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POModifying mutable object in completion handler
 

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    <p>I have a question about thread safety of the following code example from Apple (from GameKit programming guide)</p> <p>This is to load achievements from game center and save it locally:</p> <p>Step 1) Add a mutable dictionary property to your class that report achievements. This dictionary stores the collection of achievement objects.</p> <pre><code>@property(nonatomic, retain) NSMutableDictionary *achievementsDictionary; </code></pre> <p>Step 2) Initialize the achievements dictionary.</p> <pre><code>achievementsDictionary = [[NSMutableDictionary alloc] init]; </code></pre> <p>Step 3) Modify your code that loads loads achievement data to add the achievement objects to the dictionary.</p> <pre><code>{ [GKAchievement loadAchievementsWithCompletionHandler:^(NSArray *achievements, NSError *error) { if (error == nil) { for (GKAchievement* achievement in achievements) [achievementsDictionary setObject: achievement forKey: achievement.identifier]; } }]; </code></pre> <p>My question is as follows - achievementsDictionary object is being modified in the completion handler, without any locks of sort. Is this allowed because completion handlers are a block of work that will be guaranteed by iOS to be executed as unit on the main thread? And never run into thread safety issues?</p> <p>In another Apple sample code (GKTapper), this part is handled differently:</p> <pre><code>@property (retain) NSMutableDictionary* earnedAchievementCache; // note this is atomic </code></pre> <p>Then in the handler:</p> <pre><code>[GKAchievement loadAchievementsWithCompletionHandler: ^(NSArray *scores, NSError *error) { if(error == NULL) { NSMutableDictionary* tempCache= [NSMutableDictionary dictionaryWithCapacity: [scores count]]; for (GKAchievement* score in scores) { [tempCache setObject: score forKey: score.identifier]; } self.earnedAchievementCache= tempCache; } }]; </code></pre> <p>So why the different style, and is one way more correct than the other?</p>
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