On Aug 26, 2011, at 11:40 PM, Brian Norh wrote:
> I'm working against an API which looks like this:
>
> - (void)performOperationWithBlock:(void (^)(void))block
>
> The call returns directly and the execution continues. At some point
> the block is eventually called. How would I do to block execution
> until the block have been called?
>
> [obj performOperationWithBlock:^(void) {
> // Do something here
> }];
> // Block here until the block above have been called.
If you can, put the work that you want to do after the block is called into the
block itself, perhaps dispatched to the main queue if necessary.
If you can't, use a synchronization primitive to wait. For example, you can
create a dispatch semaphore with an initial value of 0, signal it inside the
block, and wait on it after the block has been queued.
(dispatch_semaphore_create, dispatch_semaphore_signal, and
dispatch_semaphore_wait)
Similarly, you can create a dispatch group, call dispatch_group_enter() before
-performOperationWithBlock:, call dispatch_group_leave() within the block, then
call dispatch_group_wait() afterward.
NSConditionLock is another widely useful synchronization primitive with greater
flexibility.
Cheers,
Ken
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