Detecting when the 'back' button is pressed on a navbar

Question

UPDATE: According to some comments, the solution in the original answer does not seem to work under certain scenarios in iOS 8+. I can't verify that that is actually the case without further details.

For those of you however in that situation there's an alternative. Detecting when a view controller is being popped is possible by overriding willMove(toParentViewController:). The basic idea is that a view controller is being popped when parent is nil.

Check out "Implementing a Container View Controller" for further details.

---

Since iOS 5 I've found that the easiest way of dealing with this situation is using the new method - (BOOL)isMovingFromParentViewController:

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

  if (self.isMovingFromParentViewController) {
    // Do your stuff here
  }
}

- (BOOL)isMovingFromParentViewController makes sense when you are pushing and popping controllers in a navigation stack.

However, if you are presenting modal view controllers you should use - (BOOL)isBeingDismissed instead:

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

  if (self.isBeingDismissed) {
    // Do your stuff here
  }
}

As noted in this question, you could combine both properties:

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

  if (self.isMovingFromParentViewController || self.isBeingDismissed) {
    // Do your stuff here
  }
}

Other solutions rely on the existence of a UINavigationBar. Instead like my approach more because it decouples the required tasks to perform from the action that triggered the event, i.e. pressing a back button.

While viewWillAppear() and viewDidDisappear() are called when the back button is tapped, they are also called at other times. See end of answer for more on that.

Using UIViewController.parent

Detecting the back button is better done when the VC is removed from it's parent (the NavigationController) with the help of willMoveToParentViewController(_:) OR didMoveToParentViewController()

If parent is nil, the view controller is being popped off the navigation stack and dismissed. If parent is not nil, it is being added to the stack and presented.

// Objective-C
-(void)willMoveToParentViewController:(UIViewController *)parent {
     [super willMoveToParentViewController:parent];
    if (!parent){
       // The back button was pressed or interactive gesture used
    }
}


// Swift
override func willMove(toParent parent: UIViewController?) {
    super.willMove(toParent: parent)
    if parent == nil {
        // The back button was pressed or interactive gesture used
    }
}

Swap out willMove for didMove and check self.parent to do work after the view controller is dismissed.

Stopping the dismiss

Do note, checking the parent doesn't allow you to "pause" the transition if you need to do some sort of async save. To do that you could implement the following. Only downside here is you lose the fancy iOS styled/animated back button. Also be careful here with the interactive swipe gesture. Use the following to handle this case.

var backButton : UIBarButtonItem!

override func viewDidLoad() {
    super.viewDidLoad()

     // Disable the swipe to make sure you get your chance to save
     self.navigationController?.interactivePopGestureRecognizer.enabled = false

     // Replace the default back button
    self.navigationItem.setHidesBackButton(true, animated: false)
    self.backButton = UIBarButtonItem(title: "Back", style: UIBarButtonItemStyle.Plain, target: self, action: "goBack")
    self.navigationItem.leftBarButtonItem = backButton
}

// Then handle the button selection
func goBack() {
    // Here we just remove the back button, you could also disabled it or better yet show an activityIndicator
    self.navigationItem.leftBarButtonItem = nil
    someData.saveInBackground { (success, error) -> Void in
        if success {
            self.navigationController?.popViewControllerAnimated(true)
            // Don't forget to re-enable the interactive gesture
            self.navigationController?.interactivePopGestureRecognizer.enabled = true
        }
        else {
            self.navigationItem.leftBarButtonItem = self.backButton
            // Handle the error
        }
    }
}

More on view will/did appear

If you didn't get the viewWillAppear viewDidDisappear issue, Let's run through an example. Say you have three view controllers:

1. ListVC: A table view of things
2. DetailVC: Details about a thing
3. SettingsVC: Some options for a thing

Lets follow the calls on the detailVC as you go from the listVC to settingsVC and back to listVC

List > Detail (push detailVC) Detail.viewDidAppear <- appear
Detail > Settings (push settingsVC) Detail.viewDidDisappear <- disappear

And as we go back...
Settings > Detail (pop settingsVC) Detail.viewDidAppear <- appear
Detail > List (pop detailVC) Detail.viewDidDisappear <- disappear

Notice that viewDidDisappear is called multiple times, not only when going back, but also when going forward. For a quick operation that may be desired, but for a more complex operation like a network call to save, it may not.

Those who claim that this doesn't work are mistaken:

override func viewWillDisappear(_ animated: Bool) {
    super.viewWillDisappear(animated)
    if self.isMovingFromParent {
        print("we are being popped")
    }
}

That works fine. So what is causing the widespread myth that it doesn’t?

The problem seems to be due to an incorrect implementation of a different method, namely that the implementation of willMove(toParent:) forgot to call super.

If you implement willMove(toParent:) without calling super, then self.isMovingFromParent will be false and the use of viewWillDisappear will appear to fail. It didn't fail; you broke it.

NOTE: The real problem is usually the second view controller detecting that the first view controller was popped. Please see also the more general discussion here: Unified UIViewController "became frontmost" detection?

EDIT A comment suggests that this should be viewDidDisappear rather than viewWillDisappear.

First Method

- (void)didMoveToParentViewController:(UIViewController *)parent
{
    if (![parent isEqual:self.parentViewController]) {
         NSLog(@"Back pressed");
    }
}

Second Method

-(void) viewWillDisappear:(BOOL)animated {
    if ([self.navigationController.viewControllers indexOfObject:self]==NSNotFound) {
       // back button was pressed.  We know this is true because self is no longer
       // in the navigation stack.  
    }
    [super viewWillDisappear:animated];
}