處理managedObjectContext的核心數據錯誤

Question

以下是我遇到錯誤的代碼行。錯誤也在下面。我明白爲什麼會出現錯誤，因爲managedObjectContext爲零。我不知道的是如何解決這個問題。

NSEntityDescription *entity = [NSEntityDescription 
             entityForName:@"Customer" inManagedObjectContext:managedObjectContext]; 

錯誤

Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '+entityForName: nil is not a legal NSManagedObjectContext parameter searching for entity name 'Customer'' 

下面是一些更相關的代碼。

- (NSManagedObjectContext *)managedObjectContext 
{ 
    if (_managedObjectContext != nil) { 
     return _managedObjectContext; 
    } 

    NSPersistentStoreCoordinator *coordinator = [self persistentStoreCoordinator]; 
    if (coordinator != nil) { 
     _managedObjectContext = [[NSManagedObjectContext alloc] init]; 
     [_managedObjectContext setPersistentStoreCoordinator:coordinator]; 
    } 
    return _managedObjectContext; 
} 
- (NSManagedObjectModel *)managedObjectModel 
{ 
    if (_managedObjectModel != nil) { 
     return _managedObjectModel; 
    } 
    NSURL *modelURL = [[NSBundle mainBundle] URLForResource:@"Customer" withExtension:@"momd"]; 
    _managedObjectModel = [[NSManagedObjectModel alloc] initWithContentsOfURL:modelURL]; 
    return _managedObjectModel; 
} 

// 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; 
    } 

    NSURL *storeURL = [[self applicationDocumentsDirectory] URLByAppendingPathComponent:@"rolo.sqlite"]; 

    NSError *error = nil; 
    _persistentStoreCoordinator = [[NSPersistentStoreCoordinator alloc] initWithManagedObjectModel:[self managedObjectModel]]; 
    if (![_persistentStoreCoordinator addPersistentStoreWithType:NSSQLiteStoreType configuration:nil URL:storeURL 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. 

     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. 


     If the persistent store is not accessible, there is typically something wrong with the file path. Often, a file URL is pointing into the application's resources directory instead of a writeable directory. 

     If you encounter schema incompatibility errors during development, you can reduce their frequency by: 
     * Simply deleting the existing store: 
     [[NSFileManager defaultManager] removeItemAtURL:storeURL error:nil] 

     * Performing automatic lightweight migration by passing the following dictionary as the options parameter: 
     @{NSMigratePersistentStoresAutomaticallyOption:@YES, NSInferMappingModelAutomaticallyOption:@YES} 

     Lightweight migration will only work for a limited set of schema changes; consult "Core Data Model Versioning and Data Migration Programming Guide" for details. 

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

    return _persistentStoreCoordinator; 
} 
- (NSURL *)applicationDocumentsDirectory 
{ 
    return [[[NSFileManager defaultManager] URLsForDirectory:NSDocumentDirectory inDomains:NSUserDomainMask] lastObject]; 
} 

Answer 1

你不告訴我們你是如何分配variable managedObjectContext您使用

我想你不想反正:)

=>您可能希望屬性self.managedObjectContext

NSEntityDescription *entity = [NSEntityDescription entityForName:@"Customer" inManagedObjectContext:self.managedObjectContext];