forKey

forKey is a naming convention used in several programming APIs to indicate the key that identifies a value within a mapping, such as a dictionary or a coding container. It signals that the corresponding argument specifies which value to retrieve, store, or encode.

In the Cocoa and Cocoa Touch ecosystems, forKey appears in many core APIs. For example, NSDictionary and NSMutableDictionary use objectForKey: to fetch a value by its key and setObject:forKey: to insert or update a value associated with a key. NSCoder uses encodeObject:forKey: and decodeObjectForKey: to serialize and deserialize values with associated keys. Key-Value Coding (KVC) also employs valueForKey: and setValue:forKey: to access properties or dictionary-like entries by key.

The forKey pattern emphasizes a mapping from a key to a value rather than a positional access. Keys are typically strings or objects that conform to the framework’s requirements (such as conforming to NSCopying in Objective-C). The exact semantics of key equality and value storage depend on the container or encoding context but generally follow the principle that the key uniquely identifies the associated value.

In modern Swift, dictionaries are commonly accessed via subscripting, but when interoperating with Objective-C APIs or using Core Data and NSCoding patterns, the forKey naming remains visible and relevant. The term helps developers understand that a parameter is serving as a lookup identifier rather than a value itself.

See also: objectForKey:, valueForKey:, encodeObject:forKey:, decodeObjectForKey:, user defaults access patterns.