How to perform Callbacks in Objective-C

Question

How to perform call back functions in Objective-C?

I would just like to see some completed examples and I should understand it.

Answer 1

To keep this question up-to-date, iOS 5.0's introduction of ARC means this can be achieved using Blocks even more concisely:

@interface Robot: NSObject
+ (void)sayHi:(void(^)(NSString *))callback;
@end

@implementation Robot
+ (void)sayHi:(void(^)(NSString *))callback {
    // Return a message to the callback
    callback(@"Hello to you too!");
}
@end

[Robot sayHi:^(NSString *reply){
  NSLog(@"%@", reply);
}];

There's always F****ng Block Syntax if you ever forget Objective-C's Block syntax.