Running Tasks in Background Thread :

//Start an activity indicator here

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{

    //Call your function or whatever work that needs to be done
    //Code in this part is run on a background thread

    dispatch_async(dispatch_get_main_queue(), ^(void) {

        //Stop your activity indicator or anything else with the GUI
        //Code here is run on the main thread

    });
});