How do I create delegates in Objective-C?

n Objective-C delegate is just an object (generally any object) that has been assigned as a delegate of another. There's no special process for creating them; you simply define a class that implements the delegate methods you're interested in. (Though with delegates that use a formal protocol, you must declare your delegate to implement that protocol; see below.)

For example, suppose you have an NSWindow. If you'd like to implement its delegate's windowDidMove:method, you could create a class like this:

@implementation MyClass
- (void)windowDidMove:(NSNotification*)notification { 
    // ... 
}
@end

Then you could create an instance of MyClass and assign it as the window's delegate:

MyClass *myDelegate = [[MyClass alloc] init];
[window setDelegate: myDelegate];

On the NSWindow side, it probably has code similar to this to see if the delegate responds to the windowDidMove: message using respondsToSelector: and send it if appropriate.

if([[self delegate] respondsToSelector:@selector(windowDidMove:)]) {
    [[self delegate] windowDidMove:notification];
}

To define your own delegates, you'll have to declare their methods somewhere. There are two basic approaches, discussed in the Apple Docs on protocols.:

1) An Informal Protocol

This can be done, as NSWindow does, in a category on NSObject. For example, continuing the example above, this is paraphrased from NSWindow.h:

@interface NSObject(NSWindowNotifications)
- (void)windowDidMove:(NSNotification *)notification;
// ... other methods here
@end

You would then use respondsToSelector:, as described above, when calling this method. Delegates simply implement this method, and they're done. This method is easy and commonly used for delegates.

2) A Formal Protocol

The other option is to declare a formal protocol. The declaration would look like this:

@protocol NSWindowNotifications
@optional
- (void)windowDidMove:(NSNotification *)notification;
// ... other methods here
@end

This is analogous to an interface or abstract base class, as it creates a special type for your delegate, NSWindowNotifications in this case. Delegate implementors would have to adopt this protocol:

@interface MyDelegate < NSWindowNotifications >
// ...
@end

And then implement the methods in the protocol. For methods declared in the protocol as @optional (like most delegate methods), you still need to check with respondsToSelector: before calling a particular method on it. Apple recommends this method, because it doesn't mess with NSObject and can provide better tool support.

 

 

 

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

http://stackoverflow.com/questions/626898/how-do-i-create-delegates-in-objective-c