NSFetchedResultsController忽略fetchLimit？

Question

我有一个NSFetchedResultsController来更新带有Core Data内容的UITableView.这是非常标准的东西,我相信你们已经多次见过但是我遇到了轻微的问题.首先,这是我的代码:

NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];

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

 [fetchRequest setEntity:entity];

 [fetchRequest setFetchLimit:20];

 NSPredicate *predicate = [NSPredicate predicateWithFormat:@"(folder.hidden == NO)"];
 [fetchRequest setPredicate:predicate];

 NSSortDescriptor *sort1 = [NSSortDescriptor sortDescriptorWithKey:@"sortDate" ascending:NO];
 [fetchRequest setSortDescriptors:[NSArray arrayWithObjects:sort1, nil]];

 NSFetchedResultsController *controller = [[NSFetchedResultsController alloc]
         initWithFetchRequest:fetchRequest
         managedObjectContext:self.managedObjectContext
         sectionNameKeyPath:nil
         cacheName:nil];
 [fetchRequest release];

 controller.delegate = self;

 self.fetchedResultsController = controller;

 [controller release];

 NSError *error = nil;
 [self.fetchedResultsController performFetch:&error];
 if (error) {
  // TODO send error notification
  NSLog(@"%@", [error localizedDescription]);
 }

问题是,最初商店没有实体,因为它下载并从Web服务同步.会发生什么是NSFetchedResultsController用来自商店的超过150行实体填充表,这是webservice返回的数量.但我设置了一个20的获取限制,它似乎忽略了.但是,如果我关闭应用程序并再次使用商店中已有的数据,它可以正常工作.我是我的代表,我这样做:

#pragma mark -
#pragma mark NSFetchedResultsControllerDelegate methods

- (void)controllerWillChangeContent:(NSFetchedResultsController *)controller {
 [self.tableView beginUpdates];
}


- (void)controller:(NSFetchedResultsController *)controller didChangeSection:(id  <NSFetchedResultsSectionInfo>)sectionInfo
 atIndex:(NSUInteger)sectionIndex forChangeType:(NSFetchedResultsChangeType)type {

switch(type) {
    case NSFetchedResultsChangeInsert:
        [self.tableView insertSections:[NSIndexSet indexSetWithIndex:sectionIndex] withRowAnimation:UITableViewRowAnimationFade];
        break;

    case NSFetchedResultsChangeDelete:
        [self.tableView deleteSections:[NSIndexSet indexSetWithIndex:sectionIndex] withRowAnimation:UITableViewRowAnimationFade];
        break;
    }
}


- (void)controller:(NSFetchedResultsController *)controller didChangeObject:(id)anObject
atIndexPath:(NSIndexPath *)indexPath forChangeType:(NSFetchedResultsChangeType)type newIndexPath:(NSIndexPath *)newIndexPath {

UITableView *tableView = self.tableView;

switch(type) {

    case NSFetchedResultsChangeInsert:
        [tableView insertRowsAtIndexPaths:[NSArray arrayWithObject:newIndexPath] withRowAnimation:UITableViewRowAnimationFade];
        break;

    case NSFetchedResultsChangeDelete:
        [tableView deleteRowsAtIndexPaths:[NSArray arrayWithObject:indexPath] withRowAnimation:UITableViewRowAnimationFade];
        break;

    case NSFetchedResultsChangeUpdate:
        [self configureCell:[tableView cellForRowAtIndexPath:indexPath] atIndexPath:indexPath];
        break;

    case NSFetchedResultsChangeMove:
        [tableView deleteRowsAtIndexPaths:[NSArray arrayWithObject:indexPath] withRowAnimation:UITableViewRowAnimationFade];
        [tableView insertRowsAtIndexPaths:[NSArray arrayWithObject:newIndexPath] withRowAnimation:UITableViewRowAnimationFade];
        break;
    }
}


- (void)controllerDidChangeContent:(NSFetchedResultsController *)controller {
 [self.tableView endUpdates]; 
}

这几乎是来自Apple的开发文档的复制粘贴,任何想法是什么？

Answer 1

我知道这是一个老问题,但我有一个解决方案:

既然有一个已知的bug在NSFetchedResultsController不兑现fetchlimit的NSFetchRequest,你必须手动处理您的内记录的限制UITableViewDataSource和NSFetchedResultsControllerDelegate方法.

的tableView:numberOfRowsInSection:

- (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section {

    id <NSFetchedResultsSectionInfo> sectionInfo = [[self.fetchedResultsController sections] objectAtIndex:section];

    NSInteger numRows = [sectionInfo numberOfObjects];

    if (numRows > self.fetchedResultsController.fetchRequest.fetchLimit) {

        numRows = self.fetchedResultsController.fetchRequest.fetchLimit;
    }

    return numRows;
}

控制器:didChangeObject:atIndexPath:forChangeType:newIndexPath:

- (void)controller:(NSFetchedResultsController *)controller didChangeObject:(id)anObject atIndexPath:(NSIndexPath *)indexPath forChangeType:(NSFetchedResultsChangeType)type newIndexPath:(NSIndexPath *)newIndexPath {

    switch(type) {

        case NSFetchedResultsChangeInsert:

            if ([self.tableView numberOfRowsInSection:0] == self.fetchedResultsController.fetchRequest.fetchLimit) {
                //Determining which row to delete depends on your sort descriptors
                [self.tableView deleteRowsAtIndexPaths:[NSArray arrayWithObject:[NSIndexPath indexPathForRow:self.fetchedResultsController.fetchRequest.fetchLimit - 1 inSection:0]] withRowAnimation:UITableViewRowAnimationFade];

            }

            [self.tableView insertRowsAtIndexPaths:[NSArray arrayWithObject:newIndexPath]
                            withRowAnimation:UITableViewRowAnimationFade];
        break;
        ...
    }
}

Answer 2

这是一个老问题,但我自己也遇到了它(在iOS 5中).我想你遇到了这里描述的错误:https://devforums.apple.com/message/279576#279576.

该线程根据您是否具有sectionNameKeyPath提供解决方案.由于我(像你一样)没有,答案是将tableview与fetchedResultsController分离.例如,而不是使用它来确定行数:

- (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section 
{
        return [[[self.fetchedResultsController sections] objectAtIndex:0] numberOfObjects];

回报你的期望:

    - (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section 
{
        return fetchLimit;

并且controller:didChangeObject,如果newIndexPath在fetchLimit 中,则仅插入新对象.

Answer 3

您遇到的问题是，您在加载 fetchedResultsController 之前调用了完整的数据，因此它会向您显示您需要做的一切就是加载所有信息，然后调用 fetchedResultsController

例子

- (void)viewDidLoad {
    [super viewDidLoad];

    // Loading Articles to CoreData
    [self loadArticle];
}

- (void)ArticleDidLoadSuccessfully:(NSNotification *)notification {
    NSError *error;
    if (![[self fetchedResultsController] performFetch:&error]) {
        // Update to handle the error appropriately.
        NSLog(@"Unresolved error %@, %@", error, [error userInfo]);
        abort();  // Fail
    }
    [tableView reloadData];
}