这是使用 CoreData NSFetchedResults 和 NSPercientStore 函数的正确方法吗？

Question

我一直在尝试将预填充的数据库加载到我的设备中，我尝试从模拟器目录复制数据库并将其放入我的应用程序文件夹中，但它不起作用，所以我想知道我的 NSFetchedResultsController 是否正确获取？

- (NSFetchedResultsController *)fetchedResultsController {

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

    CoreDataMelakaAppDelegate *appDelegate = (CoreDataMelakaAppDelegate *)[[UIApplication sharedApplication] delegate];
    NSManagedObjectContext *context = [appDelegate managedObjectContext];
    // Create the fetch request for the entity.
    NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
    // Edit the entity name as appropriate.
    NSEntityDescription *entity = [NSEntityDescription entityForName:@"WhereTo" inManagedObjectContext:context];
    [fetchRequest setEntity:entity];

    // Set the batch size to a suitable number.
   // [fetchRequest setFetchBatchSize:20];

    // Edit the sort key as appropriate.
    NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"name" ascending:NO];
    NSArray *sortDescriptors = [[NSArray alloc] initWithObjects:sortDescriptor, nil];

    [fetchRequest setSortDescriptors:sortDescriptors];

    // Edit the section name key path and cache name if appropriate.
    // nil for section name key path means "no sections".
    NSFetchedResultsController *aFetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest managedObjectContext:self.managedObjectContext sectionNameKeyPath:nil cacheName:@"Root"];
    aFetchedResultsController.delegate = self;
    self.fetchedResultsController = aFetchedResultsController;

    [aFetchedResultsController release];
    [fetchRequest release];
    [sortDescriptor release];
    [sortDescriptors release];

    NSError *error = nil;
    if (![fetchedResultsController_ performFetch:&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.
         */
        NSLog(@"Unresolved error %@, %@", error, [error userInfo]);
        abort();
    }

    return fetchedResultsController_;
}    

PersistenStore

- (NSPersistentStoreCoordinator *)persistentStoreCoordinator {

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

    NSURL *storeURL = [[self applicationDocumentsDirectory] URLByAppendingPathComponent:@"CoreDataMelaka.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. 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.


         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: 
         [NSDictionary dictionaryWithObjectsAndKeys:[NSNumber numberWithBool:YES],NSMigratePersistentStoresAutomaticallyOption, [NSNumber numberWithBool:YES], NSInferMappingModelAutomaticallyOption, nil];

         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_;
}

我已经用它替换了 persistenceStore，现在无论我如何从模拟器中删除数据库或应用程序，它都会显示 2 个数据，这是我第一次尝试此方法时安装的。

- (NSPersistentStoreCoordinator *)persistentStoreCoordinator {

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

    NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
    NSString *documentsDirectory = [paths objectAtIndex:0];
    NSString *writableDBPath = [documentsDirectory stringByAppendingPathComponent:@"CoreDataMelaka.sqlite"];

    NSString *storePath = [[NSBundle mainBundle] pathForResource:@"CoreDataMelaka" ofType:@"sqlite"];


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

    NSLog(@"store URL %@", storeURL);

    // Put down default db if it doesn't already exist
    NSFileManager *fileManager = [NSFileManager defaultManager];
    if (![fileManager fileExistsAtPath:writableDBPath]) {
        NSLog(@"proceed");
        NSString *defaultStorePath = [[NSBundle mainBundle] pathForResource:@"CoreDataMelaka" ofType:@"sqlite"];
        NSLog(@"doesnt exist");
        NSLog(@"defalultStorePath %@", defaultStorePath);


        if (defaultStorePath) {
            [fileManager copyItemAtPath:defaultStorePath toPath:writableDBPath error:NULL];
            NSLog(@"storePath= %@", storePath);
        }
    }    

    NSError *error = nil;
    persistentStoreCoordinator_ = [[NSPersistentStoreCoordinator alloc] initWithManagedObjectModel:[self managedObjectModel]];
    if (![persistentStoreCoordinator_ addPersistentStoreWithType:NSSQLiteStoreType configuration:nil URL:storeURL options:nil error:&error]) {

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

    return persistentStoreCoordinator_;
}

Answer 1

您的 NSFetchedResultsController 看起来像标准的自动生成的代码。它应该可以正确获取，但我需要更多地了解您的代码才能 100% 确定。

我猜问题与查找数据库有关。您确定将其放入文档目录中吗？我假设您已将数据库复制到 Xcode 中，并且它正在将数据库复制到您的应用程序中。如果是这种情况，您的数据库将不会位于您的文档目录中。您可以使用 Xcode 中管理器的设备部分下载应用程序文件来确认这一点。您的数据库正在被复制到您的主包中，您需要从那里提取它。这里有一些代码可以做到这一点：

- (NSPersistentStoreCoordinator *)persistentStoreCoordinator {

    if (__persistentStoreCoordinator != nil)
        return __persistentStoreCoordinator;

    NSError * error = nil;
    NSURL * storeURL = [[self applicationDocumentsDirectory] URLByAppendingPathComponent:@"Store.sqlite"];
    NSFileManager * fileManager = [NSFileManager new];

    // !!!: Be sure to create a new default database if the MOM file is ever changed.

    // If there is no previous database, then a default one is used.
    if (![fileManager fileExistsAtPath:storePath]) {

        NSURL * defaultStoreURL = [[NSBundle mainBundle] URLForResource:@"DefaultStore.sqlite" 
                                                          withExtension:nil];

        // Copies the default database from the main bundle to the documents directory.
        [fileManager copyItemAtURL:defaultStoreURL
                             toURL:storeURL
                             error:&error];

        if (error) {

            // Handle error here.

            // Resets the error.
            error = nil;
        }
    }

    [fileManager release];

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

    [__persistentStoreCoordinator addPersistentStoreWithType:NSSQLiteStoreType 
                                           configuration:nil 
                                                     URL:storeURL 
                                                 options:nil 
                                                   error:&error];
    if (error) {

        // Handle error here.

        abort();
    }    

    return __persistentStoreCoordinator;
}

要找到这个和其他核心数据便利，请查看 RBCoreDataAdditions。