The NSAttributeDescription class is used to describe attributes of an entity described by an instance of NSEntityDescription.


NSAttributeDescription inherits from NSPropertyDescription, which provides most of the basic behavior. Instances of NSAttributeDescription 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 (NSUndefinedAttributeType) 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.


Getting and Setting Type Information

var attributeType: NSAttributeType

The type of the attribute.

var attributeValueClassName: String?

The name of the class used to represent the attribute.

Getting and Setting the Default Value

var defaultValue: Any?

The default value of the attribute.

Versioning Support

var versionHash: Data

The version hash for the attribute.

Value Transformers

var valueTransformerName: String?

The name of the transformer used to transform the attribute value.

External Binary Storage

var allowsExternalBinaryDataStorage: Bool

A Boolean value that indicates whether the attribute allows external binary storage.



Defines the possible types of NSAttributeType properties. These explicitly distinguish between bit sizes to ensure data store independence.