Use Key Paths in Swift

Written by Team Kodeco

Key paths in Swift are a way to refer to the properties of objects in a type-safe manner.

They allow you to dynamically extract and set the values of a property by providing a path to the property, rather than hard-coding the property name as a string. They are particularly useful when used in combination with functions like map and compactMap to extract values from an array of objects. KeyPaths can also be used with classes, structs, and enums that conform to the Codable protocol to encode and decode custom data types.

The key path syntax in Swift consists of two parts:

1. The \ operator: This operator is used to create a key path expression. It is followed by the property or properties that you want to reference in the key path. For example: \Person.address.street creates a key path that references the street property of the Address struct, which is a property of the Person struct.
2. The [keyPath: xxx] subscript syntax: This syntax is used to access the property’s value using the key path. The keyPath argument specifies the key path to the property, and xxx is a reference to the key path expression created using the \ operator. For example: person[keyPath: streetKeyPath] accesses the value of the street property of the Person struct using the key path stored in the streetKeyPath constant.

Here’s a simple example of using key paths in Swift:

struct Person {
  var name: String
  var address: Address
}

struct Address {
  var street: String
  var city: String
}

let person = Person(name: "John Doe", address: Address(street: "1 Main St", city: "San Francisco"))

let streetKeyPath = \Person.address.street
let street = person[keyPath: streetKeyPath] // "1 Main St"

The \ in \Person.address.street creates a key path to a specific property of an object. In this case, it creates a key path to the street property of the Address struct, which is a property of the Person struct.

The value of the street property can then be accessed using the key path with the person[keyPath: streetKeyPath] syntax.