ios - Coredata 跨线程访问新发现?

Question

最近读了Coredata第一三方库Magical Record作者的一篇文章链接描述,对Coredata访问有了新的疑惑，performBlockAPI 是如何保证线程安全的？ 众所周知 ， NSManagedObjectContext（简称moc） 是不允许跨线程访问的 ...

PHP中文网 · Answer

Thank you very much for your answer. But there seem to be two more questions:

1. The author’s article did not directly say “Apple’s new API uses GCD”. Where is the sentence However, now, as more of you (and the heart of MagicalRecord) is using GCD, 是说MagicalRecord的核心是用了GCD,可我翻遍MagicalRecord的源码,似乎并没有发现使用GCD调度队列来实现异步操作,核心都是用了performBlock和performBlockAndWait来实现对MOC的异步操作.所以我猜测作者的意思是performBlock会与GCD有关，不然为何明明没有用到GCD,却因为GCD的特性而弃用之前的接口 contextForCurrentThread in the original article?

2. Put aside question 1, part of Apple’s documentation says this:

The NSPrivateQueueConcurrencyType configuration creates its own queue upon initialization and can be used only on that queue. Because the queue is private and internal to the NSManagedObjectContext instance, it can only be accessed through the performBlock: and the performBlockAndWait: methods.

NSPrivateQueueConcurrencyType type MOC will create its own queue during initialization. This queue is private within this MOC. So can it be understood as:
I created a content under NSPrivateQueueConcurrencyType的MOC,它会创建一个私有队列,在线程b中去管理performBlock in thread a.

若有代码:

  NSManagedObjectContext *moc = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSPrivateQueueConcurrencyType];
    [moc performBlock:^{
        // ... do things
        NSError *error;
        [moc save:&error];
    }];

If the context is created in thread a, it should belong to thread a, but it is accessed in thread b of the queue. Isn’t it a cross-thread access?

Does

conflict with performBlock which guarantees execution in the thread to which the current context belongs? How is it guaranteed? Is b and a the same thread when creating a private queue? How to do it?

PHPz · Answer

I read the article you linked to, "But through the above article, the author of Magical Record pointed out that Apple's new api uses GCD, and the concurrency in GCD is implemented through queues." There is no such thing. I saw the description in your sentence above that "Apple's new API uses GCD".

The following is the referenced Apple document. The performBlock method will ensure that it is executed in the thread to which the current context belongs:

performBlock: and performBlockAndWait: ensure the block operations are executed on the queue specified for the context. The performBlock: method returns immediately and the context executes the block methods on its own thread. With the performBlockAndWait: method, the context still executes the block methods on its own thread, but the method doesn’t return until the block is executed.