objective c - Delegation to multiple objects

Question

Is there any way to delegate to two objects at a time in Objective-C? I know that delegation pattern implies one response at a time and for multiple listeners and broadcasting there is notification center but notification does not return any value.

If I have a heavily network-based iOS project and need to delegate to multiple listeners and required to return values from them, in this scenario what approach should be the best?

Answer 1

In every class the delegate is one, so one delegate is informed about the event. But nothing forbids you to declare a class with a set of delegates.

Or use Observation instead. A class may be observed by multiple classes.

Example

As requested from the OP, since also some code would be useful, here is a way of doing it:

@interface YourClass()

@property (nonatomic, strong, readwrite) NSPointerArray* delegates;
// The user of the class shouldn't even know about this array
// It has to be initialized with the NSPointerFunctionsWeakMemory option so it doesn't retain objects

@end  

@implementation YourClass

@synthesize delegates;

...   // other methods, make sure to initialize the delegates set with alloc-initWithOptions:NSPointerFunctionsWeakMemory

- (void) addDelegate: (id<YourDelegateProtocol>) delegate
{
    [delegates addPointer: delegate];
}

- (void) removeDelegate: (id<YourDelegateProtocol>) delegate
{
    // Remove the pointer from the array
    for(int i=0; i<delegates.count; i++) {
        if(delegate == [delegates pointerAtIndex: i]) {
            [delegates removePointerAtIndex: i];
            break;
        }
    } // You may want to modify this code to throw an exception if no object is found inside the delegates array
}

@end

This is a very simple version, you can do it in another way. I don't suggest to make public the delegates set, you never know how it could be used, and you can get an inconsistent state, specially with multithreading. Also, when you add/remove a delegate you may need to run additional code, so that's why making the delegates set private.
You may also a lot of other methods like delegatesCount for example.

PS: The code has been edited to be a NSPointerArray instead of a NSMutableSet, because as stated in the comments a delegate should be held with a weak pointer to avoid retain cycles.