Synchronizing class methods in Objective-C | Hacker News Reader

Synchronizing class methods in Objective-C

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14 pointsMishraAnurag9 comments

I think it's easy to forget that Objective-C has simple syntax for synchronized code analogous to that of Java (although not quite as flexible as Java's - we can't make a whole method synchronized simply by adding the keyword to its declaration for example). A lot is made of Objective-C's syntactical shortcomings, but little of its wins.

So, what is self inside a class method? I was under the impression it was the Class?

self in that context would be the class. The issue he was facing is that in subclasses, "self", would refer to a different class and therefore the lock wouldn't apply. What he's pointing out is that he resolved that issue by specifying the superclass explicitly.

Personally, if I had to ensure synchronous calls to a class variable, I'd prefer to just restrict calls like this to a specific queue using dispatch_sync().

> I was under the impression it was the Class?

It is the class. But it's not the class in which the method is defined it's the class from which it is called.

So in one case it's the superclass and in the other case it's the subclass.

Yes in class methods self is the Class object, so the Class is used as the locking token:

> The object passed to the @synchronized directive is a unique identifier used to distinguish the protected block

Why do you need to synchronize class methods? Global state mutation?

Lazily instantiating a shared object is the obvious reason. Doesn't need to be mutable.

dispatch_once(3)[0] can be used for lazy initialization without concern for the calling class and without synchronizing on a shared object. This is (likely) cheaper in most cases then entering a @synchronized block.

0 - https://developer.apple.com/library/mac/documentation/Darwin...

Yes. In general, either GCD or lower level atomic ops are a better choice for thread-safety than @synchronized blocks. Just pointing out that there are reasons other than global mutable state for wanting a critical section in a class method.

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