iphone - UIManagedDocument 嵌套上下文

Question

我一直在阅读苹果文档，但仍然有一个问题，我找不到答案。我有一个 UIManagedDocument 对象，它有两个嵌套上下文 - 主线程上的子上下文和私有(private)线程上的父上下文。接下来，我有一个服务器端。因此，当数据从服务器到达时，我想将其插入后台线程上的托管文档中。

线程安全吗？要创建异步队列，请在其中创建 NSManagedObjectContext 并将其设置为在主线程中创建的父 UIManagedDocument 的子上下文？

dispatch_queue_t fetchQ = dispatch_queue_create("Data fetcher", NULL);
dispatch_async(fetchQ, ^{
     //here goes some code for downloading data from the server

    NSManagedObjectContext * backgroundContext = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSPrivateQueueConcurrencyType];
    [backgroundContext setParentContext:self.eventDatabase.managedObjectContext]; // is this thread safe?

    //some code for creating objects in backgroundContext

    NSLog(@"inserting data in background thread");


});
dispatch_release(fetchQ);

换句话说，分配给在主线程上创建的私有(private)线程父级上创建的上下文是否是线程安全的？

Answer 1

您正在使用私有(private)并发类型。这意味着，您应该在其自己的队列上运行代码(通过 PerformBlock)。所以，如果你想这样做，你应该这样做......

NSManagedObjectContext * backgroundContext = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSPrivateQueueConcurrencyType];
backgroundContext.parentContext = self.eventDatabase.managedDocument;
backgroundContext.performBlock:^{
    //here goes some code for downloading data from the server
    //some code for creating objects in backgroundContext

    NSLog(@"inserting data in background thread");

    // Calling save on the background context will push the changes up to the document.
    NSError *error = nil;
    [backgroundContext save:&error];

    // Now, the changes will have been pushed into the MOC of the document, but
    // the auto-save will not have fired.  You must make this call to tell the document
    // that it can save recent changes.
    [self.eventDatabase updateChangeCount:UIDocumentChangeDone];
});

如果您想自己管理队列，您可能应该使用限制 MOC，您应该使用 NSConfinementConcurrencyType 或标准 init 进行初始化，因为这是默认设置。那么，它看起来就像这样......

dispatch_queue_t fetchQ = dispatch_queue_create("Data fetcher", NULL);
dispatch_async(fetchQ, ^{
    backgroundContext.parentContext = self.eventDatabase.managedDocument;

    //here goes some code for downloading data from the server

    NSManagedObjectContext * backgroundContext = [[NSManagedObjectContext alloc] init];

    // Everything else is as in the code above for the private MOC.
});
dispatch_release(fetchQ);