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copied!<p>I prefer to use a generic and simplistic completion block for asynchronous <em>methods</em> when possible and when suitable as in your case. There is no special "failure" block - since the failure case will be handled by the call-site directly in the completion block. See example below:</p> <pre><code>typedef void (^completion_t)(id result); -(void) loginUser:(NSString *)username withPassword:(NSString *)password completion:(completion_t)completionHandler; </code></pre> <p>(see below for a better naming of the method)</p> <p>In the documentation you state:</p> <blockquote> <p>The result parameter returns @"OK" in case of success, otherwise a NSError object containing details about the error.</p> </blockquote> <p>(Of course, you may return something different, and more suitable as the result in case of success)</p> <p>Well, then you may use it as follows:</p> <pre><code>[foo loginUser:user withPassword:pass completion:^(id result){ if (![result isKindOfClass:[NSError class]]) { ... } else { NSLog(@"Error: %@", error); } }]; </code></pre> <hr> <p>There are countless other valid and reasonable signatures for a completion handler:</p> <pre><code>typedef void (^completion_t)(NSData* data, NSError* error); </code></pre> <p>This one, apparently, returns a NSData in case of success, and a NSError in case of a failure. The documentation <em>shall</em> specify the details.</p> <hr> <p>You may define different handlers (blocks) as <em>properties</em> when you have an <em>Operation</em> object which performs an <em>asynchronous</em> task (e.g., a subclass of <code>NSOperation</code>) Since your example deals with an asynchronous <em>method</em>, there's no reason to explain that here, unless you post a new question. ;)</p> <p>Edit:</p> <p>According the naming conventions, I would suggest the following:</p> <pre><code>-(void) loginWithUsername:(NSString *)username password:(NSString *)password completion:(completion_t)completionHandler; </code></pre>

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