Core Data and UITabBar Controller - help?!

Answer 1

Your managed object context is probably not set and the entity "Event" is obviously not valid for a nil context.

I use a reference to my app delegate in all my view controllers so they can access the one managed object context. It sounds like others often use a singleton to manage Core Data and would get the managed object context from that.

UPDATE

There is a good discussion about where to keep the Core Data stack in Where to place the “Core Data Stack” in a Cocoa/Cocoa Touch application.

Here is some example code for keeping the Core Data stack in the app delegate:

Use Apple's standard Core Data stack implementation in YourAppDelegate. managedObjectContext is implemented as an example, but managedObjectModel and persistentStoreCoordinator must be implemented as well.

YourAppDelegate.h:

@interface YourAppDelegate : NSObject <UIApplicationDelegate> {
    // Core Data stuff
    NSManagedObjectModel *managedObjectModel;
    NSManagedObjectContext *managedObjectContext;       
    NSPersistentStoreCoordinator *persistentStoreCoordinator;

    // other app ivars
}

YourAppDelegate.m:

- (NSManagedObjectContext *) managedObjectContext {
    if (managedObjectContext != nil) {
        return managedObjectContext;
    }
    NSPersistentStoreCoordinator *coordinator = [self persistentStoreCoordinator];
    if (coordinator != nil) {
        managedObjectContext = [[NSManagedObjectContext alloc] init];
        [managedObjectContext setPersistentStoreCoordinator: coordinator];
    }
    return managedObjectContext;
}

In every view controller, get a reference to the app delegate and use it to get the managedObjectContext as needed. For example, when setting up the fetchedResultsController;

RootViewController.h:

@interface RootViewController : UITableViewController <NSFetchedResultsControllerDelegate> {
    NSFetchedResultsController *fetchedResultsController;

    YourAppDelegate *app;
}

RootViewController.m:

#import "RootViewController.h"
#import "YourAppDelegate.h"

@implementation RootViewController

@synthesize fetchedResultsController;

- (void)viewDidLoad {
    [super viewDidLoad];

    app = (YourAppDelegate*)[UIApplication sharedApplication].delegate;
}

- (NSFetchedResultsController *)fetchedResultsController {
    if (fetchedResultsController != nil) {
        return fetchedResultsController;
    }

    NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
    NSEntityDescription *entity = [NSEntityDescription entityForName:@"Event" inManagedObjectContext:app.managedObjectContext];
    [fetchRequest setEntity:entity];

    // setup the batch size, predicate, & sort keys, etc

    NSFetchedResultsController *aFetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest managedObjectContext:app.managedObjectContext sectionNameKeyPath:nil cacheName:@"Root"];
    aFetchedResultsController.delegate = self;
    self.fetchedResultsController = aFetchedResultsController;

    [aFetchedResultsController release];
    [fetchRequest release];

    return fetchedResultsController;
}    

Answer 2

Make sure you have your TableView (rootViewController) in IB hooked up to the outlet in your app delegate.