NSPropertyDescription class is used to define properties of an entity in a Core Data managed object model. Properties are to entities what instance variables are to classes.
- iOS 3.0+
- macOS 10.4+
- tvOS 3.0+
- watchOS 2.0+
A property describes a single value within an object managed by the Core Data Framework. There are different types of property, each represented by a subclass which encapsulates the specific property behavior—see
Note that a property name cannot be the same as any no-parameter method name of
NSManagedObject. For example, you cannot give a property the name "description". There are hundreds of methods on
NSObject which may conflict with property names—and this list can grow without warning from frameworks or other libraries. You should avoid very general words (like "font”, and “color”) and words or phrases which overlap with Cocoa paradigms (such as “isEditing” and “objectSpecifier”).
Properties—relationships as well as attributes—may be transient. A managed object context knows about transient properties and tracks changes made to them. Transient properties are ignored by the persistent store, and not just during saves: you cannot fetch using a predicate based on transients (although you can use transient properties to filter in memory yourself).
Editing Property Descriptions
Property descriptions are editable until they are used by an object graph manager (such as a persistent store coordinator). 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.