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POPreferable design pattern for model with background networking
 

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    <p>I'm developing a model class that provides data parsed from the web. Of course, I'd like my application to be responsive and thus networking should be done on separate thread/queue.</p> <p>That leads to the question: <em>how should I design the @interface of my class</em>?</p> <p>The main requirements are:</p> <ul> <li>It should deliver data from Model to Viewcontroller :) ;</li> <li>It shouldn't block the main (UI) thread;</li> <li>It should be easy to understand and follow by other developers.</li> </ul> <p>From what I've learned from WWDC2012 video, "Building Concurrent User Interfaces on iOS" Apple recommends to move the concurrent code to the class itself that uses model.</p> <p>Let's say we have Posts class (Posts.h/.m) that should provide ViewController with latest posts in NSArray* format.</p> <p><strong>Option I -- concurrency is in class users</strong></p> <p>The class itself is not concurrent, but the users are:</p> <pre><code>//Posts.h: @interface Posts : NSObject - (NSArray*)getPostsForUser:(NSString*)user; @end //ViewController.m @implementation ViewController - (void)someFunctionThatUpdatesUI { //&lt;...&gt; NSOperationQueue *q = [[NSOperationQueue alloc] init]; [q addOperationWithBlock:^{ NSArray *currentPosts = [Posts shared] getPostsForUser:@"mike"; [[NSOperationQueue mainQueue] addOperationWithBlock:^{ //UI should be updated only on main thread [self updateUIWithPosts:currentPosts]; }]; }]; //&lt;...&gt; } </code></pre> <p>The main disadvantage of this approach is the necessity to repeat almost the same code in every ViewController. What if there are dozens of them?</p> <p><strong>Option II -- concurrency with completion handlers pattern</strong></p> <p>Second option that I currently use in my app is a completion handler pattern. As the completion handler is called only <em>after</em> some long networking is performed, it doesn't block the main thread:</p> <pre><code>//Posts.h: @interface Posts : NSObject - (NSError*)getPostsForUser:(NSString*)user withCompletionHandler:(void(^)(NSArray*))handler; @end @implementation Posts - (NSError*)getPostsForUser:(NSString*)user withCompletionHandler:(void(^)(NSArray*))handler { //&lt;...&gt; dispatch_async(dipatch_get_global_queue(0, 0), ^{ //Do some may-be-long networking stuff here, //parse, etc and put it into NSArray *result dispatch_async(dipatch_get_main_queue(), ^{ handler(result); }); }); //&lt;...&gt; } //ViewController.m - (void)someFunctionThatUpdatesUI { //&lt;...&gt; [Posts shared] getPostsForUser:@"mike" withCompletionHandler:^(NSArray* posts){ //updateUI with posts }]; } </code></pre> <p>From my point of view, this way is good but the @interface is rather complicated, the method names are long and (from my point of view) obfuscated.</p> <p><strong>Option III -- delegate pattern</strong></p> <p>Another option that I see is the delegate pattern. What bothers me is that only one ViewController may be the delegate, thus it leads to necessity to set every VC as delegate in - viewWillAppear, which is easy to forget.</p> <pre><code>//Posts.h: @protocol PostUpdate&lt;NSObject&gt; - (void)updateUIWithPosts:(NSArray*)posts FromUser:(NSString*)user; @end @interface Posts - (NSError*)updatePostsFromUser:(NSString*)user; @property(nonatomic, weak) id&lt;PostUpdate&gt; delegate; @end //ViewController.m: @implementation ViewController&lt;PostUpdate&gt; - (void)viewWillAppear { //&lt;...&gt; [Posts shared].delegate = self; } - (IBAction)getLatestsPostButtonPressed:(id)sender { [[Posts shared] updatePostsFromUser:@"mike"]; } // protocol PostUpdate - (void)updateUIWithPosts:(NSArray*)posts FromUser:(NSString*)user { //updating UI with posts } @end </code></pre> <p>So here are the questions:</p> <ul> <li>What else patterns fit the requirements to deliver data from Model to Controllers in non-blocking way?</li> <li>What option would you recommend based on your experience, practise or theoretical knowledge?</li> </ul>
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