passing in a method in Objective C

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.

score 2 · Accepted Answer · 2012-10-12 20:48:13Z

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];

Chris Trahey · Accepted Answer · 2012-10-12 20:39:09Z

1

Two quick thoughts come to mind.

The short answer is called "blocks", but it's lower level than is probably recommended for what you need.

The "cleaner" solution (read: higher level) is to pass two params: and object (called "target") and a selector (called "action"). This is a very common pattern in Objective-C, so I'll only demonstrate this one. If you are interested in the blocks idea, check out this doc.

Essentially, the object should be passed as an id, and the selector as a SEL, for which we have the handy @selector() construct:

-(void) doThingWithTarget:(id) targetObj action:(SEL) actionSel {
  if([targetObj respondsToSelector:actionSel]) {
    [targetObj performSelector:actionSel withObject:self];
  }
}

// ...
[thatOtherObject doThingWithTarget:self action:@selector(myMethod:)];

// ... where

-(void) myMethod:(id) sender {
  // sender is the calling object, or should be by contract.
}

answered Oct 12, 2012 at 20:39

Chris Trahey

18.3k1 gold badge44 silver badges55 bronze badges

3 Comments

Rob Napier Over a year ago

Most target/selectors in Cocoa have been steadily moving towards blocks. Blocks simplify several inherent problems in target/selector (particularly under ARC). While target/selector is a venerable and useful approach, going back to the NeXT days, I do not generally recommend it for new code. For most problems, blocks are a better solution. The big advantage of target/selector is that they can be serialized (unlike blocks), and so are ideal for nib files.

user529758 Over a year ago

Target-action is by no means "higher-level". They're using the Objective-C runtime just like blocks do. In fact, blocks are a more elegant solution for having callbacks, event handlers et al...

Chris Trahey Over a year ago

@RobNapier I like the comment, thanks. Note that blocks have their own ARC implications going the other way (e.g. what does the block have a strong pointer to via it's closure-like mechanism?). That certainly doesn't preclude one from choosing them over target/action; and your comment alone prompts me to improve my disposition toward them (or to remember that I was growing more affectionate when I slowed the amount of Objective-C I was doing 2 months ago).

ahoff · Accepted Answer · 2012-10-12 20:37:52Z

0

Objective C uses selectors. http://developer.apple.com/library/ios/#documentation/cocoa/conceptual/objectivec/Chapters/ocSelectors.html

answered Oct 12, 2012 at 20:37

ahoff

1,37413 silver badges25 bronze badges

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