NSFetchedResultsController 并不总是为 NSFetchedResultsChangeMove 调用 didChangeObject:atIndexPath:forChangeType:newIndexPath:

我在用着NSFetchedResultsController with sortDescriptors请求在其中填充大量结果的表。我注意到，当发生更改将行从表格底部附近移动到顶部时，didChangeObject:atIndexPath:forChangeType:newIndexPath:根本没有被调用。

奇怪的是，我可以通过迭代所有获取的对象并在调用后立即访问它们的任何属性来解决这个问题performFetch.

有什么关于问题可能是什么的提示吗，或者这只是一个不起眼的苹果错误？

这是我的代码：

NSManagedObjectContext *context = [self managedObjectContext];
NSFetchRequest *request = [[NSFetchRequest alloc] init];
request.entity = [NSEntityDescription entityForName:@"MyObject" inManagedObjectContext:context];
request.sortDescriptors = @[NSSortDescriptor sortDescriptorWithKey:@"order" ascending:NO]];
request.fetchBatchSize = 20;
NSFetchedResultsController *fetched = [[NSFetchedResultsController alloc]
                                       initWithFetchRequest:request
                                       managedObjectContext:context
                                         sectionNameKeyPath:nil
                                                  cacheName:nil];
fetched.delegate = self;
NSError *error = nil;
if (![fetched performFetch:&error]) {
    NSLog(@"Unresolved error fetching objects: %@", error);
}

// Should not be necessary, but objects near the bottom won't move to the top without it.
for (MyObject *o in fetched.fetchedObjects) {
    o.someAttribute;
}

2014 年 9 月 12 日更新：

我将所有数据保存在后台托管对象上下文中，它似乎与我看到的问题有关。这是我用于将更改合并到主对象上下文的代码：

+(void)initSaveListener {
    [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(mergeChanges:)
                                                 name:NSManagedObjectContextDidSaveNotification
                                               object:[self privateContext]];
}

+(void)mergeChanges:(NSNotification*)notification {
    NSManagedObjectContext *context = [self mainContext];

    [context performBlock:^{
        [context mergeChangesFromContextDidSaveNotification:notification];
        NSError *error = nil;
        if (![context save:&error]) {
            NSLog(@"error merging changes %@, %@", error, [error userInfo]);
        }
    }];
}

事实证明，这个问题是由于以下更改引起的managedObjectContext使用从另一个上下文传播NSManagedObjectContextDidSaveNotification。这篇博文详细解释了为什么这会导致问题NSFetchedResultsController，以及如何修复它：

http://www.mlsite.net/blog/?p=518 http://www.mlsite.net/blog/?p=518

这是我上面的代码上下文中的具体修复：

+(void)mergeChanges:(NSNotification*)notification {
    NSManagedObjectContext *context = [self mainContext];

    // Fault all objects that have changed so that NSFetchedResultsController will see the changes.
    NSArray *objects = [[notification userInfo] objectForKey:NSUpdatedObjectsKey];
    for (NSManagedObject *object in objects) {
        [[context objectWithID:[object objectID]] willAccessValueForKey:nil];
    }

    [context performBlock:^{
        [context mergeChangesFromContextDidSaveNotification:notification];
        NSError *error = nil;
        if (![context save:&error]) {
            NSLog(@"error merging changes %@, %@", error, [error userInfo]);
        }
    }];
}