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    <p>As Jacob points out, while they may appear the same, they are different things. In fact, there's a significant difference in the way that they handle sending actions to the main thread if you're already running on the main thread. </p> <p>I ran into this recently, where I had a common method that sometimes was run from something on the main thread, sometimes not. In order to protect certain UI updates, I had been using <code>-performSelectorOnMainThread:</code> for them with no problems.</p> <p>When I switched over to using <code>dispatch_sync</code> on the main queue, the application would deadlock whenever this method was run on the main queue. Reading the documentation on <a href="http://developer.apple.com/library/mac/documentation/Performance/Reference/GCD_libdispatch_Ref/Reference/reference.html#//apple_ref/c/func/dispatch_sync" rel="noreferrer"><code>dispatch_sync</code></a>, we see:</p> <blockquote> <p>Calling this function and targeting the current queue results in deadlock.</p> </blockquote> <p>where for <a href="http://developer.apple.com/library/mac/documentation/Cocoa/Reference/Foundation/Classes/NSObject_Class/Reference/Reference.html#//apple_ref/occ/instm/NSObject/performSelectorOnMainThread:withObject:waitUntilDone:" rel="noreferrer"><code>-performSelectorOnMainThread:</code></a> we see </p> <blockquote> <p><em>wait</em></p> <p>A Boolean that specifies whether the current thread blocks until after the specified selector is performed on the receiver on the main thread. Specify YES to block this thread; otherwise, specify NO to have this method return immediately.</p> <p>If the current thread is also the main thread, and you specify YES for this parameter, the message is delivered and processed immediately.</p> </blockquote> <p>I still prefer the elegance of GCD, the better compile-time checking it provides, and its greater flexibility regarding arguments, etc., so I made this little helper function to prevent deadlocks:</p> <pre><code>void runOnMainQueueWithoutDeadlocking(void (^block)(void)) { if ([NSThread isMainThread]) { block(); } else { dispatch_sync(dispatch_get_main_queue(), block); } } </code></pre> <p><strong>Update:</strong> In response to Dave Dribin pointing out the <a href="http://developer.apple.com/library/mac/#documentation/Darwin/Reference/ManPages/man3/dispatch_get_current_queue.3.html" rel="noreferrer">caveats section on<code>dispatch_get_current_queue()</code></a>, I've changed to using <code>[NSThread isMainThread]</code> in the above code.</p> <p>I then use </p> <pre><code>runOnMainQueueWithoutDeadlocking(^{ //Do stuff }); </code></pre> <p>to perform the actions I need to secure on the main thread, without worrying about what thread the original method was executed on.</p>
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    1. POGrand Central Dispatch (GCD) vs. performSelector - need a better explanation
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    1. CO@Joe - `dispatch_sync()` to the main thread could still cause a deadlock within your application if something on the main thread was blocked waiting on a value from your background thread (via `dispatch_sync()` or something else). This is very unlikely, but still possible. It's the standard threading problem of two threads waiting for the other to do something, so nothing gets done.
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    2. CO@Joe - Yes, if you're running entirely on a non-main queue or thread, you don't need the code I have above. The deadlock issue I mentioned just now will be a problem with this code, too. That's more of an architectural issue. As far as the comments go, see this Meta question: http://meta.stackexchange.com/questions/43019/how-do-comment-replies-work .
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    3. COPedantry overflow: You can use `dispatch_block_t` as the argument type instead the ugly `void (^block)(void)`.
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      1. USJacob Relkin
 

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