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copied!<p><strong>If your completion block is also called on the Main Thread, it might be difficult to achieve this</strong>, because before the completion block can execute, your method need to return. You should change implementation of the asynchronous method to:</p> <ol> <li>Be synchronous.<br> or</li> <li>Use other thread/queue for completion. Then you can use Dispatch Semaphores for waiting. You initialize a semaphore with value <code>0</code>, then call <code>wait</code> on main thread and <code>signal</code> in completion.</li> </ol> <p>In any case, <strong>blocking Main Thread is very bad idea</strong> in GUI applications, but that wasn't part of your question. Blocking Main Thread may be required in tests, in command-line tools, or other special cases. In that case, read further:</p> <hr> <h3>How to wait for Main Thread callback <em>on</em> the Main Thread:</h3> <p>There is a way to do it, but could have unexpected consequences. <em>Proceed with caution!</em></p> <p>Main Thread is special. It runs <code>+[NSRunLoop mainRunLoop]</code> which handles also <code>+[NSOperationQueue mainQueue]</code> and <code>dispatch_get_main_queue()</code>. All operations or blocks dispatched to these queues will be executed within the Main Run Loop. This means, that the methods may take <em>any</em> approach to scheduling the completion block, this should work in all those cases. Here it is:</p> <pre><code>__block BOOL isRunLoopNested = NO; __block BOOL isOperationCompleted = NO; NSLog(@"Start"); [self performOperationWithCompletionOnMainQueue:^{ NSLog(@"Completed!"); isOperationCompleted = YES; if (isRunLoopNested) { CFRunLoopStop(CFRunLoopGetCurrent()); // CFRunLoopRun() returns } }]; if ( ! isOperationCompleted) { isRunLoopNested = YES; NSLog(@"Waiting..."); CFRunLoopRun(); // Magic! isRunLoopNested = NO; } NSLog(@"Continue"); </code></pre> <p>Those two booleans are to ensure consistency in case of the block finished synchronously immediately.</p> <p>In case the <code>-performOperationWithCompletionOnMainQueue:</code> is <strong>asynchronous</strong>, the output would be:</p> <blockquote> <p>Start<br> Waiting...<br> Completed!<br> Continue </p> </blockquote> <p>In case the method is <strong>synchronous</strong>, the output would be:</p> <blockquote> <p>Start<br> Completed!<br> Continue </p> </blockquote> <p>What is the <em>Magic</em>? Calling <code>CFRunLoopRun()</code> doesn’t return immediately, but only when <code>CFRunLoopStop()</code> is called. This code <em>is</em> on Main RunLoop so running the Main RunLoop <em>again</em> will resume execution of all scheduled block, timers, sockets and so on.</p> <p><strong>Warning:</strong> The possible problem is, that all other scheduled timers and block will be executed in meantime. Also, if the completion block is never called, your code will never reach <code>Continue</code> log.</p> <p>You could wrap this logic in an object, that would make easier to use this pattern repeatedy:</p> <pre><code>@interface MYRunLoopSemaphore : NSObject - (BOOL)wait; - (BOOL)signal; @end </code></pre> <p>So the code would be simplified to this:</p> <pre><code>MYRunLoopSemaphore *semaphore = [MYRunLoopSemaphore new]; [self performOperationWithCompletionOnMainQueue:^{ [semaphore signal]; }]; [semaphore wait]; </code></pre>

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