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POLearning NSBlockOperation
 

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    <p>I'm a big fan of blocks, but have not used them for concurrency. After some googling, I pieced together this idea to hide everything I learned in one place. The goal is to execute a block in the background, and when it's finished, execute another block (like UIView animation)...</p> <pre><code>- (NSOperation *)executeBlock:(void (^)(void))block completion:(void (^)(BOOL finished))completion { NSOperation *blockOperation = [NSBlockOperation blockOperationWithBlock:block]; NSOperation *completionOperation = [NSBlockOperation blockOperationWithBlock:^{ completion(blockOperation.isFinished); }]; [completionOperation addDependency:blockOperation]; [[NSOperationQueue mainQueue] addOperation:completionOperation]; NSOperationQueue *backgroundOperationQueue = [[NSOperationQueue alloc] init]; [backgroundOperationQueue addOperation:blockOperation]; return blockOperation; } - (void)testIt { NSMutableString *string = [NSMutableString stringWithString:@"tea"]; NSString *otherString = @"for"; NSOperation *operation = [self executeBlock:^{ NSString *yetAnother = @"two"; [string appendFormat:@" %@ %@", otherString, yetAnother]; } completion:^(BOOL finished) { // this logs "tea for two" NSLog(@"%@", string); }]; NSLog(@"keep this operation so we can cancel it: %@", operation); } </code></pre> <p>My questions are:</p> <ol> <li>It works when I run it, but am I missing anything ... hidden land mine? I haven't tested cancellation (because I haven't invented a long operation), but does that look like it will work?</li> <li>I'm concerned that I need to qualify my declaration of backgroundOperation so that I can refer to it in the completion block. The compiler doesn't complain, but is there a retain cycle lurking there?</li> <li>If the "string" were an ivar, what would happen if I key-value observed it while the block was running? Or setup a timer on the main thread and periodically logged it? Would I be able to see progress? Would I declare it atomic?</li> <li>If this works as I expect, then it seems like a good way to hide all the details and get concurrency. Why didn't Apple write this for me? Am I missing something important?</li> </ol> <p>Thanks.</p>
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    1. COHave you considered using GCD? Or is this purely a learning exercise? A serial queue sounds exactly like what you are looking for.
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