NSFetchedPropertyDescription class is used to define “fetched properties.” Fetched properties allow you to specify related objects through a weak, unidirectional relationship defined by a fetch request.
- iOS 3.0+
- macOS 10.4+
- tvOS 3.0+
- watchOS 2.0+
An example might be a iTunes playlist, if expressed as a property of a containing object. Songs don’t belong to a particular playlist, especially in the case that they’re on a remote server. The playlist may remain even after the songs have been deleted, or the remote server has become inaccessible. Note, however, that unlike a playlist a fetched property is static—it does not dynamically update itself as objects in the destination entity change.
The effect of a fetched property is similar to executing a fetch request yourself and placing the results in a transient attribute, although with the framework managing the details. In particular, a fetched property is not fetched until it is requested, and the results are then cached until the object is turned into a fault. You use
NSManagedObjectContext) to manually refresh the properties—this causes the fetch request associated with this property to be executed again when the object fault is next fired.
Unlike other relationships, which are all sets, fetched properties are represented by an ordered
NSArray object just as if you executed the fetch request yourself. The fetch request associated with the property can have a sort ordering. The value for a fetched property of a managed object does not support
Fetch Request Variables
Fetch requests set on an fetched property have 2 special variable bindings you can use:
$FETCHED_PROPERTY. The source refers to the specific managed object that has this property; the property refers to the
NSFetchedPropertyDescription object itself (which may have a user info associated with it that you want to use).
Editing Fetched Property Descriptions
Fetched Property 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 subjects 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.