Performing UI updates on main thread synchronously from a concurrent queue

Question

As far as I have understood GCD UI operations should always be performed on the main thread/main queue asynchronously. But the following code seems to also work without any problem. Can someone please explain why ? I am passing 2 blocks synchronously to a dispatch_async. One block downloads an image and the other displays it on the view.

    dispatch_queue_t concurrentQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);

    dispatch_async(concurrentQueue, ^{
    __block UIImage *image = nil;

    dispatch_sync(concurrentQueue, ^{ 
    /* Download the image here */
    });

    dispatch_sync(dispatch_get_main_queue(), ^{
    /* Show the image to the user here on the main queue */
    }); 
});

Answer 1

The queue is important (it has to be the main queue) but whether the gcd calls are synchronous or asynchronous is irrelevant - that just affects how the rest of your code around the gcd calls is timed. Once a block is running on a queue it doesn't matter how it was scheduled.

Synchronous dispatch can simplify your code (since it won't return until the block is executed) but does come with the risk of locking if you end up waiting for things to finish.