StackOverflow2013

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copied!<p>There's no direct language support, but you could accomplish something similar with message forwarding. Let's say you have trait classes "Foo" and "Bar", which define methods "<code>-doFoo</code>" and "<code>-doBar</code>", respectively. You could define your class to have traits, like this:</p> <pre><code>@interface MyClassWithTraits : NSObject { NSMutableArray *traits; } @property (retain) NSMutableArray* traits; -(void) addTrait:(NSObject*)traitObject; @end @implementation MyClassWithTraits @synthesize traits; -(id)init { if (self = [super init]) { self.traits = [NSMutableArray array]; } return self; } -(void) addTrait:(NSObject*)traitObject { [self.traits addObject:traitObject]; } /* Here's the meat - we can use message forwarding to re-send any messages that are unknown to MyClassWithTraits, if one of its trait objects does respond to it. */ -(NSMethodSignature*)methodSignatureForSelector:(SEL)aSelector { // If this is a selector we handle ourself, let super handle this if ([self respondsToSelector:aSelector]) return [super methodSignatureForSelector:aSelector]; // Look for a trait that handles it else for (NSObject *trait in self.traits) if ([trait respondsToSelector:aSelector]) return [trait methodSignatureForSelector:aSelector]; // Nothing was found return nil; } -(void) forwardInvocation:(NSInvocation*)anInvocation { for (NSObject *trait in self.traits) { if ([trait respondsToSelector:[anInvocation selector]]) { [anInvocation invokeWithTarget:trait]; return; } } // Nothing was found, so throw an exception [self doesNotRecognizeSelector:[anInvocation selector]]; } @end </code></pre> <p>Now, you can create instances of MyClassWithTraits, and add whatever "trait" objects you'd like:</p> <pre><code>MyClassWithTraits *widget = [[MyClassWithTraits alloc] init]; [widget addTrait:[[[Foo alloc] init] autorelease]]; [widget addTrait:[[[Bar alloc] init] autorelease]]; </code></pre> <p>You could make these calls to <code>-addTrait:</code> in MyClassWithTraits' <code>-init</code> method, if you want every instance of that class to have the same kind of traits. Or, you could do it like I've done here, which allows you to assign a different set of traits to each instance.</p> <p>And then you can call <code>-doFoo</code> and <code>-doBar</code> as if they were implemented by widget, even though the messages are being forwarded to one of its trait objects:</p> <pre><code>[widget doFoo]; [widget doBar]; </code></pre> <p>(<strong>Edit</strong>: Added error handling.)</p>

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