Question

I’ve read that an object can only have one delegate at once.
But is that really true?

Let’s say I make an object with a protocol and from that object I want to gather a lot of data from several other objects. I add every object that conforms to my protocol to an array. Then I just loop through it and call my methods on every delegate.

NSMutableArray *collectFromDelegates = [NSMutableArray alloc]init];

//in delegateArray I keep pointers to every delegate.
for(id delegate in delegateArray){
  [collectFromDelegates addObject:[delegate someProtocolMethod]];
}

Is this wrong?

Answer 1

That’s not really delegation.

Delegation is a simple and powerful pattern in which one object in a program acts on behalf of, or in coordination with, another object. The delegating object keeps a reference to the other object—the delegate—and at the appropriate time sends a message to it. The message informs the delegate of an event that the delegating object is about to handle or has just handled.

It doesn’t make much sense to have more than one object handle an event for you, since it has already been handled. The only reason I could see to have multiple delegates is that if the first fails to handle an event, it can be passed to the next, continuing until some object handles it.

In your example, the objects are acting as data sources. This makes more sense than multiple delegates, but could easily be implemented by having a single data source combine data from multiple objects, which means the object asking for the data doesn’t have to worry about how to combine it.

The other case where you would often want multiple objects is receiving notifications of an event. This is not delegation because the objects are not working for the object, just acting on something that happened to the object. This is better implemented using notifications or observing.