NSAttribute class is used to describe attributes of an entity described by an instance of
- iOS 3.0+
- macOS 10.4+
- tvOS 3.0+
- watchOS 2.0+
NSAttribute inherits from
NSProperty, which provides most of the basic behavior. Instances of
NSAttribute are used to describe attributes, as distinct from relationships. The class adds the ability to specify the attribute type, and to specify a default value. In a managed object model, you must specify the type of all attributes—you can only use the undefined attribute type (
NSUndefined) for transient attributes.
Editing Attribute Descriptions
Attribute descriptions are editable until they are used by an object graph manager. This allows you to create or modify them dynamically. However, once a description is used (when the managed object model to which it belongs is associated with a persistent store coordinator), it must not (indeed cannot) be changed. This is enforced at runtime: any attempt to mutate a model or any of its sub-objects after the model is associated with a persistent store coordinator 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.