better way of using GCD for threading - ios

Question

Ok. So i am trying to use GCD To Handle all the heavy loading before transition to next view controller. I am opening large archive Files and extracting them which takes some time.
The entire Process is like this:
Click a UICollectionViewCell>Display activity indicator>Let GCD take care of heavy loading>call transition selector using performSelector: onThread:.....
The problem is when i use mainThread, the transition occurs too fast and all the heaving loading don't come to effect until after some time and the transition looks awful and while using currentThread, well it just takes so much time, it seems plain awful of a app.
-(void)someMethod
{
//activity Indicator before transition begins
UIActivityIndicatorView *activity=[[UIActivityIndicatorView alloc] initWithActivityIndicatorStyle:UIActivityIndicatorViewStyleGray];
[activity setFrame:self.view.bounds];
[self.view addSubview:activity];
[self.view bringSubviewToFront:activity];
activity.hidesWhenStopped=YES;
[activity startAnimating];
dispatch_queue_t transitionQueue;
transitionQueue = dispatch_queue_create("com.app.transitionQueue", NULL);
dispatch_async(transitionQueue,^{
//heavy lifting code
viewerPVC=.....
dispatch_async(dispatch_get_main_queue(),^{
[activity stopAnimating];
[self transitionToMangaViewer:mReaderPVC];
});
};
}
-(void)transitionToViewer:(ViewerPVC*)viewerPVC
{
[self.navigationController pushViewController:mReaderPVC animated:YES];
}
So Tried The First Suggestion, but the transition still seems buggy since the CollectionViewController still remain On Background For Some Time after the transition

Answer 1

you shouldnt need to use NSThread when you are using gcd, try something like this instead
dispatch_async(transitionQueue,^{
//heavy lifting code
viewerPVC=..... //this should block here otherwise will not work
dispatch_async(dispatch_get_main_queue(), ^{
[activity stopAnimating];
[self transitionToAnotherViewer:viewerPVC];
});
});

Answer 2

UI Update should be done on Main Thread, no need to create New Thread for performing UI Transition. Try below code:
dispatch_async(transitionQueue,^{
//heavy lifting code
viewerPVC=.....
dispatch_async(dispatch_get_main_queue(), ^{
[activity stopAnimating];
[self performSelector:#selector(transitionToAnotherViewer:) withObject:viewerPVC waitUntilDone:YES];
});
};