How can I be notified when a dispatch_async task is complete?

I have a asynchronous task like so:

dispatch_async(dispatch_get_main_queue(), ^{
     myAsyncMethodsHere;
});

Is there a way to be notified when the background task is complete?

Or to call a method upon completion?

I've read through the documentation and have looked into dispatch_after, but it seems to be more designed to dispatch the method after a certain length of time.

Thanks for the help.