Better way to Trigger Asynchronous Callbacks in Objective-C

Question

I am looking for a better way to do this, if possible.

I have an asynchronous callback that updates a local sqlite database. I set a flag in a singleton variable (archiveUpdateComplete) when the update completes. I sleep in a do-while until the flag gets set to true, then I hydrate my tableview. Would like to remove sleep()! Thanks for any suggestions.

#define kBgQueue dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT,0)




- (void)viewDidLoad
{



  dispatch_async(kBgQueue, ^{

    //Hydrate word archive table view

       do {
             sleep(1.0);


           } while ([sharedManager archiveUpdateComplete]==NO);



      [self performSelectorOnMainThread:@selector(hydrateWordArchive) withObject:nil waitUntilDone:YES];


      //Run custom activity indicator
      dispatch_async(dispatch_get_main_queue(), ^{
        [MBProgressHUD hideHUDForView:self.view animated:YES];

      });



  });

}

Answer 1

If you need to poll

Polling/sleeping is rarely necessary or good. As an alternative:

• You can attach an NSTimer to the main thread's run loop.
• The selector the timer calls can test [sharedManager archiveUpdateComplete]
• if YES is returned, then
• • invalidate the timer
• • call [MBProgressHUD hideHUDForView:self.view animated:YES];

If you don't need to poll

There are a few immediate alternatives. Which you choose depends on what knows about what:

• If your manager knows who to message following completion, then the manager can simply message it. If that must occur on the main thread you can use -[NSObject performSelectorOnMainThread:withObject:waitUntilDone:] to forward to the main thread. You may also see this approach with delegates. In the case of a singleton, it doesn't make a lot of sense to take this route.

• If your manager does not know who is interested in the change/completion, your manager can post a NSNotification after the task has finished (on the current thread or from the main thread).

• Key Value Observing (KVO) is another option.