On Aug 31, 2012, at 1:08 AM, Andreas Grosam wrote:
> I have a number of operations that will be received serially. Each operation
> can be processed concurrently with respect to each other. I would like to
> invoke the completion blocks for each operation in the order as they have
> been received.
If you have operations A, B, C, D where which should execute concurrently, but
their results used serially then:
dispatch_queue_t serialQueue = …;
dispatch_queue_t concurrentQueue = (global queue, or a new concurrent queue);
dispatch_semaphore_t semaphoreA = dispatch_semaphore_create(0);
__block id resultsOfA = nil;
wrapA = ^{
resultsOfA = runA();
dispatch_semaphore_signal(semaphoreA);
}
doneA =^{
dispatch_semaphore_wait(semaphoreA);
completionHandlerForA(resultsOfA);
}
// the completion handlers are pushed onto a serial queue
// but will not execute until the semaphore is signaled
// after the work is done
dipatch_async(doneA, searialQueue);
dipatch_async(doneB, searialQueue);
dipatch_async(doneC, searialQueue);
dipatch_async(doneD, searialQueue);
// The work fires off concurrently
dispatch_async(wrapA, concurrentQueue);
dispatch_async(wrapB, concurrentQueue);
dispatch_async(wrapC, concurrentQueue);
dispatch_async(wrapD, concurrentQueue);
> I'm searching a simple approach to achieve this, implemented using
> NSOperationQueue, NSOperation and dispatch lib.
You can do the above with NSOperationQueue using a serial queue and concurrent
queue, but you must use dependencies between the operations to enforce the
serial execution of the completion handlers. The standard behavior of a serial
NSOpQ is that only one thing may be executed at a time, but it *does not care
which order they were submitted in*. It's first *ready* first out, not first in
first out. So although you may have a dependency that the completion handler
operation come after the work operation, if B finishes before A, B's completion
handler will run first. To stop that you need to have a dependency that A
complete before B. The trouble with this is that it ends up with a chain of
retains through the dependencies properties (dependencies aren't dropped from
the operation even if they're met), so you need to manually remove it from
within the operation itself. It's messy.
--
Seth Willits
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