Premature exit from dispatch_asycn , Grand Central Dispatch..

Question

Lets say i am running some code in dispatch async. .. is there a way to terminate the thread it creates before it completes? like when the user clicks cancel

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{

    //start doing something here.. break bofore it finishes? 


    dispatch_async(dispatch_get_main_queue(), ^{

    //main thread stuff..   

    });



});