Question

I’ve created a GCD queue like this:

dispatch_queue_t q = dispatch_queue_create("com.testcompany.myqueue", NULL);

When I dispatch tasks to that queue, it is way slower than simply executing the task on the main thread.

dispatch_async(q, ^(void) {
    [self performHeavyCalculationAndUpdateUI];
});

My suspicion is that the queue has a very low priority by default.
How can I change the priority of this queue? Or is there something else I must do?

Answer 1

If you’re doing UIKit stuff, in your block that’s running on the secondary queue, dispatch the UI updates back to the main queue from within the secondary queue via:

dispatch_async(q, ^(void) {
    [self performHeavyCalculationAndUpdateUI];

    dispatch_async(dispatch_get_main_queue(), ^{
        // do my ui update
    });
});

Just dispatch the few lines that actually update the UI back to the main queue and you’ll see it’s not a matter of queue priorities, but rather just making sure that UI updates happen in main queue.