NSFetchedResultsController忽略fetchLimit？ [英] NSFetchedResultsController ignores fetchLimit?

问题描述

我有一个NSFetchedResultsController来更新一个UITableView与Core Data的内容。这是很标准的东西，我相信你已经看到很多次，但我遇到了轻微的问题。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]);
 }

问题是，最初商店没有实体，因为它下载和同步web服务。会发生什么，NSFetchedResultsController从存储中填充超过150行的实体，这是webservice返回的数量。但我设置一个获取限制为20，它似乎是忽略。但是，如果我关闭了应用程序，并重新开始数据已经在商店，它的工作正常。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]; 
}

这几乎是从苹果的开发文档复制粘贴，任何想法什么是goin

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：

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.

由于 NSFetchedResultsController 中有一个已知的错误，因此不能满足 fetchlimit NSFetchRequest ，您必须手动处理您的 UITableViewDataSource 和 NSFetchedResultsControllerDelegate 方法。

tableView:numberOfRowsInSection:

tableView：numberOfRowsInSection： p> - （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;
}


controller：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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