Why is the CoreData stack in XCode's CoreData enabled template treated as private?

Question

In regards to XCode templates with CoreData enabled, I've read http://stackoverflow.com/questions/2729055/unclear-use-of-property-in-window-app-using-core-data which goes over the 'what' in the templates. But I am having an issue with the 'why'. By declaring the category in the implementation file, the CoreData accessors act like private methods. The problem with that is whenever you want to use CoreData elsewhere in your app, you need some extra code.

I've figured you need to either supply your own method that exposes the managed object context, such as...

- (NSManagedObjectContext *)getManagedObjectContext
{
    return self.managedObjectContext;
}

...which will allow other parts of your app to use it.

Or you would need to jam pack your app delegate with specific methods to return managed objects, ie getProducts or setUser.

Can anyone shed light on the reasoning here?

Answer 1

If your project is big and needs to access the managed object context from outside of the AppDelegate, I would just move the property declaration of managedObjectContext to the header file, as in:

@interface myAppDelegate : NSObject <UIApplicationDelegate> {
    NSManagedObjectModel *managedObjectModel;
    NSManagedObjectContext *managedObjectContext;       
    NSPersistentStoreCoordinator *persistentStoreCoordinator;
}
@property (retain,nonatomic) NSManagedObjectContext*managedObjectContext;
@end

Then the other parts of the app can just use appDelegate.managedObjectContext. There's no reason to expose managedObjectModel or persistentStoreCoordinator outside the app delegate, though.

By the way I have a few comments about your usage of Objective-C:

Don't use get in front of the getter. For a property called foo, the getter should be

-(Foo*)foo;

and the setter should be

-(void)setFoo:(Foo*)_foo;

By convention, get... is used when a pointer is fed as a method argument, as in -[NSString getCharacters:range:] (See the Apple doc).

Follow the proverb, when in Rome, do as the Romans do.

Answer 2

The reason for this is because you should be using dependency injection in your designs. This is the recommended design by the Core Data team. What is expected is that your app delegate will set the NSManagedObjectContext reference in your root view controller(s). From there the controllers will set or inject the necessary dependencies in the following view controllers.

This will lead to a more flexible design. I discussed it in depth in my article on the MDN (http://www.mac-developer-network.com/articles/cd0004.html).