Question

I have a method that updates two sections in a table that takes awhile. I want to do something like:

    dispatch_queue_t lowQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_LOW, 0);
    dispatch_queue_t mainQueue = dispatch_get_main_queue();

    dispatch_async(lowQueue, ^{
        NSArray *tempArray = // do long running task to get the data

        dispatch_async(mainQueue, ^{
            // update the main thread
            [self.activityIndicatorView stopAnimating];
            [self.reportsTableView reloadData];
        });
    });

dispatch_async(lowQueue, ^{

NSArray *tempArray2 = // same thing, do another long task

    // similarly, update the main thread

If I use the same lowQueue in the same method, is that ok? Thanks.

Answer 1

Yes, you can use lowQueue in the same method. When you grab the DISPATCH_QUEUE_PRIORITY_LOW global queue and store a reference to it in lowQueue, you can continue to enqueue additional blocks on it with multiple dispatch_async GCD calls. Every time you call dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_LOW, 0), you’ll get back a reference to the exact same dispatch queue.

Since all the global dispatch queues are concurrent queues, each block from both of your two tasks will be dequeued and executed simultaneously, provided that GCD determines this is most efficient for the system at runtime (given system load, CPU cores available, number of other threads currently executing, etc).