好的。这是与此处完全相同的问题:Why is NSFetchedResultsController loading all rows when setting a fetch batch size?

但他的解决方案并不能解决我的问题。

我的屏幕上有几千条记录，加载它们的速度很慢。我将批处理大小设置为 30(大约是屏幕上单元格的三倍)，但由于某种原因它仍然循环加载所有批处理。

这是代码

- (NSFetchedResultsController *)guestCardFetchedResultsController
{
    if (guestCardFetchedResultsController != nil) {
        return guestCardFetchedResultsController;
    }

    // SELECT * from GuestCard
    NSFetchRequest* fetchRequest = [[NSFetchRequest alloc] init];
    NSEntityDescription *entity = [NSEntityDescription entityForName:@"GuestCard" inManagedObjectContext:self.context];
    [fetchRequest setEntity:entity];
    // ORDER BY updated DESC
    NSSortDescriptor* updatedSortDescriptor = [NSSortDescriptor sortDescriptorWithKey:@"created" ascending:NO];
    [fetchRequest setSortDescriptors:@[updatedSortDescriptor]];
    fetchRequest.fetchBatchSize = 30;
    NSString *cacheName = self.isReportProblemView ? @"reportProblemGuestCardsAll" : @"guestCardsAll";

    [NSFetchedResultsController deleteCacheWithName:cacheName];
    NSFetchedResultsController *aFetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest managedObjectContext:self.context sectionNameKeyPath:@"sectionIdentifier" cacheName:cacheName];
    aFetchedResultsController.delegate = self;
    self.guestCardFetchedResultsController = aFetchedResultsController;

    // Clean up

    NSError *error = nil;
    if (![[self guestCardFetchedResultsController] performFetch:&error]) {
    }

    return self.guestCardFetchedResultsController;
}

在这种情况下，我没有做任何非常有趣的事情。下面是一些委托(delegate)代码(不包括单元格创建，我确认它只针对屏幕上的单元格数量调用):

- (NSInteger)numberOfSectionsInTableView:(UITableView *)tableView
{
    if ([self.guestCardFetchedResultsController.fetchedObjects count] == 0) {
        return 1;
    }
    // Return the number of sections.
    return [[self.guestCardFetchedResultsController sections] count];
}

- (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section
{
    if ([self.guestCardFetchedResultsController.fetchedObjects count] == 0) {
        return 1;
    }
    // Return the number of rows in the section.
    id <NSFetchedResultsSectionInfo> sectionInfo = [guestCardFetchedResultsController sections][section];
    return [sectionInfo numberOfObjects];
}

- (NSString *)tableView:(UITableView *)tableView titleForHeaderInSection:(NSInteger)section
{
    if ([self.guestCardFetchedResultsController.fetchedObjects count] == 0) {
        return @"";
    }

    return [[self.guestCardFetchedResultsController sections][section] name];
}

最佳答案

看完docs on the NSFetchedResultsController initializer ，我认为问题在于您在获取请求(创建)中有一个排序自然不会与部分键路径(sectionIdentifier)相同。我正在查看的文档中的具体句子是这样说的:

If this key path is not the same as that specified by the first sort descriptor in fetchRequest, they must generate the same relative orderings

我建议修改您的获取请求以先对 sectionIdentifier 进行排序，然后再创建。我认为这会解决您的问题。

 NSSortDescriptor* updatedSortDescriptor = [NSSortDescriptor sortDescriptorWithKey:@"created" ascending:NO];
 NSSortDescriptor* sectionSortDescriptor = [NSSortDescriptor sortDescriptorWithKey:@"sectionIdentifier" ascending:NO];
 // It's critical the sectionSortDescriptor is first
 [fetchRequest setSortDescriptors:@[sectionSortDescriptor, updatedSortDescriptor]];

请注意，如果 created 或 sectionIdentifier 属性实际上是实体类上的方法，那肯定会强制 Core Data 在排序之前加载所有数据/部分，因为它需要先在每个实体上执行该方法。

关于ios - NSFetchedResultsController 正在加载所有行，即使我已经设置了批处理，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/18906945/