If you’re not new to Objective-C, then I’m pretty sure you know about delegate. Normally we use it when the outerViewController that want to trigger some actions when user touch a button on innerViewController.

A typically example will be ListViewController & DetailViewController.

In delegate way:

ListViewController.m

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DetailViewController *controller = [[DetailViewController alloc] init];
controller.delegate = self;
[self presentViewController:controller animated:YES completion:nil];

...
- (void)detailDelegate:(DetailViewController *)controller
{
    // your actions here
}

DetailViewController.h

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...
@protocol DetailViewControllerDelegate <NSObject>
- (void)detailDelegate:(DetailViewController *)controller;
@end

@interface DetailViewController : UIViewController
@property (nonatomic, weak) id<DetailViewControllerDelegate> delegate;
...

So when user do anything with this DetailViewController

DetailViewController.m

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- (void)actionTouched:(UIButton *)sender
{
    [_delegate detailDelegate:self];
}

let say touched a button, then invoke the delegate method, so that the
ListViewController get notify from DetailViewController.

Alternative: Using block

DetailViewController.h

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- (void)doneSomething:(void (^)(void))actionHandler;

Create a method signature with an empty block handler.

DetailViewController.m

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@interface DetailViewController ()
@property (nonatomic, copy) void (^done)(void); // a property for the block
@end

...
- (void)actionTouched:(id)sender
{
    self.done();
}

- (void)doneSomething:(void (^)(void))actionHandler
{
    self.done = actionHandler;
}

When user touched on the button, then it invoke the done block, the doneSomething: is called by ListViewController

ListViewController.m

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DetailViewController *controller = [[DetailViewController alloc] init];
[controller doneSomething:^{
    // your actions will be invoked when user touch the button on DetailViewController
}];
[self presentViewController:controller animated:YES completion:nil];

References: