How to share a ManagedObjectContext when using UITabBarController with inner UINavigationControllers

Question

I have an architectural question. My App uses a TabBarController right in the application window. The ApplicationDelegate creates the managedObjectContext, although it actually doesn't need it.

Each ViewController in the TabBarController is a NavigationViewController. The first view controller for each NavigationController are my custom views. All is createde an linked via Interface Builder.

Now, how do I pass the managedObjectContext around the right way? Actually I need my views to load the data as soon as possible so that when the user chooses a tab or navigates through the NavigationControllers, the data is already there.

So my questions are:

1. How to I pass the context properly?
2. When should I fetch my data, i.e. in which method? "viewDidLoad" or "viewDidAppear"?

Thanks for all ideas!

Answer 1

You should generally stay away from getting shared objects from the app delegate. It makes it behave too much like a global variable, and that has a whole mess of problems associated with it. And singletons are just fancy global variables, so they should be avoided unless really necessary, too.

I would add a managedObjectContext property to each of your view controllers and assign that when you're creating them. That way, your view controllers don't have a tight linkage with the app delegate.

As for when to fetch the data, you should do it lazily. Core Data is really fast, so I would wait until viewWillAppear: to do your fetching. If you wait until viewDidAppear:, the view is already on the screen and there will be a flicker when the data loads. Do be aware, though, that viewWillAppear: is called every time your view will become visible (e.g. when the user taps the back button on the navigation bar, or a modal view controller is dismissed) so you might want to track whether you've already loaded the data and skip the loading on subsequent calls.