Using custom delegates in Objective-C

via: http://jonsterling.github.com/2009/08/01/using-custom-delegates-in-objective-c.html

On Stack Overflow there has been some interest in how to use the Delegate design pattern in Objective-C. Of course, the first step in any search should be to read Apple's documentation, but many people seem not to want to read the whole thing. Trust me, folks, it is worth it. But Apple's documentation on creating delegates doesn't use protocols, which are such an amazing and useful part of the Objective-C language.

That being said, I'd like to give a short demonstration of how to create a class with a delegate. For the purposes of this tutorial, we'll call our class JSTutorial. The interface to JSTutorial starts out as the following:

@interface JSTutorial: NSObject {
  NSString *title;
  NSString *body;
}


- (void)generateTutorial;


@property (nonatomic, retain) NSString *title;
@property (nonatomic, retain) NSString *body;


@end

view rawJSTutorial1.hThis Gist brought to you by GitHub.

Now, we need to modify this interface to include a delegate protocol:

@protocol JSTutorialDelegate;
@interface JSTutorial: NSObject {
  NSString *title;
  NSString *body;
  id <JSTutorialDelegate> delegate;
}


- (void)generateTutorial;


@property (nonatomic, retain) NSString *title;
@property (nonatomic, retain) NSString *body;
@property (nonatomic, assign) id <JSTutorialDelegate> delegate;


@end


@protocol JSTutorialDelegate <NSObject>
@optional
- (void)tutorialDidFinish:(JSTutorial *)tutorial;
@end

view rawJSTutorial2.hThis Gist brought to you by GitHub.

The implementation for JSTutorial might look like this:

@implementation JSTutorial
@synthesize title;
@synthesize body;
@synthesize delegate;


- (void)generateTutorial {
  // do something here?
  [[self delegate] tutorialDidFinish:self];
}


- (void)dealloc {
  [title release];
  [body release];
  [super dealloc];
}


@end

view rawJSTutorial.mThis Gist brought to you by GitHub.

Finally, the class that implements JSTutorialDelegate might have its interface declared as follows:

@interface SomeClass : SomeControllerClass <JSTutorialDelegate>
// ...
@end

view rawDelegate.hThis Gist brought to you by GitHub.

SomeClass should implement tutorialDidFinish:, but it is optional.

I hope that this has helped those who were struggling with the delegate design pattern.