“Appdelegate”没有可见的@interface 声明选择器“managedObjectContext”

Posted 2023-03-13

【中文标题】“Appdelegate”没有可见的@interface 声明选择器“managedObjectContext”

【英文标题】：No visible @interface for 'Appdelegate' declares the selector 'managedObjectContext'

【发布时间】：2017-01-02 00:58:25

【问题描述】：

我正在尝试学习核心数据并遵循 youtube 上的一些教程。在自己练习代码时，我试图通过将“managedObjectContext”消息传递给应用程序委托对象来创建“NSManagedObjectContext”对象。下面是代码：

+ (Employee *)addEmployeeDetails:(NSDictionary *)employeeData 
    AppDelegate *appDelegate = (AppDelegate *)[[UIApplication sharedApplication] delegate];
    NSManagedObjectContext *context = [appDelegate managedObjectContext];

    Employee *employeeEntity = nil;

    return employeeEntity;

我收到了这个问题标题中提到的错误。我记得我在创建项目时选择了“使用核心数据”选项。我在项目中是否遗漏了其他东西？

以下是随项目自动添加的 AppDelegate 文件的内容。

AppDelegate.h

#import <UIKit/UIKit.h>
#import <CoreData/CoreData.h>

@interface AppDelegate : UIResponder <UIApplicationDelegate>
@property (strong, nonatomic) UIWindow *window;
@property (readonly, strong) NSPersistentContainer *persistentContainer;
- (void)saveContext;
@end

AppDelegate.m

#import "AppDelegate.h"

@interface AppDelegate ()

@end

@implementation AppDelegate


- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions 
    // Override point for customization after application launch.
    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:@"Core_Data_Tutorial_1"];
            [_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

【问题讨论】：

无需定义和分配managedObjectContext。您可以通过 appDelegate.persistentContainer.viewContext 访问当前托管对象上下文。

哦，好的。好像最近有变化。我在 youtube 上参考了许多其他与核心数据相关的教程，看起来像旧的 ios 版本，应用程序委托文件是使用 3 个属性创建的，即 managedObjectContext、managedObjectModel 和 persistentStoreCoordinator。

【参考方案1】：

AppDelegate 没有名为 managedObjectContext 的属性。您需要在 AppDelegate.h

中声明它

@property (strong, nonatomic) NSManagedObjectContext *managedObjectContext;

当然，正确分配它。

【讨论】：

哈，我不知道为什么该部分没有包含在教程中。我确实检查了 AppDelegate.h 并没有找到该属性，但不确定它是在创建文件时自动添加还是开发人员必须添加它。谢谢萨曼莎！

是的，看起来很奇怪，尤其是如果他们让您在 AppDelegate 的其他地方创建上下文。编码愉快！