使用NSManagedObjectContextWillSaveNotification标识已更改对象的旧值和新值 [英] Identifying old and new values on changed objects with NSManagedObjectContextWillSaveNotification

问题描述

我正在跟踪核心数据上下文中对象的更改，跟踪已更改的属性的名称以及旧值和新值。

ve注册为 NSManagedObjectContextWillSaveNotification 在保存即将发生时接收通知，并可以从上下文中拉出插入/更新/删除的对象...然后我可以看到

但是，我在检索旧值时遇到困难...

例如，我有一个跟踪位置的对象，因此其中一个更改返回：

  po [obj changedValues] 
 {
 originX = 260; 
 originY = 180; 
} 
  

这给出了对象上已更改的属性的新值。为了尝试获得旧的值，我然后使用 changedValuesForCurrentEvent ，根据文档应该返回

一个包含自上次发布以来发生变化的持久
属性的键和旧值的字典
NSManagedObjectContextObjectsDidChangeNotification

但是，当我尝试这个，它回来为空...：

  po [obj changedValuesForCurrentEvent] 
 {
} 
  

如何捕获旧值和新值？

解决方案

NSManagedObjectContextObjectsDidChangeNotification 在您每次更改受管对象上的值时调用，即使您尚未保存更改。 NSManagedObjectContextWillSaveNotification 稍后调用时保存。所以顺序是：

1. 您更改了一些属性 - > NSManagedObjectContextObjectsDidChangeNotification ，您可以使用 changedValuesForCurrentEvent 查看已更改的内容。


2. 稍后，您保存更改。已发布 NSManagedObjectContextWillSaveNotification 。您可以调用 changedValuesForCurrentEvent ，但它没有帮助，因为它返回自上次更改通知以来的更改 。自上次更改通知后没有任何更改。如果有，你会收到另一个。

如果您希望旧的值和您希望在发布即时保存通知时获取它们，您有以下几个选项：

• 监听 NSManagedObjectContextObjectsDidChangeNotification 。缓存一些集合对象（可能 NSDictionary ）中的更改的信息。然后，当 NSManagedObjectContextWillSaveNotification 发生时，查找这些更改，处理它们，并清除更改缓存。 OR ...


• 当您获得 NSManagedObjectContextWillSaveNotification 时，创建第二个本地管理对象上下文。由于这是将保存通知，您仍然可以获取旧的值。因此，获取保存的每个对象，并比较之前和之后的值，看看有什么不同。

I am trying to track changes to objects in a core data context, tracking the name of properties that have changed along with the old and new values.

I've registered for NSManagedObjectContextWillSaveNotification to receive a notification when a save is about to occur, and can pull out the inserted/updated/deleted objects from the context... I can then see the changed values using .changedValues.

However, I am having difficulties retrieving the old values...

As an example, I have an object that tracks a position, and so one of the changes comes back with:

po [obj changedValues]
{
    originX = 260;
    originY = 180;
}

This gives me the new values for the properties that have changed on the object. To try and get the old values, I'm then using changedValuesForCurrentEvent, which according to the docs should return

"a dictionary containing the keys and old values of persistent properties that have changed since the last posting of NSManagedObjectContextObjectsDidChangeNotification"

However, when I try this, it is coming back empty...:

po [obj changedValuesForCurrentEvent]
{
}

How can I capture the old and new values?

解决方案

You're mixing up your notifications. NSManagedObjectContextObjectsDidChangeNotification gets called any time you change values on a managed object, even though you haven't saved changes yet. NSManagedObjectContextWillSaveNotification gets called later on when you save. So the sequence is:

1. You change some attributes --> NSManagedObjectContextObjectsDidChangeNotification is posted, and you can use changedValuesForCurrentEvent to see what changed.
2. Later, you save changes. NSManagedObjectContextWillSaveNotification is posted. You can call changedValuesForCurrentEvent, but it's not helpful because it returns changes since the last did-change notification. There are no changes since the last did-change notification. If there were, you would have received another one. That method is documented to be useful on a did-change notification, not on a will-save notification.

If you want the old values and you want to get them when the will-save notification is posted, you have a couple of options:

• Listen for NSManagedObjectContextObjectsDidChangeNotification. Cache information about changes in some collection object (probably NSDictionary). Then when NSManagedObjectContextWillSaveNotification happens, look up those changes, process them, and clear the change cache. OR...
• When you get NSManagedObjectContextWillSaveNotification, create a second local managed object context. Since this is a will save notification, you can still fetch the old values. So, fetch each object that's getting saved and compare the before and after values to see what's different.

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