NSFetchedResultsController 忽略 fetchLimit?

问题描述

我有一个 NSFetchedResultsController 来使用 Core Data 的内容更新 UITableView.这是相当标准的东西，我相信你们都见过很多次，但是我遇到了一些小问题.首先是我的代码:

I have a NSFetchedResultsController to update a UITableView with content from Core Data. It's pretty standard stuff I'm sure you've all seen many times however I am running into slight problem. First here's my code:

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]);
 }

问题在于，商店最初没有实体，因为它从网络服务下载和同步.发生的情况是 NSFetchedResultsController 用来自商店的 150 多行实体填充表，这就是 web 服务返回的数量.但是我设置了 20 的获取限制，这似乎被忽略了.但是，如果我关闭应用程序并使用商店中已有的数据重新开始，它就可以正常工作.我是我的代表，我这样做:

The problem is that initially the store has no entities as it downloads and syncs from a webservice. What happens is that the NSFetchedResultsController fills the table with over 150 rows of entities from the store, which is how many the webservice returns. But I am setting a fetch limit of 20 which it appears to be ignoring. However, if I close out the app and start again with data already in the store, it works fine. Im my delegate i do this:

#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 开发文档的复制粘贴，有什么想法吗?

Which is pretty much copy-paste from Apple's dev documents, any ideas what's goin on?

推荐答案

我知道这是一个老问题，但我有一个解决方案:

I know this is an old question, but I have a solution for it:

由于 NSFetchedResultsController 中存在一个不符合 NSFetchRequest 的 fetchlimit 的已知错误，因此您必须手动处理限制UITableViewDataSource 和 NSFetchedResultsControllerDelegate 方法中的记录数.

Since there is a known bug in NSFetchedResultsController that doesn't honor the fetchlimit of the NSFetchRequest, you have to manually handle the limiting of records within your UITableViewDataSource and NSFetchedResultsControllerDelegate methods.

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;
        ...
    }
}

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