valueForKey

NSOrderedSet

Returns an ordered set containing the results of invoking valueForKey:using key on each of the ordered set’s objects.

https://developer.apple.com/documentation/foundation/nsorderedset/1409378-valueforkey?language=occ

NSKeyValueCoding

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

https://developer.apple.com/documentation/objectivec/nsobject/1412591-valueforkey?language=occ

NSArray

Returns an array containing the results of invoking valueForKey: using keyon each of the array's objects.

Discussion

The returned array contains NSNull elements for each object that returns nil.

https://developer.apple.com/documentation/foundation/nsarray/1412219-valueforkey?language=occ

NSDictionary

Returns the value associated with a given key.

https://developer.apple.com/documentation/foundation/nsdictionary/1410210-valueforkey?language=occ

NSSet

Return a set containing the results of invoking valueForKey: on each of the receiving set's members.

Discussion

The returned set might not have the same number of members as the receiving set. The returned set will not contain any elements corresponding to instances of valueForKey:returning nil (note that this is in contrast with NSArray’s implementation, which may put NSNull values in the arrays it returns).

https://developer.apple.com/documentation/foundation/nsset/1418386-valueforkey?language=occ

等等。。。