NSAttributeDescription Class Reference

Inherits from	NSPropertyDescription : NSObject
Conforms to	NSCoding (NSPropertyDescription) NSCopying (NSPropertyDescription) NSObject (NSObject)
Framework	/System/Library/Frameworks/CoreData.framework
Availability	Available in Mac OS X v10.4 and later.
Declared in	NSAttributeDescription.h
Companion guides	Core Data Programming Guide Core Data Utility Tutorial
Related sample code	Core Data HTML Store CoreRecipes CustomAtomicStoreSubclass

Overview

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.

Note: Default values set for attributes are retained by a managed object model, not copied. This means that attribute values do not have to implement the NSCopying protocol, however it also means that you should not modify any objects after they have been set as default values.

Tasks

Getting and Setting Type Information

Getting and Setting the Default Value

Versioning Support

Value Transformers

Instance Methods

attributeType

Returns the type of the receiver.

- (NSAttributeType)attributeType

Return Value

The type of the receiver.

Availability

Available in Mac OS X v10.4 and later.

Related Sample Code

Declared In

NSAttributeDescription.h

attributeValueClassName

Returns the name of the class used to represent the receiver.

- (NSString *)attributeValueClassName

Return Value

The name of the class used to represent the receiver, as a string.

Availability

Available in Mac OS X v10.4 and later.

Declared In

NSAttributeDescription.h

defaultValue

Returns the default value of the receiver.

- (id)defaultValue

Return Value

The default value of the receiver.

Availability

Available in Mac OS X v10.4 and later.

Declared In

NSAttributeDescription.h

setAttributeType:

Sets the type of the receiver.

- (void)setAttributeType:(NSAttributeType)type

Parameters

type: An NSAttributeType constant that specifies the type for the receiver.

Special Considerations

This method raises an exception if the receiver’s model has been used by an object graph manager.

Availability

Available in Mac OS X v10.4 and later.

Declared In

NSAttributeDescription.h

setAttributeValueClassName:

Sets the name of the class used to represent the receiver.

- (void)setAttributeValueClassName:(NSString *)className

Parameters

className: The name of the class used to represent the receiver.

Discussion

If you set the value class name, Core Data can check the class of any instance set as the value of an attribute.

Availability

Available in Mac OS X v10.5 and later.

Declared In

NSAttributeDescription.h

setDefaultValue:

Sets the default value of the receiver.

- (void)setDefaultValue:(id)value

Parameters

value: The default value for the receiver.

Discussion

Default values are retained by a managed object model, not copied. This means that attribute values do not have to implement the NSCopying protocol, however it also means that you should not modify any objects after they have been set as default values.

Special Considerations

This method raises an exception if the receiver’s model has been used by an object graph manager.

Availability

Available in Mac OS X v10.4 and later.

Declared In

NSAttributeDescription.h

setValueTransformerName:

Sets the name of the transformer to use to transform the attribute value.

- (void)setValueTransformerName:(NSString *)string

Parameters

string: The name of the transformer to use to transform the attribute value. The transformer must output an NSData object from transformedValue: and must allow reverse transformations.

Discussion

The receiver must be an attribute of type NSTransformedAttributeType.

If this value is not set, or is set to nil, Core Data will default to using a transformer which uses NSCoding to archive and unarchive the attribute value.

Availability

Available in Mac OS X v10.5 and later.

Declared In

NSAttributeDescription.h

valueTransformerName

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

- (NSString *)valueTransformerName

Return Value

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

Discussion

The receiver must be an attribute of type NSTransformedAttributeType.

Availability

Available in Mac OS X v10.5 and later.

Related Sample Code

CustomAtomicStoreSubclass

Declared In

NSAttributeDescription.h

versionHash

Returns the version hash for the receiver.

- (NSData *)versionHash

Return Value

The version hash for the receiver.

Discussion

The version hash is used to uniquely identify an attribute based on its configuration. This value includes the versionHash information from NSPropertyDescription and the attribute type.

Availability

Available in Mac OS X v10.5 and later.

Declared In

NSAttributeDescription.h

Constants

NSAttributeType

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

typedef enum {
NSUndefinedAttributeType = 0,
NSInteger16AttributeType = 100,
NSInteger32AttributeType = 200,
NSInteger64AttributeType = 300,
NSDecimalAttributeType = 400,
NSDoubleAttributeType = 500,
NSFloatAttributeType = 600,
NSStringAttributeType = 700,
NSBooleanAttributeType = 800,
NSDateAttributeType = 900,
NSBinaryDataAttributeType = 1000,
NSTransformableAttributeType = 1800,
NSObjectIDAttributeType = 2000
} NSAttributeType;

Constants

NSUndefinedAttributeType

Specifies an undefined attribute type.

NSUndefinedAttributeType is valid for transient properties—Core Data will still track the property as an id value and register undo/redo actions, and so on. NSUndefinedAttributeType is illegal for non-transient properties.

Available in Mac OS X v10.4 and later.