Core Data 合并两个托管对象上下文 [英] Core Data merge two Managed Object Context
问题描述
我的 Cocoa/Application 在主线程上有一个托管对象上下文.当我需要更新我的数据时，我的程序将:
My Cocoa/Application has a Managed Object Context on the main thread. When I need to update my data my program will:
- 开始一个新话题
- 从服务器接收新数据
- 创建新的托管对象上下文
- 向主线程发送通知以合并两个上下文
这是在主线程上接收通知的函数
This is the function that receive the notification on the main thread
- (void)loadManagedObjectFromNotification:(NSNotification *)saveNotification
{
if ([NSThread isMainThread]) {
[self.managedObjectContext mergeChangesFromContextDidSaveNotification:saveNotification];
} else {
[self performSelectorOnMainThread:@selector(loadManagedObjectFromNotification:) withObject:saveNotification waitUntilDone:YES];
}
}
我没有收到任何错误.我的问题是合并结果，它实际上连接了两个上下文中的托管对象.
I do not receive any error. My problem is the merge result, it actually concatenate Managed Objects from both context.
我的实体是一个非常简单的属性和关系列表.
My Entity are a really simple list of attribute and relationship.
也许合并需要一些说明以了解更新的托管对象何时不是新的，而是第一个的编辑版本.我想在某处我需要指定一种方式来明确标识一个实体(例如，一个属性可以像一个 ID 一样)和一个类似于合并策略的东西(如果 2 个托管对象代表同一个对象，则采用带有 lastModificationDate 的对象更多最近).
Maybe the merge need some instructions in order to understand when an updated Managed Object IS NOT a new one, but a edited version of the first one. I imagine that somewhere I need to specify a way to univocally identify an Entity, (an attribute for example can act like an ID) and something like a merge policy (if 2 managed object represent the same object, take the one with the lastModificationDate more recent).
我只需要了解如何正确合并 2 个上下文，以便为每个对象拥有一个更新的副本.
I just need to understand how to correctly merge the 2 contexts in order to have a single updated copy for each object.
问题现在对我来说很清楚.2 上下文有很大的不同:ObjectID.当主线程上的上下文使用持久存储协调器获取 ManagedObjects 时，第二个线程通过获取远程 URL 创建这些对象.即使对象的内容相同，它们也会有2个不同的objectID.
The problem is now clear to me. The 2 context has a big difference: the ObjectID. While the context on the main thread fetched the ManagedObjects with the Persistent Store coordinator, the second thread create those object by fetching a remote URL. Even if the objects have the same contents, they will have 2 different objectID.
我的对象已经有一个唯一标识符，我可以使用 setObjectId 来设置这个值.(Apple 文档说这不是一个好主意.
My objects had already an unique identificator, I could use setObjectId in order to set this value. (Apple documentation says this is NOT a good idea).
推荐答案
为了正确合并上下文，您需要执行以下操作.首先，您不需要自己的通知.对上下文执行保存操作会自动将以下通知转发给注册的观察者:
Here is what you need to do in order to correctly merge the contexts. First, you do not need your own notification. Performing a save operation on a context automatically forwards the following notification to registered observers:
NSManagedObjectContextDidSaveNotification
因此，您需要做的就是:
Therefore, all you need to do is:
1) 在你的主线程中，可能在 viewDidLoad 方法中，注册这个通知:
1) in your main thread, may be in the viewDidLoad method, register for this notification:
[[NSNotificationCenter defaultCenter] addObserver:self
selector:@selector(contextDidSave:)
name:NSManagedObjectContextDidSaveNotification
object:nil];
2) 在主线程中实现 contextDidSave: 方法如下:
2) implement the contextDidSave: method in your main thread as follows:
- (void)contextDidSave:(NSNotification *)notification
{
SEL selector = @selector(mergeChangesFromContextDidSaveNotification:);
[managedObjectContext performSelectorOnMainThread:selector withObject:notification waitUntilDone:YES];
}
3) 在您的 dealloc 方法中添加以下内容:
3) in your dealloc method add the following:
[[NSNotificationCenter defaultCenter] removeObserver:self];
4) 使用类似以下方法在您的另一个线程中创建一个新上下文:
4) create a new context in your other thread using something like the following method:
- (NSManagedObjectContext*)createNewManagedObjectContext
{
NSManagedObjectContext *moc = [[NSManagedObjectContext alloc] init];
[moc setPersistentStoreCoordinator:[self persistentStoreCoordinator]];
[moc setUndoManager:nil];
return [moc autorelease];
}
5) 收到新数据后，处理这种情况的正确方法是使用托管对象 ID.由于托管对象 ID 是线程安全的，您可以将它们从主线程传递到另一个线程，然后使用 existingObjectWithID:error: 检索与特定 ID 关联的对象，更新它并保存上下文.现在合并将按您的预期运行.或者，如果您事先不知道必须在线程之间传递哪些托管对象 ID，那么在您的另一个线程中，您只需使用谓词获取对象以检索与从服务器检索到的对象对应的对象，然后更新它们并保存上下文.
5) upon receiving the new data, the proper way to handle this situation is the use of managed object IDs. Since managed object IDs are thread safe, you can pass them from your main thread to the other thread, then use existingObjectWithID:error: to retrieve the object associated to a specific ID, update it and save the context. Now the merge will operate as you expect. Alternatively, if you do not know in advance what managed object IDs must be passed between the threads, then in your other thread you simply fetch the objects using a predicate to retrieve the ones corresponding to the objects retrieved from the server, then you update them and save the context.
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