This is based very heavily on Alejandro Isaza's MultiDelegate.

MultiDelegate is a delegate multiplexing class for Objective-C. In other words, it will dispatch delegate methods to multiple objects, instead of being restricted to a single delegate object. You can also use it as a generic method dispatch mechanism. For more information see the blog post.

Using With Pre-existing Classes (UIScrollView, UITableView, etc.)

Suppose you have a UITableView and you want to implement the data source using two separate classes: one is the actual data source implementing the tableView:numberOfRowsInSection: method and the other one is the cell factory implementing the tableView:cellForRowAtIndexPath: method to construct the cells.

First create an ILABMultiDelegate instance. You need to keep a strong reference to this instance because most objects don't retain their delegates:

_multiDelegate = [ILABMultiDelegate delegateWithProtocol:@protocol(UITableViewDataSource)];

Then add all the actual delegates to the _multiDelegate object:

[_multiDelegate addDelegate:self];
[_multiDelegate addDelegate:_dataSource];

Finally set the table's data source as the delegate multiplexer:

self.tableView.dataSource = (id)_multiDelegate;

Using With Your Own Classes

If you are writing a class and you want to have multi-delegate support, it's pretty easy to add. Here is the header for a class that will have multi-delegate support:

#import "ILABMultiDelegate.h"

@protocol ILABYourClassDelegate;

@interface ILABYourClass : NSObject

@property (readonly) id<ILABMultiDelegateProtocol, ILABYourClassDelegate> delegate;

-(void)doSomething;

@end

@protocol ILABDelegateObjectDelegate<NSObject>

-(void)didSomething;

@end

You notice that our delegate property is readonly. That's because your delegate will actually be an instance of ILABMultiDelegate. Because it's marked with ILABMultiDelegateProtocol you can use the addDelegate: and insertDelegate: methods to actually add your delegates.

Your implementation would then look like this:

@implementation ILABYourClass

-(instancetype)init
{
    if ((self=[super init]))
    {
        _delegate=[ILABMultiDelegate delegateWithProtocol:@protocol(ILABDelegateObjectDelegate)];
    }
    
    return self;
}


-(void)doSomething
{
    [_delegate didSomething];
}

Here we are creating the instance of ILABMultiDelegate and assigning it to our _delegate ivar. Notice how we are passing the protocol of our delegate to the constructor? We do this to insure that any future delegates added conform to the protocol.

To add a delegate, your client class would:

ILABYourClass *myClass=[ILABYourClass new];
[myClass.delegate addDelegate:self];

Multiple Protocols

ILABMultiDelegate allows you to specify multiple protocols that it will handle. In strict mode, any delegate added to the ILABMultiDelegate instance must implement ALL of the protocols or it will raise an exception.

In non-strict mode, each delegate added can implement one or more of any of the protocols that the ILABMultiDelegate handles without raising an exception.

For example, let's say we have these three protocols defined:

@protocol ProtocolA <NSObject>

-(void)selector_1;

@end

@protocol ProtocolB <NSObject>

-(void)selector_2;

@end

@protocol ProtocolC <NSObject>

-(void)selector_3;

@end

Now we have two classes that will act as delegates, implementing these protocols:

@interface DelegateAB : NSObject<ProtocolA, ProtocolB>
@end


@interface DelegateC : NSObject<ProtocolC>
@end

Our class is defined:

@interface OurObject : NSObject

@property (readonly) id<ILABMultiDelegateProtocol, ProtocolA, ProtocolB, ProtocolC> delegate;

@end

Our initializer for the class:

@implementation OurObject

-(instancetype)init {
    if ((self = [super init])) {
        _delegate = [ILABMultiDelegate delegateWithProtocols:@[@protocol(ProtocolA),@protocol(ProtocolB),@protocol(ProtocolC)] strict:NO];
    }
    
    return self;
}
@end

In our initializer we are creating a new instance of the ILABMultiDelegate passing in the list of protocols a delegate should implement. Note that passing NO to strict: tells the ILABMultiDelegate that any delegate added only needs to conform to one or more of the protocols. If we had passed YES, then each delegate added would need to conform to ALL protocols.

Things of Note

• Every method invocation will be forwarded to each object in the list in the order they were added.
• If a method returns a value the return value will be from the first delegate that responded to the method. For example if object A implements method getInt by returning 1, object B implements getInt by returning 2 and object C doesn't implement getInt, calling getInt on an ILABMultiDelegate containing A, B and C (in that order) will return 1.
• ILABMultiDelegate doesn't keep strong references to the objects added to it.
• Some objects only call respondsToSelector: when you first set the delegate to improve performance, so make sure you add all your delegates to the ILABMultiDelegate before you set it as the delegate.

Installation

If you are using CocoaPods, add this to your Podfile:

pod 'ILABMultiDelegate'

Otherwise add ILABMultiDelegate.h/.m to your project.

Special Thanks

Of course, special thanks to Alejandro Isaza for writing MultiDelegate on which this is 80% lifted from.