Delegate

delegate.h

#import <UIKit/UIKit.h>

@class SomeViewController;

@protocol SomeViewControllerDelegate <NSObject>
-(void)delegateMethodName:(SomeViewController *)controller;
@end
  
@interface SomeViewController : UIViewController
@property (nonatomic, weak) id<SomeViewControllerDelegate> delegate;
@end

delegate.m

#import "delegate.h"

@implementation SomeViewController

- (void)someMethodToCallDelegate {
    if ([self.delegate respondsToSelector:@selector(delegateMethodName:)]) {
        [self.delegate delegateMethodName:self];
    }
}

@end

delegateUser.m

#import "delegate.h"

@interface SomeDelegateUser () <SomeViewControllerDelegate> @end

@implementation SomeDelegateUser

- (void)variousFoo {
   SomeViewController *controller = [[SomeViewController alloc] init];
   controller.delegate = self;
}

-(void)delegateMethodName:(SomeViewController *)controller {
   // handle the delegate being called here
}

@end