NSKeyValueCoding Protocol Reference

(informal protocol)

Framework
/System/Library/Frameworks/Foundation.framework
Availability
Available in OS X v10.0 and later.
Companion guide
Declared in
NSKeyValueCoding.h

Overview

The NSKeyValueCoding informal protocol defines a mechanism by which you can access the properties of an object indirectly by name (or key), rather than directly through invocation of an accessor method or as instance variables. Thus, all of an object’s properties can be accessed in a consistent manner.

The basic methods for accessing an object’s values are setValue:forKey:, which sets the value for the property identified by the specified key, and valueForKey:, which returns the value for the property identified by the specified key. The default implementation uses the accessor methods normally implemented by objects (or to access instance variables directly if need be).

Tasks

Getting Values

Setting Values

Changing Default Behavior

Validation

Deprecated Methods

Class Methods

accessInstanceVariablesDirectly

Returns a Boolean value that indicates whether the key-value coding methods should access the corresponding instance variable directly on finding no accessor method for a property.

+ (BOOL)accessInstanceVariablesDirectly
Return Value

YES if the key-value coding methods should access the corresponding instance variable directly on finding no accessor method for a property, otherwise NO.

Discussion

The default returns YES. Subclasses can override it to return NO, in which case the key-value coding methods won’t access instance variables.

Availability
  • Available in OS X v10.0 and later.
Related Sample Code
Declared In
NSKeyValueCoding.h

Instance Methods

dictionaryWithValuesForKeys:

Returns a dictionary containing the property values identified by each of the keys in a given array.

- (NSDictionary *)dictionaryWithValuesForKeys:(NSArray *)keys
Parameters
keys

An array containing NSString objects that identify properties of the receiver.

Return Value

A dictionary containing as keys the property names in keys, with corresponding values being the corresponding property values.

Discussion

The default implementation invokes valueForKey: for each key in keys and substitutes NSNull values in the dictionary for returned nil values.

Availability
  • Available in OS X v10.3 and later.
Declared In
NSKeyValueCoding.h

mutableArrayValueForKey:

Returns a mutable array proxy that provides read-write access to an ordered to-many relationship specified by a given key.

- (NSMutableArray *)mutableArrayValueForKey:(NSString *)key
Parameters
key

The name of an ordered to-many relationship.

Return Value

A mutable array proxy that provides read-write access to the ordered to-many relationship specified by key.

Discussion

Objects added to the mutable array become related to the receiver, and objects removed from the mutable array become unrelated. The default implementation recognizes the same simple accessor methods and array accessor methods as valueForKey:, and follows the same direct instance variable access policies, but always returns a mutable collection proxy object instead of the immutable collection that valueForKey: would return.

The search pattern that mutableArrayValueForKey: uses is described in Accessor Search Implementation Details in Key-Value Coding Programming Guide.

Availability
  • Available in OS X v10.3 and later.
Declared In
NSKeyValueCoding.h

mutableArrayValueForKeyPath:

Returns a mutable array that provides read-write access to the ordered to-many relationship specified by a given key path.

- (NSMutableArray *)mutableArrayValueForKeyPath:(NSString *)keyPath
Parameters
keyPath

A key path, relative to the receiver, to an ordered to-many relationship.

Return Value

A mutable array that provides read-write access to the ordered to-many relationship specified by keyPath.

Discussion

See mutableArrayValueForKey: for additional details.

Availability
  • Available in OS X v10.3 and later.
Declared In
NSKeyValueCoding.h

mutableOrderedSetValueForKey:

Returns a mutable ordered set that provides read-write access to the uniquing ordered to-many relationship specified by a given key.

- (NSMutableOrderedSet *)mutableOrderedSetValueForKey:(NSString *)key
Parameters
key

The name of a uniquing ordered to-many relationship.

Return Value

A mutable ordered set that provides read-write access to the uniquing to-many relationship specified by key.

Discussion

Objects added to the mutable set proxy become related to the receiver, and objects removed from the mutable set become unrelated. The default implementation recognizes the same simple accessor methods and set accessor methods as valueForKey:, and follows the same direct instance variable access policies, but always returns a mutable collection proxy object instead of the immutable collection that valueForKey: would return.

The search pattern that mutableOrderedSetValueForKey: uses is described in Accessor Search Implementation Details in Key-Value Coding Programming Guide.

Availability
  • Available in OS X v10.7 and later.
Declared In
NSKeyValueCoding.h

mutableOrderedSetValueForKeyPath:

Returns a mutable ordered set that provides read-write access to the uniquing ordered to-many relationship specified by a given key path.

- (NSMutableOrderedSet *)mutableOrderedSetValueForKeyPath:(NSString *)keyPath
Parameters
keyPath

A key path, relative to the receiver, to a uniquing ordered to-many relationship represented by a set.

Return Value

A mutable ordered set that provides read-write access to the uniquing to-many relationship specified by keyPath.

Discussion

See mutableOrderedSetValueForKey: for additional details.

Availability
  • Available in OS X v10.7 and later.
Declared In
NSKeyValueCoding.h

mutableSetValueForKey:

Returns a mutable set proxy that provides read-write access to the unordered to-many relationship specified by a given key.

- (NSMutableSet *)mutableSetValueForKey:(NSString *)key
Parameters
key

The name of an unordered to-many relationship.

Return Value

A mutable set that provides read-write access to the unordered to-many relationship specified by key.

Discussion

Objects added to the mutable set proxy become related to the receiver, and objects removed from the mutable set become unrelated. The default implementation recognizes the same simple accessor methods and set accessor methods as valueForKey:, and follows the same direct instance variable access policies, but always returns a mutable collection proxy object instead of the immutable collection that valueForKey: would return.

The search pattern that mutableSetValueForKey: uses is described in Accessor Search Implementation Details in Key-Value Coding Programming Guide.

Availability
  • Available in OS X v10.4 and later.
Declared In
NSKeyValueCoding.h

mutableSetValueForKeyPath:

Returns a mutable set that provides read-write access to the unordered to-many relationship specified by a given key path.

- (NSMutableSet *)mutableSetValueForKeyPath:(NSString *)keyPath
Parameters
keyPath

A key path, relative to the receiver, to an unordered to-many relationship.

Return Value

A mutable set that provides read-write access to the unordered to-many relationship specified by keyPath.

Discussion

See mutableSetValueForKey: for additional details.

Availability
  • Available in OS X v10.4 and later.
Declared In
NSKeyValueCoding.h

setNilValueForKey:

Invoked by setValue:forKey: when it’s given a nil value for a scalar value (such as an int or float).

- (void)setNilValueForKey:(NSString *)key
Parameters
key

The name of one of the receiver's properties.

Discussion

Subclasses can override this method to handle the request in some other way, such as by substituting 0 or a sentinel value for nil and invoking setValue:forKey: again or setting the variable directly. The default implementation raises an NSInvalidArgumentException.

Availability
  • Available in OS X v10.3 and later.
Declared In
NSKeyValueCoding.h

setValue:forKey:

Sets the property of the receiver specified by a given key to a given value.

- (void)setValue:(id)value forKey:(NSString *)key
Parameters
value

The value for the property identified by key.

key

The name of one of the receiver's properties.

Discussion

If key identifies a to-one relationship, relate the object specified by value to the receiver, unrelating the previously related object if there was one. Given a collection object and a key that identifies a to-many relationship, relate the objects contained in the collection to the receiver, unrelating previously related objects if there were any.

The search pattern that setValue:forKey: uses is described in Accessor Search Implementation Details in Key-Value Coding Programming Guide.

In a reference-counted environment, if the instance variable is accessed directly, value is retained.

Availability
  • Available in OS X v10.3 and later.
Declared In
NSKeyValueCoding.h

setValue:forKeyPath:

Sets the value for the property identified by a given key path to a given value.

- (void)setValue:(id)value forKeyPath:(NSString *)keyPath
Parameters
value

The value for the property identified by keyPath.

keyPath

A key path of the form relationship.property (with one or more relationships): for example “department.name” or “department.manager.lastName.”

Discussion

The default implementation of this method gets the destination object for each relationship using valueForKey:, and sends the final object a setValue:forKey: message.

Special Considerations

When using this method, and the destination object does not implement an accessor for the value, the default behavior is for that object to retain value rather than copy or assign value.

Availability
  • Available in OS X v10.3 and later.
Declared In
NSKeyValueCoding.h

setValue:forUndefinedKey:

Invoked by setValue:forKey: when it finds no property for a given key.

- (void)setValue:(id)value forUndefinedKey:(NSString *)key
Parameters
value

The value for the key identified by key.

key

A string that is not equal to the name of any of the receiver's properties.

Discussion

Subclasses can override this method to handle the request in some other way. The default implementation raises an NSUndefinedKeyException.

Availability
  • Available in OS X v10.3 and later.
Declared In
NSKeyValueCoding.h

setValuesForKeysWithDictionary:

Sets properties of the receiver with values from a given dictionary, using its keys to identify the properties.

- (void)setValuesForKeysWithDictionary:(NSDictionary *)keyedValues
Parameters
keyedValues

A dictionary whose keys identify properties in the receiver. The values of the properties in the receiver are set to the corresponding values in the dictionary.

Discussion

The default implementation invokes setValue:forKey: for each key-value pair, substituting nil for NSNull values in keyedValues.

Availability
  • Available in OS X v10.3 and later.
Related Sample Code
Declared In
NSKeyValueCoding.h

validateValue:forKey:error:

Returns a Boolean value that indicates whether the value specified by a given pointer is valid for the property identified by a given key.

- (BOOL)validateValue:(id *)ioValue forKey:(NSString *)key error:(NSError **)outError
Parameters
ioValue

A pointer to a new value for the property identified by key. This method may modify or replace the value in order to make it valid.

key

The name of one of the receiver's properties. The key must specify an attribute or a to-one relationship.

outError

If validation is necessary and ioValue is not transformed into a valid value, upon return contains an NSError object that describes the reason that ioValue is not a valid value.

Return Value

YES if *ioValue is a valid value for the property identified by key, or if the method is able to modify the value to *ioValue to make it valid; otherwise NO.

Discussion

The default implementation of this method searches the class of the receiver for a validation method whose name matches the pattern validate<Key>:error:. If such a method is found it is invoked and the result is returned. If no such method is found, YES is returned.

The sender of the message is never given responsibility for releasing ioValue or outError.

See “Key-Value Validation” for more information.

Availability
  • Available in OS X v10.3 and later.
Declared In
NSKeyValueCoding.h

validateValue:forKeyPath:error:

Returns a Boolean value that indicates whether the value specified by a given pointer is valid for a given key path relative to the receiver.

- (BOOL)validateValue:(id *)ioValue forKeyPath:(NSString *)inKeyPath error:(NSError **)outError
Parameters
ioValue

A pointer to a new value for the property identified by keyPath. This method may modify or replace the value in order to make it valid.

key

The name of one of the receiver's properties. The key path must specify an attribute or a to-one relationship. The key path has the form relationship.property (with one or more relationships); for example “department.name” or “department.manager.lastName”.

outError

If validation is necessary and ioValue is not transformed into a valid value, upon return contains an NSError object that describes the reason that ioValue is not a valid value.

Discussion

The default implementation gets the destination object for each relationship using valueForKey: and returns the result of a validateValue:forKey:error: message to the final object.

Availability
  • Available in OS X v10.3 and later.
Declared In
NSKeyValueCoding.h

valueForKey:

Returns the value for the property identified by a given key.

- (id)valueForKey:(NSString *)key
Parameters
key

The name of one of the receiver's properties.

Return Value

The value for the property identified by key.

Discussion

The search pattern that valueForKey: uses to find the correct value to return is described in Accessor Search Implementation Details in Key-Value Coding Programming Guide.

Availability
  • Available in OS X v10.0 and later.
Declared In
NSKeyValueCoding.h

valueForKeyPath:

Returns the value for the derived property identified by a given key path.

- (id)valueForKeyPath:(NSString *)keyPath
Parameters
keyPath

A key path of the form relationship.property (with one or more relationships); for example “department.name” or “department.manager.lastName”.

Return Value

The value for the derived property identified by keyPath.

Discussion

The default implementation gets the destination object for each relationship using valueForKey: and returns the result of a valueForKey: message to the final object.

Availability
  • Available in OS X v10.0 and later.
Related Sample Code
Declared In
NSKeyValueCoding.h

valueForUndefinedKey:

Invoked by valueForKey: when it finds no property corresponding to a given key.

- (id)valueForUndefinedKey:(NSString *)key
Parameters
key

A string that is not equal to the name of any of the receiver's properties.

Discussion

Subclasses can override this method to return an alternate value for undefined keys. The default implementation raises an NSUndefinedKeyException.

Availability
  • Available in OS X v10.3 and later.
Declared In
NSKeyValueCoding.h

Constants

Key Value Coding Exception Names

This constant defines the name of an exception raised when a key value coding operation fails.

extern NSString *NSUndefinedKeyException;
Constants
NSUndefinedKeyException

Raised when a key value coding operation fails. userInfo keys are described in “NSUndefinedKeyException userInfo Keys”

Available in OS X v10.3 and later.

Declared in NSKeyValueCoding.h.

Declared In
NSKeyValueCoding.h

NSUndefinedKeyException userInfo Keys

These constants are keys into an NSUndefinedKeyException userInfo dictionary

extern NSString *NSTargetObjectUserInfoKey;
extern NSString *NSUnknownUserInfoKey;
Constants
NSTargetObjectUserInfoKey

The object on which the key value coding operation failed.

NSUnknownUserInfoKey

The key for which the key value coding operation failed.

Discussion

For additional information see “Key Value Coding Exception Names.”

Declared In
NSKeyValueCoding.h

Array operators

These constants define the available array operators. See Set and Array Operators for more information.

NSString *const NSAverageKeyValueOperator;
NSString *const NSCountKeyValueOperator;
NSString *const NSDistinctUnionOfArraysKeyValueOperator;
NSString *const NSDistinctUnionOfObjectsKeyValueOperator;
NSString *const NSDistinctUnionOfSetsKeyValueOperator;
NSString *const NSMaximumKeyValueOperator;
NSString *const NSMinimumKeyValueOperator;
NSString *const NSSumKeyValueOperator;
NSString *const NSUnionOfArraysKeyValueOperator;
NSString *const NSUnionOfObjectsKeyValueOperator;
NSString *const NSUnionOfSetsKeyValueOperator;
Constants
NSAverageKeyValueOperator

The @avg array operator.

Available in OS X v10.4 and later.

Declared in NSKeyValueCoding.h.

NSCountKeyValueOperator

The @count array operator.

Available in OS X v10.4 and later.

Declared in NSKeyValueCoding.h.

NSDistinctUnionOfArraysKeyValueOperator

The @distinctUnionOfArrays array operator.

Available in OS X v10.4 and later.

Declared in NSKeyValueCoding.h.

NSDistinctUnionOfObjectsKeyValueOperator

The @distinctUnionOfObjects array operator.

Available in OS X v10.4 and later.

Declared in NSKeyValueCoding.h.

NSDistinctUnionOfSetsKeyValueOperator

The @distinctUnionOfSets array operator.

Available in OS X v10.4 and later.

Declared in NSKeyValueCoding.h.

NSMaximumKeyValueOperator

The @max array operator.

Available in OS X v10.4 and later.

Declared in NSKeyValueCoding.h.

NSMinimumKeyValueOperator

The @min array operator.

Available in OS X v10.4 and later.

Declared in NSKeyValueCoding.h.

NSSumKeyValueOperator

The @sum array operator.

Available in OS X v10.4 and later.

Declared in NSKeyValueCoding.h.

NSUnionOfArraysKeyValueOperator

The @unionOfArrays array operator.

Available in OS X v10.4 and later.

Declared in NSKeyValueCoding.h.

NSUnionOfObjectsKeyValueOperator

The @unionOfObjects array operator.

Available in OS X v10.4 and later.

Declared in NSKeyValueCoding.h.

NSUnionOfSetsKeyValueOperator

The @unionOfSets array operator.

Available in OS X v10.4 and later.

Declared in NSKeyValueCoding.h.

Availability
  • Available in OS X version 10.4 and later.
Declared In
NSKeyValueCoding.h