On Thu, Jan 14, 2010 at 3:15 PM, Jamie Hardt <jamieha...@gmail.com> wrote:
> On Jan 14, 2010, at 12:07 PM, Nick Zitzmann wrote:
>
>> Sorry, forgot that block operations, unlike invocation operations, don't
>> take arguments. (Guess which one I use more often?) Yes, I'd try what you're
>> proposing.
>
> It was a good though, but doing something like this...
>
> NSBlockOperation *b = [NSBlockOperation blockOperationWithBlock:^{
>
> /* do stuff */
>
> if ([b isCancelled]) {
>
> /* clean up and rejoin */
>
> }
> }];
>
> [queue addOperation:b];
>
> ...fails horribly. The debugger looks strange, I can't make sense of it.
Despite how it may look, the order of operations screws you up.
The block gets created first, and only after that happens does the
assignment to b occur. But creating the block copies the value of b.
Because that copy happens before initialization, you get the old,
uninitialized, garbage value of b.
It's easy to fix, though. Just make it so that the block can follow
changes to the value of b that happen after the block is created, by
declaring b with the __block qualifier.
Mike
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