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copied!<p>The following recursive block code will compile and run using ARC, GC, or manual memory management, without crashing, leaking, or issuing warnings (analyzer or regular):</p> <pre><code>typedef void (^CountdownBlock)(int currentValue); - (CountdownBlock) makeRecursiveBlock { CountdownBlock aBlock; __block __unsafe_unretained CountdownBlock aBlock_recursive; aBlock_recursive = aBlock = [^(int currentValue) { if(currentValue >= 0) { NSLog(@"Current value = %d", currentValue); aBlock_recursive(currentValue-1); } } copy]; #if !__has_feature(objc_arc) [aBlock autorelease]; #endif return aBlock; } - (void) callRecursiveBlock { CountdownBlock aBlock = [self makeRecursiveBlock]; // You don't need to dispatch; I'm doing this to demonstrate // calling from beyond the current autorelease pool. dispatch_async(dispatch_get_main_queue(), ^ { aBlock(10); }); } </code></pre> <p>Important considerations:</p> <ul> <li>You must copy the block onto the heap manually or else it will try to access a nonexistent stack when you call it from another context (ARC usually does this for you, but not in all cases. Better to play it safe).</li> <li>You need TWO references: One to hold the strong reference to the block, and one to hold a weak reference for the recursive block to call (technically, this is only needed for ARC).</li> <li>You must use the __block qualifier so that the block doesn't capture the as-yet unassigned value of the block reference.</li> <li>If you're doing manual memory management, you'll need to autorelease the copied block yourself.</li> </ul>

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