Question

我在核心数据上遵循了本教程，我正在努力使其适应我的项目。 https://www.appcoda.com/introduction-to-core-data/

因此，当我尝试通过以下方式访问我的托管对象上下文时：

NSManagedObjectContext *context = [self context];

我得到一个null异常错误，因为这是nil。我先前在View Controller中定义了上下文：

@interface UpdateCardsViewController ()

@property NSArray* fullCardList;
@property NSManagedObjectContext* context;

@end

@implementation UpdateCardsViewController

- (NSManagedObjectContext *) setManagedObjectContext {
    NSManagedObjectContext *context = nil;
    id delegate = [[UIApplication sharedApplication] delegate];
    if ([delegate respondsToSelector:@selector(managedObjectContext)]) {
        context = [delegate managedObjectContext];
    }
    return context;
}

我认为应该获得在AppDelegate中自动创建的ManagedObjectContext实例。

这是我的AppDelegate也存在问题：

#import "AppDelegate.h"

@interface AppDelegate ()

@end

@implementation AppDelegate


- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
    // Override point for customization after application launch.
    UITabBarController *tabBar = (UITabBarController *)self.window.rootViewController;
    tabBar.selectedIndex = 2;
    return YES;
}


- (void)applicationWillResignActive:(UIApplication *)application {
    // Sent when the application is about to move from active to inactive state. This can occur for certain types of temporary interruptions (such as an incoming phone call or SMS message) or when the user quits the application and it begins the transition to the background state.
    // Use this method to pause ongoing tasks, disable timers, and invalidate graphics rendering callbacks. Games should use this method to pause the game.
}


- (void)applicationDidEnterBackground:(UIApplication *)application {
    // Use this method to release shared resources, save user data, invalidate timers, and store enough application state information to restore your application to its current state in case it is terminated later.
    // If your application supports background execution, this method is called instead of applicationWillTerminate: when the user quits.
}


- (void)applicationWillEnterForeground:(UIApplication *)application {
    // Called as part of the transition from the background to the active state; here you can undo many of the changes made on entering the background.
}


- (void)applicationDidBecomeActive:(UIApplication *)application {
    // Restart any tasks that were paused (or not yet started) while the application was inactive. If the application was previously in the background, optionally refresh the user interface.
}


- (void)applicationWillTerminate:(UIApplication *)application {
    // Called when the application is about to terminate. Save data if appropriate. See also applicationDidEnterBackground:.
    // Saves changes in the application's managed object context before the application terminates.
    [self saveContext];
}

#pragma mark - Core Data stack

@synthesize persistentContainer = _persistentContainer;

- (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:@"Lifepoints"];
            [_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;
}

#pragma mark - Core Data Saving support

- (void)saveContext {
    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();
    }
}


@end

我可能接近这个完全错误，但我认为我已经得到了这个教程。

感谢。

Answer 1

链接教程已过时（iOS10之前），但您使用的是iOS10 +核心数据堆栈。

将您的方法更改为

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

由于您清楚地知道respondsSelector中存在核心数据堆栈，因此无需进行AppDelegate检查。并且你get不是set ting它。

也不需要属性context，只需写

即可

NSManagedObjectContext *context = [self managedObjectContext];

App Delegate中的托管对象上下文为零

1 个答案: