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    <p>IMHO, neither. </p> <p>The preferred way should be having an <em>object</em> which knowns <em>where</em> to execute its actions:</p> <pre><code>completion_block_t completionHandler = ^(id result) { ... }; AsyncOperation* op = [AsyncOperation alloc] initWithCompletion:completionHandler]; [op start]; // executes its actions on a private execution context </code></pre> <p>Then, one can wrap those AsyncOperation objects into a convenient method:</p> <pre><code>- (void) fetchUsersWithCompletion:(completion_block_t)completionHandler { NSDictionary* params = ...; self.currentOperation = [[HTTPOperation alloc] initWithParams:params completion:completionHandler]; [self.currentOperation start]; } </code></pre> <p>The client may only be interested in specifying <em>where</em> its completionHandler should be executed. The API may be enhanced as follows:</p> <pre><code>- (void) fetchUsersWithQueue:(NSOperationQueue*)handlerQueue withCompletion:(completion_block_t)completionHandler { NSDictionary* params = ...; self.currentOperation = [[HTTPOperation alloc] initWithParams:params completion:^(id result){ // As per the documentation of HTTPOperation, the handler will be executed // on an _unspecified_ execution context. // Ensure to execute the client's handler on the specified operation queue: [handlerQueue:addOperationWithBlock:^{ completionHandler(result); }]; }]; [self.currentOperation start]; } </code></pre> <p>The latter API can be used as this:</p> <pre><code>[self fetchUsersWithQueue:[NSOperation mainQueue] completion:^(id result){ self.users = result; [self.tableView reloadData]; }]; </code></pre>
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    1. POWhat is preferred: implement method with GCD inside and then just simple call, or implement method and then call it later with GCD?
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    1. COA little bit complicated for me at the moment, but thanks for your answer. Will return to it later :)
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    2. COWell, this is a general "idiom" and basically results in the code what @Wain has indicated and suggested. ;)
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    3. CO@CouchDeveloper is underscoring an important point; when implementing anything concurrent, you have to *think very carefully about your architecture*. Concurrent programming is hard; not only is it hard to get it right, but it is equally as hard to make it fast.
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