ios - NSFetchedResultsController 并不总是调用 didChangeObject :atIndexPath:forChangeType:newIndexPath: for NSFetchedResultsChangeMove

Question

我在请求中使用 NSFetchedResultsController 和 sortDescriptors 来填充包含大量结果的表。我注意到，当发生将行从表底部附近移动到顶部的更改时，根本不会调用 didChangeObject:atIndexPath:forChangeType:newIndexPath:。

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

关于问题可能是什么的任何提示，或者这只是一个模糊的 Apple 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，这似乎与我所看到的问题有关。这是我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]);
        }
    }];
}

Answer 1

事实证明，这个问题是由于对 managedObjectContext 的更改是从另一个使用 NSManagedObjectContextDidSaveNotification 的上下文传播的。这篇博文详细解释了为什么这会导致 NSFetchedResultsController 出现问题，以及如何修复它:

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

这是我上面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]);
        }
    }];
}