Objective-C中从AppDelegate传递NSPersistentContainer至根视图控制器问题

问题背景 #

我按照《Setting Up a Core Data Stack》文档搭建Objective-C版本的Core Data栈，但文档仅提供Swift实现。应用结构为：Application Entry Point->UINavigationController->TodoListTableViewController。

在-[UIApplicationDelegate application:didFinishLaunchingWithOptions:]中编写了初始化并传递PersistentContainer的代码，但在TodoListTableViewController的viewDidLoad中检查时触发了“missing container”异常。

相关代码片段 #

AppDelegate的启动方法代码 #

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
    // Override point for customization after application launch.
    if (self.persistentContainer == nil) {
        [NSException raise:@"did not initialize persistent container in app delegate" format:@"no init"];
    }

    UIStoryboard *mainStoryboard = [UIStoryboard storyboardWithName:@"Main" bundle:nil];
    TodoListTableViewController *todoListVC = [mainStoryboard instantiateViewControllerWithIdentifier:@"TodoListTableViewController"];
    todoListVC.persistentContainer = self.persistentContainer;
    UINavigationController *navController = [[UINavigationController alloc] initWithRootViewController:todoListVC];
    self.window.rootViewController = navController;
    
    return YES;
}

TodoListTableViewController的viewDidLoad代码 #

- (void)viewDidLoad {
   [super viewDidLoad];
   if (self.persistentContainer == nil) {
     [NSException raise:@"missing container" format:@"missing container"];
   }
}

AppDelegate中Xcode生成的Core Data模板代码 #

@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:@"ToDoList"];
            [_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();
    }
}

AppDelegate头文件代码 #

@property (readonly, strong) NSPersistentContainer *persistentContainer;

问题原因与解决方案 #

核心原因 #

你手动创建了新的TodoListTableViewController实例并传递PersistentContainer，但Storyboard中已经初始化了导航控制器及其根视图控制器（即实际显示的TodoListTableViewController），两者不是同一个对象，导致真正显示的控制器没有拿到PersistentContainer。

修复步骤 #

1. 修改AppDelegate的启动代码，直接获取Storyboard中已有的导航控制器和对应的根视图控制器：

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
    if (self.persistentContainer == nil) {
        [NSException raise:@"did not initialize persistent container in app delegate" format:@"no init"];
    }

    UIStoryboard *mainStoryboard = [UIStoryboard storyboardWithName:@"Main" bundle:nil];
    // 获取Storyboard中设置的初始导航控制器
    UINavigationController *navController = [mainStoryboard instantiateInitialViewController];
    // 从导航控制器中取出实际显示的根视图控制器
    TodoListTableViewController *todoListVC = (TodoListTableViewController *)navController.topViewController;
    todoListVC.persistentContainer = self.persistentContainer;
    
    self.window.rootViewController = navController;
    return YES;
}

2. 确保TodoListTableViewController正确声明属性，在TodoListTableViewController.h中添加：

@property (strong, nonatomic) NSPersistentContainer *persistentContainer;

3. 验证Storyboard配置：

• 确认UINavigationController已勾选Is Initial View Controller
• 确认TodoListTableViewController是导航控制器的根视图控制器

额外说明 #

loadPersistentStoresWithCompletionHandler:是异步执行，但persistentContainer的getter方法中通过@synchronized保证了初始化的同步性，因此didFinishLaunching中访问时容器已经初始化完成，不存在异步导致的空值问题。

内容的提问来源于stack exchange，提问作者reactor