4 ms·

Synchronizing class methods in Objective-C

14 points by MishraAnurag 12y ago

WoodenChair 12y ago
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.
deleted 12y ago
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stickydink 12y ago
So, what is self inside a class method? I was under the impression it was the Class?
- wattson12 12y ago
  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
- marcomonteiro 12y ago
  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().
- masklinn 12y ago
  > 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.
tomlu 12y ago
Why do you need to synchronize class methods? Global state mutation?
- chrisdevereux 12y ago
  Lazily instantiating a shared object is the obvious reason. Doesn't need to be mutable.
  - jonhohle 12y ago
    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/Reference/Manpages/man3/dispatch_once.3.html https://developer.apple.com/library/mac/documentation/Darwin...
    - chrisdevereux 12y ago
      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.

14 points by MishraAnurag 12y ago

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.

deleted 12y ago

[deleted]

stickydink 12y ago

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

wattson12 12y ago
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
marcomonteiro 12y ago
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().
masklinn 12y ago
> 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.

tomlu 12y ago

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

chrisdevereux 12y ago
Lazily instantiating a shared object is the obvious reason. Doesn't need to be mutable.
- jonhohle 12y ago
  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/Reference/Manpages/man3/dispatch_once.3.html https://developer.apple.com/library/mac/documentation/Darwin...
  - chrisdevereux 12y ago
    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.