+ entityForName：nil不是合法的NSManagedObjectContext参数搜索实体名称'分数'

Question

我是新来的目标C，iOS和CoreData，我目前正试图保存一些数据到我的CoreData实体。

当试图找到insertNewObjectForEntityForName的实体时，它看起来像基于下面的错误返回nil。

任何帮助将不胜感激！

错误：

2015-04-06 17:46:12.274 Bugland[18623:607] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', 
reason: '+entityForName: nil is not a legal NSManagedObjectContext parameter searching for entity name 'Scores'' 

LeaderboardViewController.m

JPAppDelegate *JPAppDelegate = [[UIApplication sharedApplication] delegate]; 

    NSManagedObjectContext *context =[JPAppDelegate managedObjectContext]; 

    NSManagedObject *newScore; 
    newScore = [NSEntityDescription 
       insertNewObjectForEntityForName:@"Scores" 
       inManagedObjectContext:context]; 
    [newScore setValue: scoreToAddAsString forKey:@"score"]; 
    NSError *error; 
    [context save:&error]; 

Answer 1

好了，所以，不就是应用程序的委托应该拥有核心数据堆栈，但此刻你拥有了它这样做，拥有核心数据堆栈的实例需要创建堆栈 - 目前没有这样做。

所有你必须是

@synthesize managedObjectContext = _managedObjectContext; 
@synthesize managedObjectModel = _managedObjectModel; 
@synthesize persistentStoreCoordinator = _persistentStoreCoordinator; 

刚刚产生了一些访问方法。什么都没有设置这些实例变量。

你应该有一些像下面的庞然大物的代码，另见this。

- (NSManagedObjectContext *)managedObjectContext { 

    if (_managedObjectContext != nil) { 
     return _managedObjectContext; 
    } 

    NSPersistentStoreCoordinator *coordinator = [self persistentStoreCoordinator]; 
    if (coordinator != nil) { 
     _managedObjectContext = [NSManagedObjectContext new]; 
     [_managedObjectContext setPersistentStoreCoordinator: coordinator]; 
    } 
    return _managedObjectContext; 
} 

/** 
Returns the managed object model for the application. 
If the model doesn't already exist, it is created by merging all of the models found in the application bundle. 
*/ 
- (NSManagedObjectModel *)managedObjectModel { 

    if (_managedObjectModel != nil) { 
     return _managedObjectModel; 
    } 
    _managedObjectModel = [NSManagedObjectModel mergedModelFromBundles:nil]; 
    return _managedObjectModel; 
} 

/** 
Returns the URL to the application's documents directory. 
*/ 
- (NSURL *)applicationDocumentsDirectory 
{ 
    return [[[NSFileManager defaultManager] URLsForDirectory:NSDocumentDirectory inDomains:NSUserDomainMask] lastObject]; 
} 

/** 
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 *)persistentStoreCoordinator { 

    if (_persistentStoreCoordinator != nil) { 
     return _persistentStoreCoordinator; 
    } 

    // copy the default store (with a pre-populated data) into our Documents folder 
    // 
    NSString *documentsStorePath = 
     [[[self applicationDocumentsDirectory] path] stringByAppendingPathComponent:@"Recipes.sqlite"]; 

    // if the expected store doesn't exist, copy the default store 
    if (![[NSFileManager defaultManager] fileExistsAtPath:documentsStorePath]) { 
     NSString *defaultStorePath = [[NSBundle mainBundle] pathForResource:@"Recipes" ofType:@"sqlite"]; 
     if (defaultStorePath) { 
      [[NSFileManager defaultManager] copyItemAtPath:defaultStorePath toPath:documentsStorePath error:NULL]; 
     } 
    } 

    _persistentStoreCoordinator = 
     [[NSPersistentStoreCoordinator alloc] initWithManagedObjectModel:[self managedObjectModel]]; 

    // add the default store to our coordinator 
    NSError *error; 
    NSURL *defaultStoreURL = [NSURL fileURLWithPath:documentsStorePath]; 
    if (![_persistentStoreCoordinator addPersistentStoreWithType:NSSQLiteStoreType 
                configuration:nil 
                  URL:defaultStoreURL 
                 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. If it is not possible to recover from the error, display an alert panel that instructs the user to quit the application by pressing the Home button. 

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

    // setup and add the user's store to our coordinator 
    NSURL *userStoreURL = [[self applicationDocumentsDirectory] URLByAppendingPathComponent:@"UserRecipes.sqlite"]; 
    if (![_persistentStoreCoordinator addPersistentStoreWithType:NSSQLiteStoreType 
                    configuration:nil 
                       URL:userStoreURL 
                      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. If it is not possible to recover from the error, display an alert panel that instructs the user to quit the application by pressing the Home button. 

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

    return _persistentStoreCoordinator; 
}