//interface file is SampleController.h

//in SampleController.h


@protocol SampleControllerDelegate;


@interface SampleController : UIViewController <> {
  id<SampleControllerDelegate> delegate;
}

@property(nonatomic,assign) id<SampleControllerDelegate> delegate;

@end

@protocol SampleControllerDelegate <NSObject>

- (void)testFunction:(SampleController*)controller;

@end


//in SampleController.m file

@synthesize delegate



// implement SampleControllerDelegate like this

- (void)yourCustomFunction:(id)sender {
  [delegate testFunction:self];
}

writing a custom delegate

Objective C