What are keypaths?

Swift version: 5.10

Paul Hudson @twostraws May 28th 2019

Swift keypaths are a way of storing uninvoked references to properties, which is a fancy way of saying they refer to a property itself rather than to that property’s value.

Here’s an example struct storing a name and maximum warp speed of a starship:

struct Starship {
    var name: String
    var maxWarp: Double
}

let voyager = Starship(name: "Voyager", maxWarp: 9.975)

Keypaths let us refer to the name or maxWarp properties without reading them directly, like this:

let nameKeyPath = \Starship.name
let warpKeyPath = \Starship.maxWarp

If you want to read those keypaths on a specific starship, Swift will return you the actual values attached to those properties:

print(voyager[keyPath: nameKeyPath])
print(voyager[keyPath: warpKeyPath])

In practice, this means you can refer to the same property in multiple places all using the same keypath – and if you decide you want a different property you can change it in just one place.