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    <p>All of these perform the same task, that is make the <code>doStuff</code> method on <code>anObject</code> execute synchronously on the <strong>current</strong> thread:</p> <pre><code>// 1 [anObject doStuff]; // 2 [anObject performSelector:@selector(doStuff)]; // 3 objc_msgSend(anObject, @selector(doStuff)); // 4 IMP imp = [anObject methodForSelector:@selector(doStuff)]; imp(anObject, @selector(doStuff)); </code></pre> <ol> <li>Is how you normally should go about to do stuff.</li> <li>Is for dynamically dispatching a message. Use if the selector is unknown, or provided by a client, for example if you implement an target-action pattern. or if the class of <code>anObject</code> is unknown, usually used by first asking if the object has the method with <code>-[NSObject respondsToSelector:]</code>.</li> <li>Is what no 1. is actually compiled down to. Usually never any real need to do this.</li> <li>Cached the actual <code>IMP</code> <em>(implementation)</em> for a method, and then call it directly. Can sometimes be faster than 1. if used in a tight loop. Just remember; <em>premature optimization is evil</em>.</li> </ol> <p>What you need to grasp is that in Objective-C methods are more important than classes/interfaces. Usually you do not query an object if it belongs to a particular class, or conforms to any protocol, that is for the compiler to complain about. At run-time you instead query for specific methods.</p> <p>In short: <strong>It does not matter what you are, just what you can do.</strong></p> <p>As a convenience <code>NSObject</code> also have several siblings to <code>performSelector</code> that are asynchronios. Most notably:</p> <ul> <li><code>performSelector:withObject:afterDelay:</code> - To execute the method on the current thread after a delay.</li> <li><code>performSelectorInBackground:withObject:</code> - To execute the method on a new background thread.</li> <li><code>performSelectorOnMainThread:withObject:waitUntilDone:</code> - To execute the method on the main thread.</li> <li><code>performSelector:onThread:withObject:waitUntilDone:</code> - To execute the method on any thread.</li> </ul> <p>The asynchronous performers all depend on a <code>NSRunLoop</code> to function. This is not something you need to worry about unless you spawn a thread yourself. If you do then you need to also run the new threads run loop. Just skip that for now.</p>
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    1. POWhat does the performSelector method do?
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    1. POWhat does the performSelector method do?
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    1. COYou can use the direct method call when you don't know the class of an object -- `id foo; if ([foo respondsToSelector:@selector(bar)]) [foo bar];` When you do need `performSelector:` is if you don't know the selector to call at compile-time (e.g. it's sent in from a different object, or it's user-selectable, etc).
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      1. USDaniel Dickison
    2. CO@Daniel Dickison: Good point, I have updated my answer.
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    3. COnice explanation thanks.
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