On Apr 14, 2011, at 7:44 PM, David Duncan wrote:
> On Apr 14, 2011, at 10:26 AM, WT wrote:
>
>> On Apr 14, 2011, at 2:09 PM, David Duncan wrote:
>>
>>> On Apr 14, 2011, at 10:02 AM, WT wrote:
>>>
>>>> I looked at dispatch_once() at one point, but I'm still confused by how it
>>>> works.
>>>
>>> dispatch_once uses a predicate (just a flag) to determine if it should run
>>> the block or not. If that flag is false, then it sets the flag to true and
>>> executes the block. If the flag is true, it does nothing. What makes
>>> dispatch_once useful over a simple if statement is that it ensures that if
>>> you execute dispatch_once concurrently from multiple threads that flag gets
>>> updated exactly once, and the block gets called exactly once.
>>
>> Thanks for explaining it, David. The bit that was throwing me off is the
>> predicate part. I understand now that all I need is to declare a static
>> variable of the appropriate type and pass a pointer to it to dispatch_once().
>
> 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)
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...)
Thanks,
Jason
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