Introduction to Swift Keypaths


released Fri, 29 Mar 2019
Swift Version 5.0


In the previous example code, we always defined User struct instances. Which meant that the actual instance we instantiated also had to be mutable (var):

var firstUser = User(username: String)

firstUser[keyPath: \User.username] = \"Ok\"

If this had be a let firstUser, it would not have worked, because let instances are immutable. However, if our User is a class type, we could still mutate it just fine:

class User {

   var username: String = \"Nothing\"


let firstUser = User()

// This works

firstUser[keyPath: \User.username] = \"Something\"

in The example above, the username property can still be modified because User is a class. Swift distinguishes between keypaths for reference (class) types and value types. Keypaths to reference types are of the type ReferenceWritableKeyPath<Root, Value>.

It is important to note that ReferenceWritableKeyPaths are subclasses of WritableKeyPath, so any function that accepts a WritableKeyPath can also accept a ReferenceWritableKeyPath.

The next KeyPath we want to look at is the PartialKeyPath, however, before we do so, we'll have a detour and look at a short example to better understand the need for it and to see some of what we've seen so far in action.