managedObjectContext is nil - ios

Question

I'm trying to add Core Data to an existing project. I've:
1)added the Core Data framework
2)added the accessors and properties to the AppDelegate
3)created the data model file
Now when I try to call
NSManagedObjectContext *context = [self managedObjectContext];
from a view controller the context is nil and the managedObjectContext never fires.
Here is the AppDelegate:
#import "XXXAppDelegate.h"
#import <CoreData/CoreData.h>
#implementation XXXAppDelegate
#synthesize window=_window;
#synthesize navigationController=_navigationController;
#synthesize managedObjectContext = _managedObjectContext;
#synthesize managedObjectModel = _managedObjectModel;
#synthesize persistentStoreCoordinator = _persistentStoreCoordinator;
- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
// Override point for customization after application launch.
// Add the navigation controller's view to the window and display.
self.window.rootViewController = self.navigationController;
[self.window makeKeyAndVisible];
return YES;
}
// Explicitly write Core Data accessors
- (NSManagedObjectContext *) managedObjectContext {
if (managedObjectContext != nil) {
return managedObjectContext;
}
NSPersistentStoreCoordinator *coordinator = [self persistentStoreCoordinator];
if (coordinator != nil) {
managedObjectContext = [[NSManagedObjectContext alloc] init];
[managedObjectContext setPersistentStoreCoordinator: coordinator];
}
return managedObjectContext;
}
- (NSManagedObjectModel *)managedObjectModel {
if (managedObjectModel != nil) {
return managedObjectModel;
}
managedObjectModel = [NSManagedObjectModel mergedModelFromBundles:nil];
return managedObjectModel;
}
- (NSPersistentStoreCoordinator *)persistentStoreCoordinator {
if (persistentStoreCoordinator != nil) {
return persistentStoreCoordinator;
}
NSURL *storeUrl = [NSURL fileURLWithPath: [[self applicationDocumentsDirectory]
stringByAppendingPathComponent: #"<Project Name>.sqlite"]];
NSError *error = nil;
persistentStoreCoordinator = [[NSPersistentStoreCoordinator alloc]
initWithManagedObjectModel:[self managedObjectModel]];
if(![persistentStoreCoordinator addPersistentStoreWithType:NSSQLiteStoreType
configuration:nil URL:storeUrl options:nil error:&error]) {
/*Error for store creation should be handled in here*/
}
return persistentStoreCoordinator;
}
- (NSString *)applicationDocumentsDirectory {
return [NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES) lastObject];
}
#end
EDIT: here is my view controller code
NSManagedObjectContext *context = [self managedObjectContext];
NSManagedObject *cardSet = [NSEntityDescription insertNewObjectForEntityForName:#"CardSet" inManagedObjectContext:context];
[cardSet setValue:#"Set 1" forKey:#"cardSetName"];

Answer 1

Try adding these condition to check if your managedObjectContext is nil or not wherever you want to use it. If its nil copy it from Appdelegate file.
if (managedObjectContext == nil)
{
managedObjectContext = [(AppDelegate *)[[UIApplication sharedApplication] delegate] managedObjectContext];
NSLog(#"After managedObjectContext: %#", managedObjectContext);
}

Answer 2

Assuming your properties are declared... you are synthesizing them to ivars with an underscore in front. That's a good thing. However, the only place you want to access them with the underscore is in the implementation of the getter/setter for the property. Unfortunately, that's not happening in any of these. Change it to...
- (NSManagedObjectContext *) managedObjectContext {
if (_managedObjectContext != nil) {
return _managedObjectContext;
}
NSPersistentStoreCoordinator *coordinator = [self persistentStoreCoordinator];
if (coordinator != nil) {
_managedObjectContext = [[NSManagedObjectContext alloc] init];
[_managedObjectContext setPersistentStoreCoordinator: coordinator];
}
return _managedObjectContext;
}
- (NSManagedObjectModel *)managedObjectModel {
if (_managedObjectModel != nil) {
return _managedObjectModel;
}
_managedObjectModel = [NSManagedObjectModel mergedModelFromBundles:nil];
return _managedObjectModel;
}
- (NSPersistentStoreCoordinator *)persistentStoreCoordinator {
if (_persistentStoreCoordinator != nil) {
return _persistentStoreCoordinator;
}
NSURL *storeUrl = [NSURL fileURLWithPath: [[self applicationDocumentsDirectory]
stringByAppendingPathComponent: #"<Project Name>.sqlite"]];
NSError *error = nil;
_persistentStoreCoordinator = [[NSPersistentStoreCoordinator alloc]
initWithManagedObjectModel:[self managedObjectModel]];
if(![_persistentStoreCoordinator addPersistentStoreWithType:NSSQLiteStoreType
configuration:nil URL:storeUrl options:nil error:&error]) {
/*Error for store creation should be handled in here*/
}
return _persistentStoreCoordinator;
}

Answer 3

If you look at the Master-Detail Application template in Xcode you see they pass the ManagedObjectContext in the AppDelegate like this:
#import "AppDelegate.h"
#import "MasterViewController.h"
#implementation AppDelegate
#synthesize managedObjectContext = _managedObjectContext;
#synthesize managedObjectModel = _managedObjectModel;
#synthesize persistentStoreCoordinator = _persistentStoreCoordinator;
- (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;
NSLog(#"navigationController viewControllers: %#",[navigationController viewControllers]);
NSLog(#"navigationController.topViewController: %#",navigationController.topViewController);
controller.managedObjectContext = self.managedObjectContext;
return YES;
}
If you need a TabBarViewController in front of your app the code looks like this:
#import "AppDelegate.h"
#import "MasterViewController.h"
#implementation AppDelegate
#synthesize managedObjectContext = _managedObjectContext;
#synthesize managedObjectModel = _managedObjectModel;
#synthesize persistentStoreCoordinator = _persistentStoreCoordinator;
- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
// Override point for customization after application launch.
UITabBarController *tabController = (UITabBarController *)self.window.rootViewController;
UINavigationController *navigationController = (UINavigationController *)[[tabController viewControllers] objectAtIndex:0];
MasterViewController *controller = (MasterViewController *)[[navigationController viewControllers] objectAtIndex:0];
controller.managedObjectContext = self.managedObjectContext;
navigationController = (UINavigationController *)[[tabController viewControllers] objectAtIndex:1];
controller = (MasterViewController *)[[navigationController viewControllers] objectAtIndex:0];
controller.managedObjectContext = self.managedObjectContext;
navigationController = (UINavigationController *)[[tabController viewControllers] objectAtIndex:2];
controller = (MasterViewController *)[[navigationController viewControllers] objectAtIndex:0];
controller.managedObjectContext = self.managedObjectContext;
return YES;
}

Answer 4

I don't know how you got past without compiler errors, but your #synthesizes use underscore'd variables that your getters never access. Do it like this:
- (NSManagedObjectContext *) managedObjectContext {
if (_managedObjectContext != nil) {
return _managedObjectContext;
}
NSPersistentStoreCoordinator *coordinator = [self persistentStoreCoordinator];
if (coordinator != nil) {
_managedObjectContext = [[NSManagedObjectContext alloc] init];
[_managedObjectContext setPersistentStoreCoordinator: coordinator];
}
return _managedObjectContext;
}
Take note of _managedObjectContext (with underscore). Do the same fixes with managedObjectModel and persistentStoreCoordinator.