What is a KeyPath used for?

Objective-C has the ability to reference a property dynamically rather than directly. These references, called keypaths. They are distinct from direct property accesses because they don't actually read or write the value, they just stash it away for use.

Let define a struct called Cavaliers and a struct called Player, then create one instance of each:

// an example struct
struct Player {
    var name: String
    var rank: String
}

// another example struct, this time with a method
struct Cavaliers {
    var name: String
    var maxPoint: Double
    var captain: Player

    func goTomaxPoint() {
        print("\(name) is now travelling at warp \(maxPoint)")
    }
}

// create instances of those two structs
let james = Player(name: "Lebron", rank: "Captain")
let irving = Cavaliers(name: "Kyrie", maxPoint: 9.975, captain: james)

// grab a reference to the `goTomaxPoint()` method
let score = irving.goTomaxPoint

// call that reference
score()

The last lines create a reference to the goTomaxPoint() method called score. The problem is, we can't create a reference to the captain's name property but keypath can do.

let nameKeyPath = \Cavaliers.name
let maxPointKeyPath = \Cavaliers.maxPoint
let captainName = \Cavaliers.captain.name
let cavaliersName = irving[keyPath: nameKeyPath]
let cavaliersMaxPoint = irving[keyPath: maxPointKeyPath]
let cavaliersNameCaptain = irving[keyPath: captainName]

Please test with Xcode 9 or capable snapshot.