I love design patterns. When understood and used properly they can make your life as a developer so much easier. One of my favorites is the delegate pattern which is used all over the place in Cocoa Touch. Delegation is a design pattern which allows an object to “delegate” work to another helper object. This design pattern is great because it allows you to provide custom behavior without having to subclass.

But there are times when you need to subclass a class that has already implemented a delegate and add additional delegate methods. As a matter of fact, one of the most used classes in iOS does this… UITableView. We’ll take a look at UITableView and see how it adds delegate methods.

UITableView is a subclass of UIScrollView as you can see from its header file.

UIKIT_CLASS_AVAILABLE(2_0) @interface UITableView : UIScrollView <NSCoding>

If we look at the UIScrollView header we’ll see

UIKIT_CLASS_AVAILABLE(2_0) @interface UIScrollView : UIView <NSCoding> {
id           _delegate; // A delegate instance variable is declared of type id
}
// The delegate will be required to conform to the UIScrollViewDelegate protocol
@property(nonatomic,assign) id<UIScrollViewDelegate> delegate;
@end

You can see that the “delegate” instance variable should conform to the UIScrollViewDelegate protocol. So as a subclass, UITableView wants to also use the delegate pattern and has to add the additional methods it needs. It’s actually pretty simple to do so.

Step 1) Create your delegate protocol and extend the parent classes delegate protocol (in your .h)

// This is the syntax to extend another protocol
@protocol UITableViewDelegate<NSObject, UIScrollViewDelegate>

// Your delegate methods go here...

@optional
// Display customization
- (void)tableView:(UITableView *)tableView willDisplayCell:(UITableViewCell *)cell forRowAtIndexPath:(NSIndexPath *)indexPath;
@end

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

Step 3) Tell the compiler that the delegate property will be provided at runtime (by the superclass in our case) (in your .m)

@dynamic delegate;

That’s all there really is to it. Other classes can now conform to the delegate protocol, set themselves as the delegate, and receive delegate calls from UITableView and UIScrollView. So if you ever need to add delegate methods in a subclass, remember UITableView is a great example.

*You can learn more about design patterns in iOS in Apples documentation.