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 questi开发者_开发百科ons are:

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

Thanks for all ideas!

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.

I've ran into this same problem, i'll share my solution.

First you need a reference to the Nav Controller in the Tab Bar in the nib file, make sure you connect it up.

IBOutlet UINavigationController *navigationController;

Then, get the Controller as recommended in the support docs and send it the managedObjectContext:

SavedTableViewController *saved = (SavedTableViewController *)[navigationController topViewController];
saved.managedObjectContext = self.managedObjectContext;

Alex is right, "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."

You can get it from the app delegate at any time like this:

myApp *d = [[UIApplication sharedApplication] delegate];
NSManagedObjectContext *managedObjectContext = d.managedObjectContext;

Or variations of the above. Other than that you can add a property to all your viewcontrollers and pass it around or you can create a singleton and reference that globally.

Swift

You should not share a NSManagedObjectContext, but you can share the NSPersistentStoreCoordinator.

Thus, you can create a new managed object context for each view, each sharing the same store. It is the preferred method, and allows concurrent, multithreaded access. In the example below, I am assuming that your AppDelegate, *if created with a recent version of Xcode with Use Core Data checked*, has a property named persistentStoreCoordinator:

lazy var managedObjectContext:NSManagedObjectContext? = {
    // This property is optional since there are legitimate error conditions that could cause the creation of the context to fail.

    if let appDelegate = UIApplication.sharedApplication().delegate as? AppDelegate {
        let coordinator = appDelegate.persistentStoreCoordinator
        var managedObjectContext = NSManagedObjectContext(concurrencyType: .PrivateQueueConcurrencyType)
        managedObjectContext.persistentStoreCoordinator = coordinator
        return managedObjectContext
    }
    }()