FastElegantDelegation

© 2013 Yan Rabovik (@rabovik on twitter)

FastElegantDelegation solves 3 problems:

1. Elegant single delegation without annoying respondsToSelector: checks;
2. Implementing multi-delegate pattern in your own code;
3. Assigning multiple delegates for a single third-party source.

Single delegation

Delegation is often implemented like this:

@protocol SomeDelegateProtocol <NSObject>
@optional
-(void)someOptionalDelegateMethod;
// ...
@end

@interface MyClass : NSObject
@property (nonatomic,weak) id<SomeDelegateProtocol> delegate;
@end

@implementation MyClass
-(void)someMethod{
    if ([self.delegate respondsToSelector:@selector(someOptionalDelegateMethod)]) {
        [self.delegate someOptionalDelegateMethod];
    }
}
// ...
@end

With FastElegantDelegation you may get rid of annoying respondsToSelector: checks:

@implementation MyClass
fed_use_proxy_for_delegate // just add this macro
-(void)someMethod{
    [self.delegate someOptionalDelegateMethod];
}
// ...
@end

How it works

• fed_use_proxy_for_delegate macro synthesizes delegate and setDelegate methods;
• delegate returns a FEDProxy class that forwards messages to the real delegate. If delegate does not respond to an optional protocol method, proxy just does nothing like when the message is sent to nil.

Custom delegate names

You may use any name you like for the delegate property, but you should specify getter and setter names in the macro:

@interface MyClass : NSObject
@property (nonatomic,weak) id<SomeDelegateProtocol> myCoolDelegate;
@end

@implementation MyClass
fed_use_proxy_for_property(myCoolDelegate,setMyCoolDelegate)
// ...
@end

Return values

Delegate methods may return any value.

@implementation MyClass
fed_use_proxy_for_delegate
-(void)someMethod{
    BOOL returnValue = [self.delegate methodReturningBOOL];
}
// ...
@end

Safeness

If the property is declared as weak then delegate is not retained. When delegate deallocates, proxy is automatically deallocated too. You will never get into a situation when proxy returned by self.delegate is alive, but real delegate is already deallocated.

FEDProxy does not break required/optional paradigm. If you try to call required method that is not implemented in the delegate, you'll get an exception.

Strong delegates

If for some reason you need to declare delegate as strong, it is OK. FEDProxy will be strongly stored in the ivar and will retain real delegate.

@interface MyClass : NSObject
@property (nonatomic, strong) id<SomeDelegateProtocol> myStrongDelegate;
@end

Performance

FEDProxy uses fast message forwarding (via forwardingTargetForSelector:) and a cache to prevent multiple internal respondsToSelector: checks.

Multi-delegate pattern in your own code

With fed_synthesize_multi_delegates macro you may implement multi-delegate pattern in your own class so that the clients of your class may easily add and remove delegates.

Example:

@class MyClass;
@protocol MyClassDelegate <NSObject>
-(void)myClassDidStartSomeJob;
@end

@interface MyClass : NSObject
-(void)addDelegate:(id<MyClassDelegate>)delegate;
-(void)removeDelegate:(id<MyClassDelegate>)delegate;
@end

@implementation MyClass
fed_synthesize_multi_delegates(MyClassDelegate)
-(void)someJob{
    [self.delegates myClassDidStartSomeJob]; // will be sent to each delegate
}
@end

fed_synthesize_multi_delegates synthesizes addDelegate:, removeDelegate: and delegates methods.

Custom method names

You can name methods as you like. For example you may use listener name instead of delegate:

@class MyClass;
@protocol MyClassListener <NSObject>
-(void)myClassDidStartSomeJob;
@end

@interface MyClass : NSObject
-(void)addListener:(id<MyClassListener>)delegate;
-(void)removeListener:(id<MyClassListener>)delegate;
@end

@implementation MyClass
fed_synthesize_multiproxy(MyClassListener, addListener, removeListener, listeners)
-(void)someJob{
    [self.listeners myClassDidStartSomeJob];
}
@end

Return values

If a method returns a value the return value will be from the first delegate in the list that responds to the selector.

Multiple delegates for a single third-party source

FEDMultiProxy class is a NSProxy subclass that allows to add multiple delegates to a single third-party source.

For example, you may assign multiple delegates to a UIScrollView:

__typeof(self) __weak weakSelf = self;
FEDMultiProxy *multiProxy = [FEDMultiProxy proxyWithDelegates:@[firstDelegate, secondDelegate]
                                                     protocol:@protocol(UIScrollViewDelegate)
                                          retainedByDelegates:YES
                                                    onDealloc:^{
                                                        weakSelf.scrollView.delegate = nil;
                                                    }];
self.scrollView.delegate = multiProxy;

You do not need to keep a strong reference to FEDMultiProxy object. It is automatically retained by each delegate and will be deallocated when all delegates die. onDealloc block will be called at that moment and you may set targets delegate to nil there.

CocoaPods

Add FastElegantDelegation to your Podfile.

Requirements

• iOS 5.0+
• ARC
• MAObjCRuntime

License

MIT License.