Question

In C# you can create a delegate method, assign it to a variable or pass it into a method as if it were a variable. For example:

public delegate int Lookup(String s);
//...
public static int Evaluate(String exp, Lookup variableEvaluator)
{
//...
}

I heard that in C you can create a pointer to any method and then pass that pointer to a method.
Can anyone give me a simple example of doing that in Objective-C? Of course, I can create an object with a singe method and pass that object into a method. But I am curious if there is a way of doing that similar to that of C# or C.

Answer 1

Lots of ways.

One: the good. Use blocks (closures, lambda calculus, however you call it):

typedef void (^MyCallback)();

- (void)callTheCallback:(MyCallback)blockToInvoke
{
    blockToInvoke();
}

MyCallback cb = ^{
    NSLog(@"I was called! :D");
};
[self callTheCallback:cb];

Two: the bad. Grab a pointer to the method function itself and call that. (Warning: if you use this approach, I’ll sue you.)

- (void)callTheCallback:(IMP)funcPtrToCall withObject:(id)obj selector:(SEL)sel
{
     funcPtrToCall(obj, sel);
}

- (void)someCallbackMethod
{
    NSLog(@"I was called! :D");
}

IMP implemt = [[self class] instanceMethodForSelector:@selector(someCallbackMethod)];
[self callTheCallback:implemt withObject:self selector:@selector(someCallbackMethod)];

Three: the ugly. Use a delegate:

- (void)delegateMethodOfSomeObject:(SomeObject *)obj
{
    NSLog(@"I was called! :D");
}

SomeObject *obj = [[SomeObject alloc] init];
obj.delegate = self;
[obj makeThisObjectSomehowCallItsDelegateThatIsCurrentlySelf];