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    <blockquote> <p>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> </blockquote> <p>This is definitely not the case here. The documentation for <code>-loadAchievementsWithCompletionHandler:</code> explicitly warns that the completion handler might be called on a thread other than the one you started the load from.</p> <p>Apple's "Threading Programming Guide" classifies <code>NSMutableDictionary</code> among thread-unsafe classes, but qualifies this with, "In most cases, you can use these classes from any thread as long as you use them from only one thread at a time."</p> <p>So, if both apps are designed such that nothing will be working with the achievement cache till the worker task has finished updating it, then no synchronization would be necessary. This is the only way in which I can see the first example as being safe, and it's a tenuous safety.</p> <p>The latter example looks like it's relying on the atomic property support to make the switcheroo between the old cache and the new cache. This should be safe, provided all access to the property is via its accessors rather than direct ivar access. This is because the accessors are synchronized with respect to each other, so you do not risk seeing a half-set value. Also, the getter retains and autoreleases the returned value, so that code with the old version will be able to finish working with it without crashing because it was released in the middle of its work. A nonatomic getter simply returns the object directly, which means that it could be deallocated out from under your code if a new value were set for that property by another thread. Direct ivar access can run into the same problem.</p> <p>I would say the latter example is both correct and elegant, though perhaps a bit over-subtle without a comment explaining how crucial the atomicity of the property is.</p>
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    1. USJeremy W. Sherman
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    1. COthanks Jeremy. i was also questioning about block completion handlers in general. If documentation doesn't explicitly say where completion handler is called, is it safe to assume that it's not called in main thread? If it is specified to be called from main queue, then is it safe to do anything inside without worrying about thread safety (assuming all access are from main thread or compl. handler called on main queue).
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      1. USSean S Lee
    2. COIf it doesn't specify that your block will be called on the main queue/thread, then you have to take your own measures to ensure it is, if that's what you need. It might actually be called on the main thread, but at that point, that fact is an implementation detail, not part of the API contract, and might change without warning. If it is specified to be called from the main queue, then you can take full advantage of that.
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    3. COOne last question then - if I change first completion handler to use dispatch_async(dispatch_get_main_queue(), ^{ for (GKAchievement* achievement in achievements) [achievementsDictionary setObject: achievement forKey: achievement.identifier]; });) will this code be threadsafe as well? I remember reading something about this not being safe..
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