Question

I usually read a word ‘delegate’ in the apple document and their library book. What exactly meaning about this word? Any special meaning in iPhone?

Thank you very much.

Answer 1

Delegates are a design pattern in object-oriented languages that allow an object to “call out” to unknown code to perform activities or calculations that that object cannot effectively do on its own. Let’s say you have a class Dog:

@protocol DogDelegate;

@interface Dog : Wolf <Domesticated>
- (void)bark;
- (void)tiltHeadAdorably;

- (void)playWithToy: (Toy *)aToy;
@property (readonly) Toy *favoriteChewToy;

@property (readwrite, assign) id <DogDelegate> delegate; // "DELEGATE" PROPERTY DECLARED HERE
@end

The delegate object is generally supplied by the code that instantiates Dog, and is called upon by that instance to do things that the dog itself can’t do. For instance, consider this interface of the DogDelegate protocol, which defines what the Dog‘s delegate object is expected to do:

@protocol DogDelegate <NSObject>
@required - (void)letDogOut: (Dog *)aDog;
@required - (void)letDogIn: (Dog *)aDog;
@optional - (void)scratchDog: (Dog *)aDog forTimeInterval: (NSTimeInterval)duration;
@end

In this case, an instance of DogDelegate is often the owner of the Dog (and, in Objective-C, a delegate often owns an object, so this lines up nicely with the metaphor.) The dog, when it needs to go out for… dog activities… will ask its delegate to perform the -letDogOut: method, placing the dog in the backyard. When done, it will ask its delegate to perform the -letDogIn: method, bringing the dog back inside. When the dog wants affection, if its delegate is able to, it will ask the delegate to scratch it for some period of time using -scratchDog:forTimeInterval:.