swift 键路径

struct Address {
    var street: String
    var city: String
    var zipCode: Int
}
struct Person {
    let name: String
    var address: Address
}

let streetKeyPath = \Person.address.street // Swift.WritableKeyPath<Person, Swift.String>
let nameKeyPath = \Person.name // Swift.KeyPath<Person, Swift.String>

let simpsonResidence = Address(street: "1094 Evergreen Terrace",
                               city: "Springfield", zipCode: 97475)
var lisa = Person(name: "Lisa Simpson", address: simpsonResidence)
lisa[keyPath: nameKeyPath] // Lisa Simpson

lisa[keyPath: streetKeyPath] = "742 Evergreen Terrace"

var bart = Person(name: "Bart Simpson", address: simpsonResidence)
let people = [lisa, bart]
people[keyPath: \.[1].name] // Bart Simpson


// 等效于 "Hello".count
"Hello"[keyPath: \.count]

摘录来自: Chris Eidhof. “Swift 进阶。”