Checking if a UIViewController is about to get Pop

问题:

I need to know when my view controller is about to get popped from a nav stack so I can perform an action.

I can't use -viewWillDisappear, because that gets called when the view controller is moved off screen for ANY reason (like a new view controller being pushed on top).

I specifically need to know when the controller is about to be popped itself.

Any ideas would be awesome, thanks in advance.

回答1:

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?

回答2:

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.

回答3:

Try overriding willMoveToParentViewController: (instead of viewWillDisappear:) in your custom subclass of UIViewController.

Called just before the view controller is added or removed from a container view controller.

- (void)willMoveToParentViewController:(UIViewController *)parent
{
    [super willMoveToParentViewController:parent];
    if (!parent) {
        // `self` is about to get popped.
    }
}

回答4:

Fortunately, by the time the viewWillDisappear method is called, the viewController has already been removed from the stack, so we know the viewController is popping because it's no longer in the self.navigationController.viewControllers

- (void)viewWillDisappear:(BOOL)animated {
    [super viewWillDisappear:animated];
    if ((self.navigationController) && (![self.navigationController.viewControllers containsObject:self])) {
        NSLog(@"I've been popped!");
    }
}

回答5:

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");
    }
}

回答6:

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.

回答7:

I have the same problem. I tried with viewDisDisappear, but I don't have the function get called :( (don't know why, maybe because all my VC is UITableViewController). The suggestion of Alex works fine but it fails if your Navigation controller is displayed under the More tab. In this case, all VCs of your nav controllers have the navigationController as UIMoreNavigationController, not the navigation controller you have subclassed, so you will not be notified by the nav when a VC is about to popped.
Finaly, I solved the problem with a category of UINavigationController, just rewrite - (UIViewController *)popViewControllerAnimated:(BOOL)animated

- (UIViewController *)popViewControllerAnimated:(BOOL)animated{
   NSLog(@"UINavigationController(Magic)");
   UIViewController *vc = self.topViewController;
   if ([vc respondsToSelector:@selector(viewControllerWillBePopped)]) {
      [vc performSelector:@selector(viewControllerWillBePopped)];
   }
   NSArray *vcs = self.viewControllers;
   UIViewController *vcc = [vcs objectAtIndex:[vcs count] - 2];
   [self popToViewController:vcc animated:YES];
   return vcc;}

It works well for me :D

回答8:

I tried this:

- (void) viewWillDisappear:(BOOL)animated {
    // If we are disappearing because we were removed from navigation stack
    if (self.navigationController == nil) {
        // YOUR CODE HERE
    }

    [super viewWillDisappear:animated];
}

The idea is that at popping, the view controller's navigationController is set to nil. So if the view was to disappear, and it longer has a navigationController, I concluded it was popped. (might not work in other scenarios).

Can't vouch that viewWillDisappear will be called upon popping, as it is not mentioned in the docs. I tried it when the view was top view, and below top view - and it worked in both.

Good luck, Oded.

回答9:

You can use this one:

if(self.isMovingToParentViewController)
{
    NSLog(@"Pushed");
}
else
{
    NSLog(@"Popped");
}

回答10:

Subclass UINavigationController and override popViewController:

Swift 3

protocol CanPreventPopProtocol {
    func shouldBePopped() -> Bool
}

class MyNavigationController: UINavigationController {
    override func popViewController(animated: Bool) -> UIViewController? {
        let viewController = self.topViewController

        if let canPreventPop = viewController as? CanPreventPopProtocol {
            if !canPreventPop.shouldBePopped() {
                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
    //
    //tested on iOS 9.3.2 not others
    func gestureRecognizerShouldBegin(_ gestureRecognizer: UIGestureRecognizer) -> Bool {
        let viewController = self.topViewController

        if let canPreventPop = viewController as? CanPreventPopProtocol {
            if !canPreventPop.shouldBePopped() {
                return false
            }
        }

        return true
    }

}

回答11:

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

回答12:

Try making this check in viewwilldisappear if ([self.navigationController.viewControllers indexOfObject:self] == NSNotFound) { //popping of this view has happend. }