核心数据 NSFetchedResultsController 批量大小

Question

NSFetchResultController with NSFetchRequest with fetchBatchSize = 20 always return 所有实体。它可以是什么？我没有使用 sectionKeyPath 并尝试了不同的排序描述符，但它仍然是 return 所有对象。

感谢回复！我会详细说明的。我有一个包含两个字段的实体 - 距离和时间。我创建了 NSFetchResultController:

func noticesFetcher() -> NSFetchedResultsController {

    let fetchRequest = NSFetchRequest()

    let defaultStore = RKManagedObjectStore.defaultStore()

    let entity = NSEntityDescription.entityForName("Notice", inManagedObjectContext: defaultStore.mainQueueManagedObjectContext)
    fetchRequest.entity = entity

    let distanceSortDescriptor = NSSortDescriptor(key: "distance", ascending: true)
    let timeSortDescriptor = NSSortDescriptor(key: "time", ascending: false)
    let sortDescriptors = [distanceSortDescriptor, timeSortDescriptor]
    fetchRequest.sortDescriptors = sortDescriptors

    fetchRequest.fetchBatchSize = 20

    let resultFetcher = NSFetchedResultsController(fetchRequest: fetchRequest, managedObjectContext: defaultStore.mainQueueManagedObjectContext, sectionNameKeyPath: nil, cacheName: nil)

    return resultFetcher
}

但是当我执行 fetcher 时，我的所有实体总是在数据库中。(100)

func performFetcher(fetcher: NSFetchedResultsController?, filter: NSPredicate?, success: (() -> ())?, failure: ((NSError) -> (NSError))?) -> Bool {

    NSFetchedResultsController.deleteCacheWithName(nil)

    var resultPerform = false

    fetcher?.fetchRequest.predicate = filter

    do {
        try fetcher?.performFetch()
        resultPerform = true
        if success != nil {
            success!();
        }
    }
    catch let error as NSError {
        if failure != nil {
            failure!(error)
        }
    }

    return resultPerform
}

会是什么？我想要得到的结果是分页getter。我知道我可以通过限制和偏移来做到这一点，但这里有什么问题？谢谢

Answer 1

好吧，这取决于你的意思 "return all entities." 我怀疑 returns 一个包含所有实体的数组完全实现了。

批量大小一次只能获取（在您的情况下）20 个实体。继续查看 MOC 的注册对象集，您可以轻松验证发生了什么。

您还可以启动仪器并观察各个核心数据的提取过程。

No, batch size fetch not "only 20 entities", it fetches ALL entities. Can you make a test project and test this batch size?I'm sure you will have the same issue – Serd

@Serd，答案提供者随时为您提供帮助，但我们有时会出错。如果您怀疑他们的回答，那么 you 应该创建一个测试用例来证明他们所说的是否正确。为什么他们要花额外的时间来这样做，当你是问题所在，而他们是免费提供帮助的人？

如果您对答案有疑问，那么您完成作业，如果您发现他们的答案有误，则提供更正或证明答案不正确的证据。

不过，你是新来的，希望你能从这次经历中吸取教训，我会为你提供一个简短的演示。

通过将 NUM_OBJECTS 个实例添加到数据库来为测试设置 MOC。

NSUInteger const NUM_OBJECTS = 1000;
NSUInteger const INIT_BATCH_SIZE = 100;

- (void)setUp {
    [super setUp];
    helper = [[TestHelper alloc] init];
    url = [[[NSURL fileURLWithPath:NSTemporaryDirectory()] URLByAppendingPathComponent:[[NSUUID UUID] UUIDString]] URLByAppendingPathComponent:@"foo.sqlite"];
    [[NSFileManager defaultManager] createDirectoryAtURL:[url URLByDeletingLastPathComponent] withIntermediateDirectories:YES attributes:nil error:NULL];
    @autoreleasepool {
        NSEntityDescription *noticeEntity = [[NSEntityDescription alloc] init];
        noticeEntity.name = @"Notice";
        NSAttributeDescription *distance = [[NSAttributeDescription alloc] init];
        distance.name = @"distance";
        distance.attributeType = NSDoubleAttributeType;
        NSAttributeDescription *time = [[NSAttributeDescription alloc] init];
        time.name = @"time";
        time.attributeType = NSDoubleAttributeType;
        noticeEntity.properties = @[distance, time];
        NSManagedObjectModel *model = [[NSManagedObjectModel alloc] init];
        model.entities = @[noticeEntity];

        NSPersistentStoreCoordinator *psc = [[NSPersistentStoreCoordinator alloc] initWithManagedObjectModel:model];
        [psc addPersistentStoreWithType:NSSQLiteStoreType configuration:nil URL:url options:nil error:NULL];
        moc = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSMainQueueConcurrencyType];
        moc.persistentStoreCoordinator = psc;
        for (NSUInteger count = 0; count < NUM_OBJECTS; ) {
            @autoreleasepool {
                for (NSUInteger batchCount = 0; batchCount < INIT_BATCH_SIZE && count < NUM_OBJECTS; ++batchCount, ++count) {
                    NSManagedObject *notice = [NSEntityDescription insertNewObjectForEntityForName:@"Notice" inManagedObjectContext:moc];
                    double distance = ((double)arc4random_uniform(100000)) / (arc4random_uniform(100)+1);
                    double time = distance / (arc4random_uniform(100)+1);
                    [notice setValue:@(distance) forKey:@"distance"];
                    [notice setValue:@(time) forKey:@"time"];
                }
                [moc save:NULL];
                [moc reset];
            }
        }
        [moc save:NULL];
        [moc reset];
    }
}

测试后清理...

- (void)tearDown {
    [super tearDown];
    [[NSFileManager defaultManager] removeItemAtURL:[url URLByDeletingLastPathComponent] error:NULL];
}

几个辅助方法...

- (NSArray*)executeFetchWithBatchSize:(NSUInteger)batchSize {
    NSFetchRequest *fetchRequest = [NSFetchRequest fetchRequestWithEntityName:@"Notice"];
    fetchRequest.sortDescriptors = @[[NSSortDescriptor sortDescriptorWithKey:@"distance" ascending:YES],
                                     [NSSortDescriptor sortDescriptorWithKey:@"time" ascending:NO]];
    fetchRequest.fetchBatchSize = batchSize;
    return [moc executeFetchRequest:fetchRequest error:NULL];
}

- (NSUInteger)numberOfFaults {
    NSUInteger numFaults = 0;
    for (NSManagedObject *object in moc.registeredObjects) {
        if (object.isFault) ++numFaults;
    }
    return numFaults;
}

使用默认批量大小的测试

- (void)testThatFetchRequestWitDefaultBatchSizeFetchesEverything {
    XCTAssertEqual(0, moc.registeredObjects.count);

    NSArray *fetched = [self executeFetchWithBatchSize:0];

    XCTAssertEqual(NUM_OBJECTS, moc.registeredObjects.count);
    XCTAssertEqual(NUM_OBJECTS, fetched.count);
    XCTAssertEqual(NUM_OBJECTS, [self numberOfFaults]);

    [[fetched objectAtIndex:1] valueForKey:@"distance"];
    XCTAssertEqual(NUM_OBJECTS, moc.registeredObjects.count);
    XCTAssertEqual(NUM_OBJECTS, fetched.count);
    XCTAssertEqual(NUM_OBJECTS-1, [self numberOfFaults]);
}

使用非默认批量大小的测试

- (void)testThatFetchRequestWithExplicitBatchSizeOnlyFetchesTheNumberRequested {
    XCTAssertEqual(0, moc.registeredObjects.count);

    NSUInteger const BATCH_SIZE = 20;
    NSArray *fetched = [self executeFetchWithBatchSize:BATCH_SIZE];

    XCTAssertEqual(0, moc.registeredObjects.count);
    XCTAssertEqual(NUM_OBJECTS, fetched.count);
    XCTAssertEqual(0, [self numberOfFaults]);

    [[fetched objectAtIndex:1] valueForKey:@"distance"];
    XCTAssertEqual(BATCH_SIZE, moc.registeredObjects.count);
    XCTAssertEqual(NUM_OBJECTS, fetched.count);
    XCTAssertEqual(BATCH_SIZE-1, [self numberOfFaults]);
}

Thanks for your code! But i have a problem with NSFetchResultController and his NSFetchRequest, that have batch size(It's fetches all entities – Serd

习惯很难改掉。您不应该做出该断言，而应该采用我给您的代码并稍微修改它以使用获取的结果控制器进行测试，然后您将自己确认您的断言 "It's fetches all entities" 不正确。例如...

稍微修改一下助手...

- (NSFetchRequest*)fetchRequestWithBatchSize:(NSUInteger)batchSize {
    NSFetchRequest *fetchRequest = [NSFetchRequest fetchRequestWithEntityName:@"Notice"];
    fetchRequest.sortDescriptors = @[[NSSortDescriptor sortDescriptorWithKey:@"distance" ascending:YES],
                                     [NSSortDescriptor sortDescriptorWithKey:@"time" ascending:NO]];
    fetchRequest.fetchBatchSize = batchSize;
    return fetchRequest;
}

- (NSArray*)executeFetchWithBatchSize:(NSUInteger)batchSize {
    return [moc executeFetchRequest:[self fetchRequestWithBatchSize:batchSize] error:NULL];
}

- (NSFetchedResultsController*)executeFetchUsingFetchedResultsControllerWithBatchSize:(NSUInteger)batchSize {
    NSFetchedResultsController *frc = [[NSFetchedResultsController alloc] initWithFetchRequest:[self fetchRequestWithBatchSize:batchSize] managedObjectContext:moc sectionNameKeyPath:nil cacheName:nil];
    [frc performFetch:NULL];
    return frc;
}

并稍微更改现有测试以添加 FRC 测试。

- (void)testThatFetchRequestWitDefaultBatchSizeFetchesEverythingEvenWithFetchedResultsController {
    XCTAssertEqual(0, moc.registeredObjects.count);

    NSFetchedResultsController *frc = [self executeFetchUsingFetchedResultsControllerWithBatchSize:0];

    XCTAssertEqual(NUM_OBJECTS, moc.registeredObjects.count);
    XCTAssertEqual(NUM_OBJECTS, frc.fetchedObjects.count);
    XCTAssertEqual(NUM_OBJECTS, [self numberOfFaults]);

    [[frc.fetchedObjects objectAtIndex:1] valueForKey:@"distance"];
    XCTAssertEqual(NUM_OBJECTS, moc.registeredObjects.count);
    XCTAssertEqual(NUM_OBJECTS, frc.fetchedObjects.count);
    XCTAssertEqual(NUM_OBJECTS-1, [self numberOfFaults]);
}

- (void)testThatFetchRequestWithExplicitBatchSizeOnlyFetchesTheNumberRequestedEvenWithFetchedResultsController {
    XCTAssertEqual(0, moc.registeredObjects.count);

    NSUInteger const BATCH_SIZE = 20;
    NSFetchedResultsController *frc = [self executeFetchUsingFetchedResultsControllerWithBatchSize:20];
    XCTAssertEqual(moc, frc.managedObjectContext);

    XCTAssertEqual(0, moc.registeredObjects.count);
    XCTAssertEqual(NUM_OBJECTS, frc.fetchedObjects.count);
    XCTAssertEqual(0, [self numberOfFaults]);

    [[frc.fetchedObjects objectAtIndex:1] valueForKey:@"distance"];
    XCTAssertEqual(BATCH_SIZE, moc.registeredObjects.count);
    XCTAssertEqual(NUM_OBJECTS, frc.fetchedObjects.count);
    XCTAssertEqual(BATCH_SIZE-1, [self numberOfFaults]);
}

Answer 2

fetchBatchSize 只是 Core Data 用来确保一次获得尽可能少的记录的一种效率度量。坦率地说，这在 99% 的情况下是完全没有必要的，因此您通常可以安全地删除那行多余的代码。

如果你想将你的获取限制在一定数量的记录（类似于 SQL LIMIT 参数，通常用于排序），你必须设置 fetchLimit 属性获取请求。

fetchRequest.fetchLimit = 20

核心数据 NSFetchedResultsController 批量大小

Core Data NSFetchResultController batch size

ios

core-data

nsfetchedresultscontroller

nsfetchrequest