Question

I have several threads and once they are all finished working, I need to call a myMergeBlock method exactly once per action. I can’t use dispatch_once because I want to be able to call myMergeBlock at a later time.

Some pseudo code looks like this but is not yet thread safe:

BOOL worker1Finished, worker2Finished, worker3Finished;

void (^mergeBlock)(void) = ^{
    if (worker1Finished && worker2Finished && worker3Finished)
        dispatch_async(queue, myMergeBlock);    // Must dispatch this only once
}

void (^worker1)(void) = ^{
    ...
    worker1Finished = YES;
    mergeBlock();
}

void (^worker2)(void) = ^{
    ...
    worker2Finished = YES;
    mergeBlock();
}

void (^worker3)(void) = ^{
    ...
    worker3Finished = YES;
    mergeBlock();
}

Also, based on the way the workers are called, I do not call them directly, but instead pass them into a function as arguments.

Answer 1

You want to use dispatch groups. First you create a group, schedule the three workers in the group, then add a notification block to the group.
It should look something like this:

//create dispatch group
dispatch_group_t myWorkGroup = dispatch_group_create();

//get one of the global concurrent queues
dispatch_queue_t myQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, NULL);

//submit your work blocks
dispatch_group_async(myWorkGroup, myQueue, worker1);
dispatch_group_async(myWorkGroup, myQueue, worker2);
dispatch_group_async(myWorkGroup, myQueue, worker3);

//set the mergeBlock to be submitted when all the blocks in the group are completed
dispatch_group_notify(myWorkGroup, myQueue, mergeBlock);

//release the group as you no longer need it
dispatch_release(myWorkGroup);

You could hang on the the group and reuse it later if you prefer. Be sure to schedule the work before the notification. If you try to schedule the notification first it will be dispatched immediately.

I haven’t tested this code but I do use dispatch_groups in my projects.