UICollectionView rendering and activityindicator - ios

Question

I loaded a UIView from a UIViewController. This UIView contains a (big) UICollectionView.
The transition from the first UIView to the second UIView is very slow: It seems that when the rendering of all collection's cells is done the second view can show up.
In the second UIView, I tried.
- (void)viewDidAppear:(BOOL)animated
{
[activityView stopAnimating];
NSLog(#"did appear %#",[NSDate date]);
}
- (void)viewWillAppear:(BOOL)animated
{
[super viewWillAppear:animated];
[activityView startAnimating];
NSLog(#"will appear %#",[NSDate date]);
}
In the NSLog, there is no time difference between the two events, and in fact the second UIView shows up in about 1 second after the event viewDidAppear.
At this point, I would start a UIActivityIndicator, as in the code. But the indicator is never shown.
Any hint?

Answer 1

Your problem here is that you're probably blocking the main thread by maybe doing some disk IO or network activity or heavy computations, and that is why you're experiencing this delay.
I'd recommend that you do all this on a secondary thread while showing a UIActivityIndicator. On the completion you can then hide the activity indicator and show the collection view.
EDIT:
N.B. There is probably a better way to go, but i'm not very familiar with collection views.
A really easy fix would be to keep a BOOL ivar in the view controller where you load the collection view. Call it shouldLoadData and set it to NO in your viewDidLoad method. Then all you need to do is to return 0 to your UICollectionViewDelegate methods numberOfSectionsInCollectionView: and collectionView:numberOfItemsInSection:.
Finally in your viewDidAppear method, you set shouldLoadData to YES and call reloadData on your collectionView. The tricky part at this point is to figure out a way to tell when the collection view finished reloading its data so that you can stop the activity indicator.
I found out that it is not even that tricky, reloadData just queues up on the main thread, so you can just queue another task on the main thread after you make the call to reloadData. Just do:
[self.collectionView reloadData];
dispatch_async(dispatch_get_main_queue(), ^{
[self.activity stopAnimating];
});
And you'll get the desired behaviour. You should be aware, however, that this would still block the main thread.
E.g. if you have a back button, it could not be pressed until the data is fully loaded (it could actually be pressed, but it would not have any visible effect until then).