Detect when a presented view controller is dismissed

Let's say, I have an instance of a view controller class called VC2. In VC2, there is a "cancel" button that will dismiss itself. But I can't detect or receive any callback when the "cancel" button got trigger. VC2 is a black box.

A view controller (called VC1) will present VC2 using presentViewController:animated:completion: method.

What options does VC1 have to detect when VC2 was dismissed?

Edit: From the comment of @rory mckinnel and answer of @NicolasMiari, I tried the following:

In VC2:

-(void)cancelButton:(id)sender
{
    [self dismissViewControllerAnimated:YES completion:^{

    }];
//    [super dismissViewControllerAnimated:YES completion:^{
//        
//    }];
}

In VC1:

//-(void)dismissViewControllerAnimated:(BOOL)flag completion:(void (^)(void))completion
- (void)dismissViewControllerAnimated:(BOOL)flag
                           completion:(void (^ _Nullable)(void))completion
{
    NSLog(@"%s ", __PRETTY_FUNCTION__);
    [super dismissViewControllerAnimated:flag completion:completion];
//    [self dismissViewControllerAnimated:YES completion:^{
//        
//    }];
}

But the dismissViewControllerAnimated in the VC1 was not getting called.

Solution 1:

There is a special Boolean property inside UIViewController called isBeingDismissed that you can use for this purpose:

override func viewWillDisappear(_ animated: Bool) {
    super.viewWillDisappear(animated)
    if isBeingDismissed {
        // TODO: Do your stuff here.
    }
}

Solution 2:

According to the docs, the presenting controller is responsible for the actual dismiss. When the presented controller dismisses itself, it will ask the presenter to do it for it. So if you override dismissViewControllerAnimated in your VC1 controller I believe it will get called when you hit cancel on VC2. Detect the dismiss and then call the super classes version which will do the actual dismiss.

As found from discussion this does not seem to work. Rather than rely on the underlying mechanism, instead of calling dismissViewControllerAnimated:completion on VC2 itself, call dismissViewControllerAnimated:completion on self.presentingViewController in VC2. This will then call your override directly.

A better approach altogether would be to have VC2 provide a block which is called when the modal controller has completed.

So in VC2, provide a block property say with the name onDoneBlock.

In VC1 you present as follows:

• In VC1, create VC2

• Set the done handler for VC2 as: VC2.onDoneBlock={[VC2 dismissViewControllerAnimated:YES completion:nil]};

• Present the VC2 controller as normal using [self presentViewController:VC2 animated:YES completion:nil];

• In VC2, in the cancel target action call self.onDoneBlock();

The result is VC2 tells whoever raises it that it is done. You can extend the onDoneBlock to have arguments which indicate if the modal comleted, cancelled, succeeded etc....

Solution 3:

Use a Block Property

Declare in VC2

var onDoneBlock : ((Bool) -> Void)?

Setup in VC1

VC2.onDoneBlock = { result in
    // Do something
}

Call in VC2 when you're about to dismiss

onDoneBlock!(true)

Solution 4:

Both the presenting and presented view controller can call dismissViewController:animated: in order to dismiss the presented view controller.

The former option is (arguably) the "correct" one, design-wise: The same "parent" view controller is responsible for both presenting and dismissing the modal ("child") view controller.

However, the latter is more convenient: typically, the "dismiss" button is attached to the presented view controller's view, and it has said view controller set as its action target.

If you are adopting the former approach, you already know the line of code in your presenting view controller where the dismissal occurs: either run your code just after dismissViewControllerAnimated:completion:, or within the completion block.

If you are adopting the latter approach (presented view controller dismisses itself), keep in mind that calling dismissViewControllerAnimated:completion: from the presented view controller causes UIKit to in turn call that method on the presenting view controller:

Discussion

The presenting view controller is responsible for dismissing the view controller it presented. If you call this method on the presented view controller itself, UIKit asks the presenting view controller to handle the dismissal.

(source: UIViewController Class Reference)

So, in order to intercept such event, you could override that method in the presenting view controller:

override func dismiss(animated flag: Bool,
                         completion: (() -> Void)?) {
    super.dismiss(animated: flag, completion: completion)

    // Your custom code here...
}

Solution 5:

extension Foo: UIAdaptivePresentationControllerDelegate {
    func presentationControllerDidDismiss(_ presentationController: UIPresentationController) {
        //call whatever you want
    }
}

vc.presentationController?.delegate = foo