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    <p>You can assign a variable of type <code>id</code> to any object type. If you know that it must implement a given protocol you can assign it to a variable of that type within the method and invoke the method on that variable.</p> <p>As a design point, I would say that it is better to make the protocol explicit and externalise it to the caller so that the compiler can do type checking properly. Some parts of Apple's code are better than others at this: from what you say GameKit is very much at the unhelpful end of things.</p> <p>Defining a category is not what you want to do, because that tells the compiler that you will add the method to every NSObject.</p> <p>If you have this protocol:</p> <pre><code>@protocol YourProtocol &lt;NSObject&gt; - (UIViewController *)viewControllerWithIdentifier:(NSString *)identifier; @end </code></pre> <p>And define your method as:</p> <pre><code>- (void)requestViewControllerWithIdentifier:(NSString *)identifier fromObject:(id &lt;YourProtocol&gt;)object; </code></pre> <p>It will probably do what you want. It may not be strictly necessary in this case but it's usually a good idea to have your protocol extend the <code>NSObject</code> protocol (as above) so that you can call useful stuff like <code>-retain</code>, <code>-release</code>, <code>-autorelease</code>, and <code>-respondsToSelector</code>. The alternative of declaring the method as follows prevents the user from using an <code>NSProxy</code>-rooted object as the <code>object</code> parameter.</p> <pre><code>- (void)requestViewControllerWithIdentifier:(NSString *)identifier fromObject:(NSObject &lt;YourProtocol&gt; *)object; </code></pre> <p>If it's just for your own use and you aren't using proxy objects this can be quite convenient, but you should avoid it in public APIs.</p>
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    1. CODefining a category actually *will* add the method to every object, with the implementation you provide for the category.
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    2. COYou don't have to implement categories you define; I don't think Rits was suggesting that he would.
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