没有在dispatch_async中调用UITableView endUpdates [英] UITableView endUpdates not being called in dispatch_async

问题描述

我有几个表视图，这些表视图将 JSON 请求发送到服务器，将结果存储在核心数据中，并使用NSFetchedResultsController显示它们.我正在对GCD进行如下实验:

I have several table views that send JSON requests to a server, store the results in core data, and display them using an NSFetchedResultsController. I was experimenting with GCD as follows:

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{
    // Perform JSON requests.
    dispatch_async(dispatch_get_main_queue(), ^{
        [theTableView reloadData];
    });
});

但是，这会导致UI中发生一些奇怪的事情.新的托管对象将呈现空白单元格，删除的托管对象将导致单元格重叠等.

However, this would cause some weird things to happen in the UI. New managed objects would render blank cells, deleted managed objects would cause cells to overlap, etc.

但是，我发现如果执行此操作，一切都将正确呈现.

However, I found that if I did this, everything would render correctly.:

- (void)controllerDidChangeContent:(NSFetchedResultsController *)controller
{
    dispatch_async(dispatch_get_main_queue(), ^{
        [theTableView endUpdates];
    });
}

我想知道的是，为什么这是必要的?由于它是[theTableView reloadData]的结果而触发，为什么它不自动包含在主队列中?我以为也许是因为我没有明确地称呼它.在那种情况下，我是否必须同样包装所有功能?

What I wanted to know is, why is this necessary? Since it fires as a result of [theTableView reloadData], why isn't it automatically included in the main queue? I thought maybe that it was because I didn't call it explicitly. In that case, do I have to wrap all my functions similarly?

推荐答案

我假定，您使用的是单独线程中的主NSManagedObjectContext， 不允许.对于后台导入，您必须创建一个单独的托管对象上下文， 导入该上下文中的对象，然后将更改保存/合并到主上下文中.

I assume that you use the main NSManagedObjectContext from a separate thread, which is not allowed. For a background import, you have to create a separate managed object context, import the objects in that context, and then save/merge the changes to the main context.

一种可能性是使用主要管理对象上下文的子上下文:

One possibility is to use a child context of the main managed object context:

NSManagedObjectContext *childContext = [[NSManagedObjectContext alloc]
                         initWithConcurrencyType:NSPrivateQueueConcurrencyType];
childContext.parentContext = mainManagedObjectContext;
[childContext performBlock:^{
    // Perform JSON requests.
    // Save objects to childContext.

    NSError *error;
    BOOL success = [childContext save:&error];
}];

私有并发类型"的上下文创建其自己的队列/线程，以便执行performBlock 在后台(而且您不必/不必自己创建队列).

A context of the "private concurrency type" creates its own queue/thread, so that the performBlock is executed in the background (and you need/must not create a queue yourself).

保存childContext会将更改合并到父级mainManagedObjectContext. 对于获取的结果控制器来说，这应该足够了 更改，并更新表格视图.

Saving childContext merges the changes up to the parent mainManagedObjectContext. This should be sufficent for the fetched results controller to get notified of the changes, and update the table view.

请参阅 OS X v10.7和iOS 5.0的核心数据发行说明 有关托管对象上下文并发的更多信息， 和嵌套上下文.

See Core Data Release Notes for OS X v10.7 and iOS 5.0 for more information about managed object context concurrency, and nested contexts.

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