CoreData UITableViewController managedObjectContext Error

Question

I have started playing around with CoreData and taking a new project with CoreData and building those foundations into my own project. I have reached a stage where I have more or less identically duplicated the fresh project, however I am getting an error.

This line, controller.managedObjectContext = self.managedObjectContext; is causing me problems. When I comment it out, the app simply launches with a blank UITableView, however if I include the line, it causes this error:

Universal[24718:707] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[UITableViewController setManagedObjectContext:]: unrecognized selector sent to instance 0x1521a0'
*** First throw call stack:
(0x344558bf 0x346a51e5 0x34458acb 0x34457945 0x343b2680 0x2413 0x378367eb 0x378303bd 0x377fe921 0x377fe3bf 0x377fdd2d 0x30c30df3 0x34429553 0x344294f5 0x34428343 0x343ab4dd 0x343ab3a5 0x3782f457 0x3782c743 0x2331 0x22c8)
terminate called throwing an exception

I you need anymore code to look at, I can provide it and I hope you have an idea of whats happening. I can't see anything which would cause this, I am not getting any errors, but I get this log as that line causes the entire app to crash.

The line resides in the didFinishLaunchingWithOptions method of the app delegate, just like in a fresh core data project.

As requested, the header file for the table view controller:

#import <UIKit/UIKit.h>

#import <CoreData/CoreData.h>

@interface myTableViewController : UITableViewController <NSFetchedResultsControllerDelegate>

@property (strong, nonatomic) NSFetchedResultsController *fetchedResultsController;
@property (strong, nonatomic) NSManagedObjectContext *managedObjectContext;

@end

Answer 1

The reason why you are getting the error is in your AppDelegate.m

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
    // Override point for customization after application launch.
    UINavigationController *navigationController = (UINavigationController *)self.window.rootViewController;
    MasterViewController *controller = (MasterViewController *)navigationController.topViewController;
    controller.managedObjectContext = self.managedObjectContext;
    return YES;
}

When using the default CoreData template the managed object context is assigned to the topViewController which no longer is MasterViewController as you've inserted a different view as your starting point. Thus the unrecognized selector sent to instance.

You have two options:

1) Remove the code that assigns the managed object context and, in the view that needs it get it like this

 [[[UIApplication sharedApplication] delegate] mainManagedObjectContext];

2) Keep the code (correct the class name), add declaration for managedObjectContext, and pass your NSManagedObjectContext object throughout all your app between all your UIViewControllers

Answer 2

You're missing some wiring that links UIViewControllers (or subclasses thereof) up to NSFetchedResultsController. You get that for free when you set up a new project with Core Data enabled, but if you're adding Core Data to en existing project, you need to wire up a few things yourself.

For more info, please see http://wiresareobsolete.com/wordpress/2009/12/adding-core-data-existing-iphone-projects/