What's the difference between the “global queue” and the “main queue” in GCD?

Question

Among some other ways, there are these two ways to get queues in GCD:

dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);

dispatch_g         


        

Answer 1

Yes. You can run code like this on a device to test it:

dispatch_async(
    dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
            NSLog(@"Block 1a");
            NSAssert(![NSThread isMainThread], @"Wrong thread!");
            NSLog(@"Block 1b");
        });
dispatch_async(
    dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
            dispatch_async(dispatch_get_main_queue(), ^{
                    NSLog(@"Block 2a");
                    NSAssert([NSThread isMainThread], @"Wrong thread!");
                    NSLog(@"Block 2b");
                });
        });