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    <p>The various links in the comments don't say "it's better not to do it." They say you can't do it. You must either pass the queue you want or dispatch to a known queue. Dispatch queues don't have the concept of "current." Blocks often feed from one queue to another (called "targeting"). By the time you're actually running, the "current" queue is not really meaningful, and relying on it can (and historically did) lead to dead-lock. <code>dispatch_get_current_queue()</code> was never meant for dispatching; it was a debugging method. That's why it was removed (since people treated it as if it meant something meaningful).</p> <p>If you need that kind of higher-level book-keeping, use an <code>NSOperationQueue</code> which tracks its original queue (and has a simpler queuing model that makes "original queue" much more meaningful).</p> <p>There are several approaches used in UIKit that are appropriate:</p> <ul> <li>Pass the call-back dispatch_queue as a parameter (this is probably the most common approach in new APIs). See <code>[NSURLConnection setDelegateQueue:]</code> or <code>addObserverForName:object:queue:usingBlock:</code> for examples. Notice that <code>NSURLConnection</code> expects an <code>NSOperationQueue</code>, not a <code>dispatch_queue</code>. Higher-level APIs and all that.</li> <li>Call back on whatever queue you're on and leave it up to the receiver to deal with it. This is how callbacks have traditionally worked.</li> <li>Demand that there be a runloop on the calling thread, and schedule your callbacks on the calling runloop. This is how <code>NSURLConnection</code> historically worked before queues.</li> <li>Always make your callbacks on one of the well-known queues (particularly the main queue) unless told otherwise. I don't know of anywhere that this is done in UIKit, but I've seen it commonly in app code, and is a very easy approach most of the time.</li> </ul>
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    1. COI have some gripes with your last statement: if this is a callback (say a completion, error or progress handler) then the *main queue* should be the least candidate to be selected as the default execution context. Using the main queue increases the probability for dead locks. This is a bad habit from library designers, stemming from the stupid and false assumption, that the client of the async task want's to do stuff on the main thread "anyway". But that makes it just worse.
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