下面的代码在没有最后一条语句的情况下运行良好。但是，对于最后一行，Xcode 停止并显示以下消息:

CoreData`+[NSManagedObjectContext Multithreading_Violation_AllThatIsLeftToUsIsHonor]:

 NSManagedObjectContext *context = GLOBAL_appDelegate.coreDataHelper.contextBackground;
    [context performBlock:^{
         NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
         [fetchRequest setEntity:[NSEntityDescription entityForName:@"CdTag" inManagedObjectContext:context]];
         NSPredicate *predicate = [NSPredicate predicateWithFormat:@"title BEGINSWITH[cd] %@", @"webpages"];
         [fetchRequest setPredicate:predicate];
         NSArray *cdTags = [context executeFetchRequest:fetchRequest error:nil];
         NSLog(@"number of tags: %li", cdTags.count);
         CdTag *cdTag = [cdTags objectAtIndex:0];

         // Accessing a property causes the assertion warning to be shown
         cdTag.title;
    }];

我使用的是 Xcode 7 beta 5，并且启用了多线程断言。

上下文定义如下:

_model = [NSManagedObjectModel mergedModelFromBundles:nil];
_coordinator = [[NSPersistentStoreCoordinator alloc] initWithManagedObjectModel:_model];
_context = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSMainQueueConcurrencyType];
[_context setPersistentStoreCoordinator:_coordinator];

_contextBackground = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSPrivateQueueConcurrencyType];
[_contextBackground setParentContext:_context];

这是 Xcode 错误还是我做错了什么？

最佳答案

您能否也添加上下文初始化发生的代码？

您收到的错误与从线程/队列而不是初始化线程/队列访问托管对象上下文有关。

通常发生的情况是，托管对象上下文是使用默认/遗留的“限制类型”并发类型初始化的，

performBlock: 

和

performBlockAndWait:

方法实际上不会做他们宣传的事情，即确保上下文在初始化时关联的队列上被访问。

如果我上面建议的是实际发生的事情，您可以(假设其他一切都已到位)通过像这样初始化您的上下文来解决这个问题:

_managedObjectContext = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSMainQueueConcurrencyType];

或

_managedObjectContext = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSPrivateQueueConcurrencyType];

这是关于并发策略的文档:

When you create a managed object context using initWithConcurrencyType:, you have three options for its thread (queue) association

Confinement (NSConfinementConcurrencyType)

For backwards compatibility, this is the default. You promise that context will not be used by any thread other than the one on which you created it. In general, to make the behavior explicit you’re encouraged to use one of the other types instead.

You can only use this concurrency type if the managed object context’s parent store is a persistent store coordinator.

Private queue (NSPrivateQueueConcurrencyType)

The context creates and manages a private queue.

Main queue (NSMainQueueConcurrencyType)

The context is associated with the main queue, and as such is tied into the application’s event loop, but it is otherwise similar to a private queue-based context. You use this queue type for contexts linked to controllers and UI objects that are required to be used only on the main thread.

If you use contexts using the confinement pattern, you send the contexts messages directly; it’s up to you to ensure that you send the messages from the right queue.

You use contexts using the queue-based concurrency types in conjunction with performBlock: and performBlockAndWait:.

Apple Docs

附带说明一下，您收到的错误消息是 Cocoa 中最后几个“彩蛋”之一。

关于ios - 为什么在尝试访问已获取对象的属性时会出现 CoreData 多线程冲突？，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/31916154/