CoreData NSManagedObjectContext not found

Question

New to iOS development. I am trying to use the core data stack in a sample app, which fetches data over network and simply updates the UI. Whilst creating the project, the "Use Core Data" checkbox was ticked which generates bunch of boilerplate code. However, on XCode 8.2.1, the I only see the NSPersistentContainer implementation within the AppDelegate.m file.

- (NSPersistentContainer *)persistentContainer {
    // The persistent container for the application. This implementation creates and returns a container, having loaded the store for the application to it.
    @synchronized (self) {
        if (_persistentContainer == nil) {
            _persistentContainer = [[NSPersistentContainer alloc] initWithName:@"ergast_coredata_objc"];
            [_persistentContainer loadPersistentStoresWithCompletionHandler:^(NSPersistentStoreDescription *storeDescription, NSError *error) {
                if (error != nil) {
                    // Replace this implementation with code to handle the error appropriately.
                    // abort() causes the application to generate a crash log and terminate. You should not use this function in a shipping application, although it may be useful during development.

                    /*
                     Typical reasons for an error here include:
                     * The parent directory does not exist, cannot be created, or disallows writing.
                     * The persistent store is not accessible, due to permissions or data protection when the device is locked.
                     * The device is out of space.
                     * The store could not be migrated to the current model version.
                     Check the error message to determine what the actual problem was.
                    */
                    NSLog(@"Unresolved error %@, %@", error, error.userInfo);
                    abort();
                }
            }];
        }
    }

    return _persistentContainer;
}

There is no boilerplate code for the NSManagedObjectContext as most of the tutorials would have you believe.

Hence, the following block within the ViewController.m class fails with a Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[AppDelegate managedObjectContext]: unrecognized selector sent to instance

-(NSArray *)getSeasonsList{
    NSFetchRequest *request = [NSFetchRequest fetchRequestWithEntityName:@"SeasonData"];
    NSError *error = nil;
    NSArray *results = [[self getManagedObjectContext] executeFetchRequest:request error:&error];
    if (!results) {
        NSLog(@"Error fetching objects: %@\n%@", [error localizedDescription], [error userInfo]);
        abort();
    }
    return results;
}




- (NSManagedObjectContext *)getManagedObjectContext {
    NSManagedObjectContext *context = nil;
    id delegate = [[UIApplication sharedApplication] delegate];
    context = [delegate managedObjectContext];
    return context;

}

Tried looking online but most of the tutorials are Swift based it seems. What am i missing here? Is the developer supposed to provide the implementation for the NSManagedObjectContext? Would be much appreciated if someone can provide with an Objective-c based sample code.

Thanks!

Answer 1

Modify the "getManagedObjectContext" method as below

- (NSManagedObjectContext *)getManagedObjectContext {
    AppDelegate *delegate = (AppDelegate *)[[UIApplication sharedApplication] delegate];
    NSManagedObjectContext *context = [[delegate persistentContainer] viewContext];
    return context;
}

I didn't tested this, but hope this will work

Answer 2

I have a category for appdelegate that was included in one of the old Stanford CS193P that uses objective-c. Some of it might be deprecated, though. Also check out https://developer.apple.com/library/content/documentation/Cocoa/Conceptual/CoreData/InitializingtheCoreDataStack.html#//apple_ref/doc/uid/TP40001075-CH4-SW1 has objective-c sample. Post back if that helps.

//
//  PhotomaniaAppDelegate+MOC.m
//  Photomania
//
//  This code comes from the Xcode template for Master-Detail application.

#import "PhotomaniaAppDelegate+MOC.h"
#import <CoreData/CoreData.h>

@implementation PhotomaniaAppDelegate (MOC)

#pragma mark - Core Data

- (void)saveContext:(NSManagedObjectContext *)managedObjectContext
{
    NSError *error = nil;
    if (managedObjectContext != nil) {
        if ([managedObjectContext hasChanges] && ![managedObjectContext save:&error]) {
            // Replace this implementation with code to handle the error appropriately.
            // abort() causes the application to generate a crash log and terminate. You should not use this function in a shipping application, although it may be useful during development.
            NSLog(@"Unresolved error %@, %@", error, [error userInfo]);
            abort();
        }
    }
}

// Returns the managed object context for the application.
// If the context doesn't already exist, it is created and bound to the persistent store coordinator for the application.
- (NSManagedObjectContext *)createMainQueueManagedObjectContext
{
    NSManagedObjectContext *managedObjectContext = nil;
    NSPersistentStoreCoordinator *coordinator = [self createPersistentStoreCoordinator];
    if (coordinator != nil) {
        managedObjectContext = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSMainQueueConcurrencyType];
        [managedObjectContext setPersistentStoreCoordinator:coordinator];
    }
    return managedObjectContext;
}

// Returns the managed object model for the application.
// If the model doesn't already exist, it is created from the application's model.
- (NSManagedObjectModel *)createManagedObjectModel
{
    NSManagedObjectModel *managedObjectModel = nil;
    NSURL *modelURL = [[NSBundle mainBundle] URLForResource:@"Photomania" withExtension:@"momd"];
    managedObjectModel = [[NSManagedObjectModel alloc] initWithContentsOfURL:modelURL];
    return managedObjectModel;
}

// Returns the persistent store coordinator for the application.
// If the coordinator doesn't already exist, it is created and the application's store added to it.
- (NSPersistentStoreCoordinator *)createPersistentStoreCoordinator
{
    NSPersistentStoreCoordinator *persistentStoreCoordinator = nil;
    NSManagedObjectModel *managedObjectModel = [self createManagedObjectModel];

    NSURL *storeURL = [[self applicationDocumentsDirectory] URLByAppendingPathComponent:@"MOC.sqlite"];

    NSError *error = nil;
    persistentStoreCoordinator = [[NSPersistentStoreCoordinator alloc] initWithManagedObjectModel:managedObjectModel];
    if (![persistentStoreCoordinator addPersistentStoreWithType:NSSQLiteStoreType configuration:nil URL:storeURL options:nil error:&error]) {
        /*
         Replace this implementation with code to handle the error appropriately.

         abort() causes the application to generate a crash log and terminate. You should not use this function in a shipping application, although it may be useful during development.

         Typical reasons for an error here include:
         * The persistent store is not accessible;
         * The schema for the persistent store is incompatible with current managed object model.
         Check the error message to determine what the actual problem was.


         If the persistent store is not accessible, there is typically something wrong with the file path. Often, a file URL is pointing into the application's resources directory instead of a writeable directory.

         If you encounter schema incompatibility errors during development, you can reduce their frequency by:
         * Simply deleting the existing store:
         [[NSFileManager defaultManager] removeItemAtURL:storeURL error:nil]

         * Performing automatic lightweight migration by passing the following dictionary as the options parameter:
         @{NSMigratePersistentStoresAutomaticallyOption:@YES, NSInferMappingModelAutomaticallyOption:@YES}

         Lightweight migration will only work for a limited set of schema changes; consult "Core Data Model Versioning and Data Migration Programming Guide" for details.

         */
        NSLog(@"Unresolved error %@, %@", error, [error userInfo]);
        abort();
    }

    return persistentStoreCoordinator;
}

// Returns the URL to the application's Documents directory

- (NSURL *)applicationDocumentsDirectory
{
    return [[[NSFileManager defaultManager] URLsForDirectory:NSDocumentDirectory inDomains:NSUserDomainMask] lastObject];
}

@end

Also, check out this project https://github.com/m2mtech/photomania-2013-14/tree/master/Photomania/CoreDataTableViewController/MOC More info how Core Data was used with this implementation.

Answer 3

See in saveContext method

pragma mark - Core Data Saving support

- (void)saveContext {
    // OBSERVE HERE
    NSManagedObjectContext *context = self.persistentContainer.viewContext; 
    NSError *error = nil;
    if ([context hasChanges] && ![context save:&error]) {
        // Replace this implementation with code to handle the error appropriately.
        // abort() causes the application to generate a crash log and terminate. You should not use this function in a shipping application, although it may be useful during development.
        NSLog(@"Unresolved error %@, %@", error, error.userInfo);
        abort();
    }
}

Whenever you need NSManagedObjectContext instance, access like this

NSManagedObjectContext *context = self.persistentContainer.viewContext;

self.persistentContainer OR classObject.persistentContainer