Swift Academy · Lesson

Writable and Reference KeyPaths

Read and write through key paths.

Read-Only vs Writable Key Paths

A plain KeyPath can only read a value. To write through a key path you need a writable variant. Swift has two of them, one for value types and one for reference types.

This lesson covers WritableKeyPath for structs and ReferenceWritableKeyPath for classes, and how the [keyPath:] subscript becomes settable.

struct Person {
    var name: String
    var age: Int
}

let writable: WritableKeyPath<Person, Int> = \Person.age
print(type(of: writable))

WritableKeyPath for Value Types

A WritableKeyPath targets a mutable stored property of a value type. To set through it, the instance itself must be a var, because mutating a struct property mutates the whole value.

Use the [keyPath:] subscript on the left side of an assignment to write.

struct Person {
    var name: String
    var age: Int
}

var alice = Person(name: "Alice", age: 30)
let agePath = \Person.age
alice[keyPath: agePath] = 31
print(alice.age)

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