this problem. In a view controller's -viewWillDisappear: method, I need to find a way to determine whether it is because a view controller is being pushed onto the navigation controller's stack, or whether it is because the view controller is disappearing because it has been popped.

At the moment I'm setting flags such as isShowingChildViewController but it's getting fairly complicated. The only way I think I can detect it is in the -dealloc method.

Was it helpful?

Solution

You can use the following.

- (void)viewWillDisappear:(BOOL)animated {
  [super viewWillDisappear:animated];
  NSArray *viewControllers = self.navigationController.viewControllers;
  if (viewControllers.count > 1 && [viewControllers objectAtIndex:viewControllers.count-2] == self) {
    // View is disappearing because a new view controller was pushed onto the stack
    NSLog(@"New view controller was pushed");
  } else if ([viewControllers indexOfObject:self] == NSNotFound) {
    // View is disappearing because it was popped from the stack
    NSLog(@"View controller was popped");
  }
}

This is, of course, possible because the UINavigationController's view controller stack (exposed through the viewControllers property) has been updated by the time that viewWillDisappear is called.

OTHER TIPS

I think the easiest way is:

 - (void)viewWillDisappear:(BOOL)animated
{
    if ([self isMovingFromParentViewController])
    {
        NSLog(@"View controller was popped");
    }
    else
    {
        NSLog(@"New view controller was pushed");
    }
    [super viewWillDisappear:animated];
}

Swift:

override func viewWillDisappear(animated: Bool)
{
    if isMovingFromParentViewController
    {
        print("View controller was popped")
    }
    else
    {
        print("New view controller was pushed")
    }
    super.viewWillDisappear(animated)
}

From Apple's Documentation in UIViewController.h :

"These four methods can be used in a view controller's appearance callbacks to determine if it is being presented, dismissed, or added or removed as a child view controller. For example, a view controller can check if it is disappearing because it was dismissed or popped by asking itself in its viewWillDisappear: method by checking the expression ([self isBeingDismissed] || [self isMovingFromParentViewController])."

- (BOOL)isBeingPresented NS_AVAILABLE_IOS(5_0);

- (BOOL)isBeingDismissed NS_AVAILABLE_IOS(5_0);

- (BOOL)isMovingToParentViewController NS_AVAILABLE_IOS(5_0);

- (BOOL)isMovingFromParentViewController NS_AVAILABLE_IOS(5_0);

So yes, the only documented way to do this is the following way :

- (void)viewWillDisappear:(BOOL)animated
{
    [super viewWillDisappear:animated];
    if ([self isBeingDismissed] || [self isMovingFromParentViewController]) {
    }
}

Swift 3 version:

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

    if self.isBeingDismissed || self.isMovingFromParentViewController { 
    }
}

If you just want to know whether your view is getting popped, I just discovered that self.navigationController is nil in viewDidDisappear, when it is removed from the stack of controllers. So that's a simple alternative test.

(This I discover after trying all sorts of other contortions. I'm surprised there's no navigation controller protocol to register a view controller to be notified on pops. You can't use UINavigationControllerDelegate because that actually does real display work.)

Swift 4

override func viewWillDisappear(_ animated: Bool)
    {
        super.viewWillDisappear(animated)
        if self.isMovingFromParent
        {
            //View Controller Popped
        }
        else
        {
            //New view controller pushed
        }
    }

In Swift: 

 override func viewWillDisappear(animated: Bool) {
    if let navigationController = self.navigationController {
        if !contains(navigationController.viewControllers as! Array<UIViewController>, self) {
        }
    }

    super.viewWillDisappear(animated)

}

I find Apple's documentation on this is hard to understand. This extension helps see the states at each navigation.

extension UIViewController {
    public func printTransitionStates() {
        print("isBeingPresented=\(isBeingPresented)")
        print("isBeingDismissed=\(isBeingDismissed)")
        print("isMovingToParentViewController=\(isMovingToParentViewController)")
        print("isMovingFromParentViewController=\(isMovingFromParentViewController)")
    }
}

This question is fairly old but I saw it by accident so I want to post best practice (afaik)

you can just do

if([self.navigationController.viewControllers indexOfObject:self]==NSNotFound)
 // view controller popped
}

This applies to iOS7, no idea if it applies to any other ones. From what I know, in viewDidDisappear the view already has been popped. Which means when you query self.navigationController.viewControllers you will get a nil. So just check if that is nil.

TL;DR

 - (void)viewDidDisappear:(BOOL)animated
 {
    [super viewDidDisappear:animated];
    if (self.navigationController.viewControllers == nil) {
        // It has been popped!
        NSLog(@"Popped and Gone");
    }
 }

Segues can be a very effective way of handling this problem in iOS 6+. If you have given the particular segue an identifier in Interface Builder you can check for it in prepareForSegue.

- (void)prepareForSegue:(UIStoryboardSegue *)segue sender:(id)sender
{
    if ([segue.identifier isEqualToString:@"LoginSegue"]) {
       NSLog(@"Push");
       // Do something specific here, or set a BOOL indicating
       // a push has occurred that will be checked later
    }
}

I assume you mean that your view is being moved down the navigation controller's stack by the pushing a new view when you say pushed onto the stack. I would suggest using the viewDidUnload method to add a NSLog statement to write something to the console so you can see what is going on, you may want to add a NSLog to viewWillDissappeer.

Here is a category to accomplish the same thing as sbrocket's answer:

Header:

#import <UIKit/UIKit.h>

@interface UIViewController (isBeingPopped)

- (BOOL) isBeingPopped;

@end

Source:

#import "UIViewController+isBeingPopped.h"

@implementation UIViewController (isBeingPopped)

- (BOOL) isBeingPopped {
    NSArray *viewControllers = self.navigationController.viewControllers;
    if (viewControllers.count > 1 && [viewControllers objectAtIndex:viewControllers.count-2] == self) {
        return NO;
    } else if ([viewControllers indexOfObject:self] == NSNotFound) {
        return YES;
    }
    return NO;
}

@end
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