Does dispatch_async(dispatch_get_main_queue(), ^{…}); wait until done?

Question

I have a scenario in my app, where I want to do some time consuming task which consists of some data processing as well as UI update, in a method. My method looks like this,

Answer 1

dispatch_queue_t queue = dispatch_queue_create("com.example.MyQueue", NULL);
dispatch_async(queue, ^{
  // Do some computation here.

  // Update UI after computation.
  dispatch_async(dispatch_get_main_queue(), ^{
    // Update the UI on the main thread.
  });
});