On Apr 14, 2011, at 4:40 PM, Jason Harris wrote:
> On Apr 14, 2011, at 7:44 PM, David Duncan wrote:
>
>> The lifetime of the predicate needs to match that of the initialization done
>> by the block. If the block's initialization is done for an instance
>> variable, then the predicate should also be an instance variable. If the
>> initialization is for a global, the predicate should also be a global.
>
> So I just want to confirm this... Are you saying that one can do something
> like:
>
> @interface MyObject
> {
> dispatch_once_t initilizeComputedResource;
> ComputedResource* theResource;
> }
>
> - (ComputedResource*) resource;
> @end
>
> @implementation MyObject
> - (ComputedResource*) resource
> {
> dispatch_once(&initilizeComputedResource, ^{
> theResource = [self doSomeBigComputation];
> });
> return theResource;
> }
> @end
>
> and later I can call
>
> {
> MyObject* anObject = [[MyObject alloc]init];
> ComputedResource* r = [anObject resource];
> [r doSomething];
> }
>
> and its legal? (I have GC switched on)
Yes, that's legal and should work.
> Looking at
> http://developer.apple.com/library/ios/documentation/Performance/Reference/GCD_libdispatch_Ref/Reference/reference.html#//apple_ref/c/func/dispatch_once
> and the bit where it says:
>
> "The predicate must point to a variable stored in global or static scope. The
> result of using a predicate with automatic or dynamic storage is undefined."
>
> Seemed to me to indicate that it wasn't valid to have the kind of code I have
> above... I would like to use the code I have above so I can ensure something
> is run *once* for a given object. (Otherwise of course I just fall back on
> @synchronized but thats longer and not as fast...)
I think that note in the documentation is overzealous. The requirements are
that the predicate storage must exist _and be initialized to zero_ prior to any
call to dispatch_once using it and persist until after all calls to
dispatch_once using it have completed. Obviously, any calls to dispatch_once
which are meant to protect the same thing must use the same predicate storage.
An instance variable predicate protecting the initialization of another
instance variable satisfies those requirements in any sane scenario (where
you've ensured that no method of the instance is in-progress on any thread when
the instance is deallocated).
Regards,
Ken
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