ios - App Delegate 的托管对象上下文为零

Question

我遵循了有关 Core Data 的本教程，并且正在尝试使其适应我的项目。 https://www.appcoda.com/introduction-to-core-data/

所以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，因为它是 nil。我在此处的 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;
}

我认为应该获取在 AppDelegate 中自动创建的 ManagedObjectContext 实例。

这也是我的 AppDelegate，以防出现问题:

#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

我可能完全错误地处理了这个问题，但我认为到目前为止我已经掌握了教程。

谢谢。

Answer 1

链接的教程已过时(iOS10 之前)，但您正在使用 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];
}

respondsSelector 检查不是必需的，因为您清楚地知道 Core Data 堆栈存在于 AppDelegate 中。并且您正在获取上下文而不是设置它。

属性context也不需要，直接写

NSManagedObjectContext *context = [self managedObjectContext];