ios - 核心数据中可变和不可变托管对象模型有什么区别？

Question

阅读 RestKit 文档后 RKManagedObjectStore我对 createPersistentStoreCoordinator 方法感到困惑，因为在该方法的描述中有下一个警告:

**Warning:** Creating the persistent store coordinator will
render the managed object model    immutable. Attempts to 
use functionality that requires  a mutable managed object model 
after the persistent store coordinator has been created
will raise an application error.

我不明白 immutable 托管对象模型是什么意思？无论是在官方 Core Data 文档中还是在 Internet 上，我都找不到关于这个主题的任何信息。有人可以给我解释一下 mutable 和 immutable 托管对象模型之间的区别吗？为什么创建持久存储协调器会呈现不可变的托管对象模型？什么功能需要可变的托管对象模型？

提前致谢。

Answer 1

您可以在代码中更改 NSManagedObjectModel(例如添加实体和属性)，它被称为可变的。但是一旦您将对象模型附加到持久存储协调器，您就不能再更改它了 - 它已经变得不可变了。

NSManagedObjectModel 的文档中也对此进行了描述:

Editing Models Programmatically Managed object models are editable until they are used by an object graph manager (a managed object context or a persistent store coordinator). This allows you to create or modify them dynamically. However, once a model is being used, it must not be changed. This is enforced at runtime—when the object manager first fetches data using a model, the whole of that model becomes uneditable. Any attempt to mutate a model or any of its sub-objects after that point causes an exception to be thrown. If you need to modify a model that is in use, create a copy, modify the copy, and then discard the objects with the old model.