Completion Handler for Uinavigationcontroller "Pushviewcontroller:Animated"

Completion block for popViewController

I know an answer has been accepted over two years ago, however this answer is incomplete.

There is no way to do what you're wanting out-of-the-box

This is technically correct because the UINavigationController API doesn't offer any options for this. However by using the CoreAnimation framework it's possible to add a completion block to the underlying animation:

[CATransaction begin];
[CATransaction setCompletionBlock:^{
    // handle completion here
}];

[self.navigationController popViewControllerAnimated:YES];

[CATransaction commit];

The completion block will be called as soon as the animation used by popViewControllerAnimated: ends. This functionality has been available since iOS 4.

How to detect pop/push animation ended within a navigationController

UINavigationControllerDelegate

func navigationController(_ navigationController: UINavigationController, didShow viewController: UIViewController, animated: Bool) {

}

func navigationController(_ navigationController: UINavigationController, willShow viewController: UIViewController, animated: Bool) {

}

How to push a view controller safely from a completion block?

Try something like:

[query findObjectsInBackgroundWithBlock:^(NSArray *objects, NSError *error){
    if (self.navigationController.topViewController == self) {
        Model *model = [[Model alloc] initWithPFObject:[objects objectAtIndex:0]];
        controller.annotation = model;
        [self.navigationController pushViewController:controller animated:YES];
    }
}];

This way it won't push if you already have another view controller on top of self. And also make sure that you are doing all this on the main thread.

While an existing transition or presentation is occurring; the navigation stack will not be updated

This warning indicates that you are trying use UINavigationController wrongly:

pushViewController:animated: called on while an existing transition or presentation is occurring; the navigation stack will not be updated

You mentioned in the comments that you are trying to pop the
ViewController using

navigationController?.popViewControllerAnimated(false)

inside completion block of UIAlertController. Therefore, you are trying to unwind from the wrong view, UIAlertController is not part of the UINavigationController stack.

Try and close the UIAlertController first, then pop the current ViewController. In other words, remove the pop from the completion block and put it inside the OK block. or use unwind segue before the alert.

Another possibility, is that you have an unused or identical duplicate in storyboard. Hence, if the unwinding operation is triggered by storyboard button, select this button and check the connectivity inspector and removed unwanted connections.

For example: the red x marked was unnecessary in my case.

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