objective-c – 如何创建一个小孩NSManagedObjectContext？

NSManagedObjectContext *moc = [[NSManagedObjectContext alloc]
       initWithConcurrencyType:NSConfinementConcurrencyType];

您通过设置其属性将MOC归属到另一个MOC：

moc.parentContext = someOtherMocThatIsNowMyParent;

在这里,孩子选择了父母.我确信我的孩子希望他们是NSManagedObjectContexts.父上下文必须是NSPrivateQueueConcurrencyType或NSMainQueueConcurrencyType.

您可以创建一个“绑定”到私有队列的MOC：

NSManagedObjectContext *moc = [[NSManagedObjectContext alloc]
       initWithConcurrencyType:NSPrivateQueueConcurrencyType];

这意味着您只能通过performBlock或performBlockAndWait API访问它.您可以从任何线程调用这些方法,因为它们将确保正确序列化块中的代码.例如…

NSManagedObjectContext *moc = [[NSManagedObjectContext alloc]
       initWithConcurrencyType:NSPrivateQueueConcurrencyType];
moc.parentContext = someOtherMocThatIsNowMyParent;
[moc performBlock:^{
    // All code running in this block will be automatically serialized
    // with respect to all other performBlock or performBlockAndWait
    // calls for this same MOC.
    // Access "moc" to your heart's content inside these blocks of code.
}];

performBlock和performBlockAndWait之间的区别在于,performBlock将创建一个代码块,并使用Grand Central Dispatch进行调度,以便将来在某些未知线程上被异步执行.方法调用将立即返回.

performBlockAndWait将与所有其他performBlock调用进行一些魔术同步,并且当在此之前呈现的所有块完成时,该块将执行.调用线程将挂起,直到此调用完成.

您还可以创建一个“绑定”到主线程的MOC,就像私有并发.

NSManagedObjectContext *moc = [[NSManagedObjectContext alloc]
       initWithConcurrencyType:NSMainQueueConcurrencyType];
moc.parentContext = someOtherMocThatIsNowMyParent;
[moc performBlock:^{
    // All code running in this block will be automatically serialized
    // with respect to all other performBlock or performBlockAndWait
    // calls for this same MOC.  Furthermore,it will be serialized with
    // respect to the main thread as well,so this code will run in the
    // main thread -- which is important if you want to do UI work or other
    // stuff that requires the main thread.
}];

这意味着如果你知道你在主线程,或者通过performBlock或performBlockAndWait API,你应该直接访问它.

现在,您可以通过performBlock方法使用“主并发”MOC,或者直接知道您已经在主线程中运行.