Anonymous delegate implementation in Objective-C?

Question

Is it possible to declare anonymous implementations of things like Delegates in Objective-C. I think I have the terminology right, but here's a java example:

myClass.addListener(new FancyInterfaceListener({
    void onListenerInterestingAction(Action a){
        ....interesting stuff here
    }
});

So for example to handle an UIActionSheet call I have to declare another method in the same class, which seems a bit silly if I want to pass it data, because I'd have to store that data as a global variable. Here's an example of deleting something with a confirmation dialog asking you if your sure:

-(void)deleteItem:(int)indexToDelete{
    UIActionSheet *confirm = [[UIActionSheet alloc] initWithTitle:@"Delete Item?" delegate:self cancelButtonTitle:@"Cancel" destructiveButtonTitle:@"Delete" otherButtonTitles:nil];
    [confirm showInView:self.view];
    [confirm release];
}

and the UIActionSheetDelegate in the same class:

- (void)actionSheet:(UIActionSheet *)actionSheet clickedButtonAtIndex:(NSInteger)buttonIndex{
    if (buttonIndex == 0){
        [[Settings sharedSettings] removeItemAtIndex:/*need index variable here*/];
        [drinksTable reloadData];
    }
}

What I want to be able to do is declare it inline, just like I did in the java example at the top. Is this possible?

Answer 1

There is no way to do this in Objective-C currently. Apple has published some work on their efforts to add blocks (really more like lambda closures than anonymous classes) to the language. You would likely be able to do something similar to the anonymous delegate with those.

In the mean time, most Cocoa programmers add the delegate methods to a separate category on the delegate class. This helps to keep the code more organized. In the .m file for the class in your example, I would do something like this:

@interface MyClass (UIActionSheetDelegate)
- (void)actionSheet:(UIActionSheet*)actionSheet clickedButtonAtIndex:(NSInteger)buttonIndex;
@end

@implementation MyClass
//... normal stuff here
@end

@implementation MyClass (UIActionSheetDelegate)
- (void)actionSheet:(UIActionSheet *)actionSheet clickedButtonAtIndex:(NSInteger)buttonIndex{
    if (buttonIndex == 0){
        [[Settings sharedSettings] removeItemAtIndex:/*need index variable here*/];
        [drinksTable reloadData];
    }
}
@end

Xcode's method popup in the editor window will separate the category's declaration and implementation from the main class'.

Answer 2

Objective-C doesn't have a notion of anonymous classes like Java's, so you can't create a class "inline" like in the Java code.

Answer 3

I was looking for something different when I came across this but if you do a search for UIALERTVIEW+BLOCKS you will find several hits for doing inline UIALERTVIEWs. This is the one I've been using: ALERTVIEW w/blocks

Answer 4

I believe that anonymous classes can be implemented in Objective-C, but it will take a lot of NSProxy magic and IMP madness. This is one of my current projects.

Answer 5

How about a class implementing the delegate interface. On initialization it would take a block. In the delegate definition it calls this block.

This allows multiple UIActionSheets to exist simultaneously without having to compare on identity.