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Delegate: A person sent or authorized to represent others
Delegation

First, DELEGATE is problematic. It’s not a common word, and most people don’t know what it means, and beyond that it’s NOT CLEAR what it means in a software context. It’s a bad metaphor.

The use of the word DELEGATE is different than in C#. A delegate in C# is just a function pointer. A DELEGATE in Objective-C is an OBJECT that implements a bunch of functions (very similar to overriding methods in inheritance), BUT THESE FUNCTIONS AREN’T OVERRIDING FUNCTIONS IN A BASE CLASS. Instead, they are implementing A “PROTOCOL” (not a C++ programming concept, its an objective-c concept). OC PROTOCOLS are like C++ INTERFACES (abstract base classes), only weirder. Delegate classes contain just a bunch of stubs of functionality that would get called by the default Apple UIApplication object, if that object respondsToSelector, for example.

Some Foundation Framework classes are like big assholes, like UIApplication for example. They go about their business in a VERY routine way, and WHEN THEY FEEL LIKE IT, they “delegate” little tasks out to YOUR DELEGATE OBJECT. When you program with Foundation classes, you provide that itty-bitty, gopher-boy, go-do-what-I-said, shoe-shine, lackey-boy, I-have-no-money-please-help-me-sir object (called a DELEGATE). The big king UIApplication Foundation class WILL CALL YOUR DELEGATE WHEN THE TIME COMES.

So your little shitty delegate just implements willDoStep3 here, and arguments may be passed by Framework object. UIApplication has a bunch of points at which it “needs” a delegate to call: applicationDidFinishLaunching, applicationWillResignActive, etc.

So app delegation is like fake inheritance. In normal inheritance, you would OVERRIDE applicationDidFinishLaunching, and provide your own method stub. In Objective C, you “provide a delegate object that implements a protocol”. It is conceptually almost the same thing.

I don’t understand why Apple decided to invent this new language and concepts of protocols and delegates, when C++’s concepts of interfaces and inheritance do just fine.


As an example of implementing a delegate protocol, let’s implement GKMatchDelegate. All the information you need is in the documentation there.

// SFMatchDelegate : inherits from NSObject< Implements GKMatchDelegate protocol >
@interface SFMatchDelegate : NSObject< GKMatchDelegate >
{
}

// any @property here

@end



// Usually in another file:
@implementation SFMatchDelegate

//any @synthesize statements

// "override"/implement protocol "Tasks"

//Receiving Data from Other Players
- (void)match:(GKMatch *)match didReceiveData:(NSData *)data fromPlayer:(NSString *)playerID
{
}

//Receiving State Notifications About Other Players
- (void)match:(GKMatch *)match player:(NSString *)playerID didChangeState:(GKPlayerConnectionState)state
{
}

//Handling Errors
- (void)match:(GKMatch *)match didFailWithError:(NSError *)error
{
}

//Reinviting a Player
- (BOOL)match:(GKMatch *)match shouldReinvitePlayer:(NSString *)playerID
{
  return YES ;
}

@end

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