Background Fetch and asynchronous UICollectionView - ios

Question

I'm trying to implement the iOS Background Fetch API in an app. The app downloads JSON from a server, calls -[UICollectionView reloadData], and for every cell, and image is downloaded asynchronously in -collectionView:cellForItemAtIndexPath:.
In my initial implementation, I would call the completion handler passed by the system into application:performFetchWithCompletionHandler after I called reloadData. The app snapshot in the multitasking view would then display empty cells, because the images wouldn't have been downloaded yet. To solve that, I removed the completion handler call after reloadData, wrote a little structure keeping track of which cells' images have been downloaded, and only after a certain number have been downloaded, I would call the completion handler.
I did this using a completionBlock property on the view controller that reloads the images. The app delegate sets that property, then calls a reload method on that view controller, which then calls its completion handler property. I looks like this:
- (void)application:(UIApplication *)application performFetchWithCompletionHandler:(void (^)(UIBackgroundFetchResult))completionHandler
{
UINavigationController *navController = (UINavigationController *)self.window.rootViewController;
WSViewController *viewController = (WSViewController *)navController.topViewController;
viewController.completionBlock = ^(BOOL success, BOOL newData) {
if (!success) {
completionHandler(UIBackgroundFetchResultFailed);
} else if (success) {
if (newData) {
completionHandler(UIBackgroundFetchResultNewData);
} else {
completionHandler(UIBackgroundFetchResultNoData);
}
}
};
[viewController reload];
}
During testing, I found that performing a background fetch in relatively rapid succession would cause the completion handler not to be called. That's easily explained, because the completionBlock property is overwritten and the old one won't get called.
So, as advised in WWDC 2013 Session 204 "What's New With Multitasking", I removed the property and decided to pass the completion handler all the way through my code. -reload is now -reloadWithCompletionBlock: etc.
But now I'm stuck on how to implement that in the App Delegate. The View Controller has a delegate, and one of its methods, - (void)didFinishDownloadingImages, is implemented by the App Delegate. That's the point where I want to call the completion handler. But I can't, since there is no way to get to the completion handler without storing it in a property, defeating why I was doing it this way in the first place.
Any thoughts on how to solve this?

Answer 1

use NSOperationQueue and move your blocks of code into a subclass of NSOperation -- this will help you handle situations like blocking, discarding multiple requests and otherwise handling all those types of situations.
So, rather than write inline blocks, I'd move to an operation queue which seems like some lifting, but actually makes this much easier to handle properly.