I love this. Use a dispatch_semaphore_t to limit how many blocks you dispatch to a queue.

semaphore.m

dispatch_semaphore_t semaphore = dispatch_semaphore_create(1);


// Then in some loop ...
if(dispatch_semaphore_wait(semaphore, DISPATCH_TIME_NOW) == 0) {
    dispatch_async(queue, ^{
        // Some code that needs to run but doesn't need to fill up the queue.
        // 
        dispatch_semaphore_signal(semaphore);
    });
}

If you want to allow 2 blocks in the queue at a time, create the semaphore with (2)!