KeyPath Composition

The last keypath concept that we should tackle is keypath composition: Swift allows you to dynamically combine KeyPath types at runtime if the types match up.

In order to showcase this, we will go back to our User and Address struct:

struct User {

   let address: Address

}



struct Address {

   let street: String

}

Example

Based on this structure, we will take two different keypaths; first, one to the address property on the User, and then one on the String property on the Address:

let addressKeyPath = \User.address



let streetKeyPath = \Address.street

Given these two variables, we can now compose them to manifest a new keypath at runtime that goes from User to the street:

let newKeyPath = addressKeyPath.appending(path: streetKeyPath)

Here, we created a new KeyPath<User, String> at runtime by joining a KeyPath<User, Address> and a KeyPath<Address, String>. However, what should Swift do if you try to merge a KeyPath<User, String> and a KeyPath<House, Int>. Obviously, there's no relationship between these types. Swift solves this by introducing laws of keypath composition. Lets have a look at them.