类“NSManagedObject”的 NSManagedObject 必须具有有效的 NSEntityDescription

Question

我正在使用Objective-CCoreData，但遇到以下错误...

*** 由于未捕获的异常“NSInvalidArgumentException”而终止应用程序，原因：“类“NSManagedObject”的 NSManagedObject 必须具有有效的 NSEntityDescription。”

我添加CoreData到我的项目中。我saveData在 中创建了方法viewControllerA并在 中调用了它viewControllerB。

在这种情况下，如果我调用 saveData 方法，它会正常工作，viewDidLoad我会收到错误。viewControllerA

In AppDelegate.h

@property (readonly, strong, nonatomic) NSManagedObjectContext *managedObjectContext;
@property (readonly, strong, nonatomic) NSManagedObjectModel *managedObjectModel;
@property (readonly, strong, nonatomic) NSPersistentStoreCoordinator *persistentStoreCoordinator;

In AppDelegate.m

#pragma mark - Core Data stack

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

- (NSURL *)applicationDocumentsDirectory {
// The directory the application uses to store the Core Data store file. This code uses a directory named "com.brninfotech._607_introToCoreDataFinal" in the application's documents directory.
return [[[NSFileManager defaultManager] URLsForDirectory:NSDocumentDirectory inDomains:NSUserDomainMask] lastObject];
}

- (NSManagedObjectModel *)managedObjectModel {
// The managed object model for the application. It is a fatal error for the application not to be able to find and load its model.
if (_managedObjectModel != nil) {
    return _managedObjectModel;
}
NSURL *modelURL = [[NSBundle mainBundle] URLForResource:@"VehicleNumberDataBase" withExtension:@"momd"];
_managedObjectModel = [[NSManagedObjectModel alloc] initWithContentsOfURL:modelURL];
return _managedObjectModel;
}

- (NSPersistentStoreCoordinator *)persistentStoreCoordinator {
// The persistent store coordinator for the application. This implementation creates and returns a coordinator, having added the store for the application to it.
if (_persistentStoreCoordinator != nil) {
    return _persistentStoreCoordinator;
}

// Create the coordinator and store

_persistentStoreCoordinator = [[NSPersistentStoreCoordinator alloc] initWithManagedObjectModel:[self managedObjectModel]];
NSURL *storeURL = [[self applicationDocumentsDirectory] URLByAppendingPathComponent:@"VehicleNumberDataBase.sqlite"];


//    NSLog(@"StoreURL is %@",storeURL);
NSError *error = nil;
NSString *failureReason = @"There was an error creating or loading the application's saved data.";
if (![_persistentStoreCoordinator addPersistentStoreWithType:NSSQLiteStoreType configuration:nil URL:storeURL options:nil error:&error]) {
    // Report any error we got.
    NSMutableDictionary *dict = [NSMutableDictionary dictionary];
    dict[NSLocalizedDescriptionKey] = @"Failed to initialize the application's saved data";
    dict[NSLocalizedFailureReasonErrorKey] = failureReason;
    dict[NSUnderlyingErrorKey] = error;
    error = [NSError errorWithDomain:@"YOUR_ERROR_DOMAIN" code:9999 userInfo:dict];
    // Replace this 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();
}

return _persistentStoreCoordinator;
}


- (NSManagedObjectContext *)managedObjectContext {
// Returns the managed object context for the application (which is already bound to the persistent store coordinator for the application.)
if (_managedObjectContext != nil) {
    return _managedObjectContext;
}

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

#pragma mark - Core Data Saving support

- (void)saveContext {
NSManagedObjectContext *managedObjectContext = self.managedObjectContext;
if (managedObjectContext != nil) {
    NSError *error = nil;
    if ([managedObjectContext hasChanges] && ![managedObjectContext 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();
    }
}
}


in viewControllerA viewDidLoad 

self.ad = (AppDelegate *)[[UIApplication sharedApplication]delegate];
self.VehicleNumberED = [NSEntityDescription entityForName:@"VehicleNumberEntity" inManagedObjectContext:self.ad.managedObjectContext];

- (void)saveData {

NSManagedObject * managedObj = [[NSManagedObject alloc]initWithEntity:self.VehicleNumberED insertIntoManagedObjectContext:self.ad.managedObjectContext];

[managedObj setValue:self.textFieldString forKey:@"vehicleNumberAtt"];

NSError * errorObj;
[self.ad.managedObjectContext save:&errorObj];

if (errorObj) {
    NSLog(@"Something goes wrong");
}else {
    NSLog(@"Saved Successfully");
}

NSFetchRequest * fetReq = [NSFetchRequest fetchRequestWithEntityName:@"VehicleNumberEntity"];

NSError * fetchErrorObj;

self.storedDataArray = [self.ad.managedObjectContext executeFetchRequest:fetReq error:&fetchErrorObj];


NSLog(@"array count is %lu", self.storedDataArray.count);

for (int i=0; i<self.storedDataArray.count; i++) {

    self.storedManagedObj = [self.storedDataArray objectAtIndex:i];

    self.vehicleNumberArray = [self.storedManagedObj valueForKey:@"vehicleNumberAtt"];

}

NSLog(@"Vehicle number is : %@", [self.storedManagedObj valueForKey:@"vehicleNumberAtt"]);

}

in ViewVontrollerB 

- (IBAction)saveVehicleNumberButton:(UIButton *)sender {

VehicleDetailsViewController *vedvc = [[VehicleDetailsViewController alloc]init];
vedvc = [self.storyboard instantiateViewControllerWithIdentifier:@"VeDVC"];
vedvc.textFieldString = self.vehicleNumberTextField.text;
[vedvc saveData];

[self.navigationController pushViewController:vedvc animated:YES];
}

Answer 1

这里的问题是您在显示视图控制器[vedvc saveData] 之前vedvc执行。此时，vedvc还没有显示出来，所以它还viewDidLoad没有被执行。因此self.ad和self.VehicleNumberED都为零。因此出现错误：实体描述为零。

最简单的解决方法是将这两个变量 (ad和VehicleNumberED)的初始化移至saveData方法中。但您可能会更好地重新考虑您的代码结构。