什么是KeyPath用于？

Question

在Swift 4中,许多基金会团队已经讨论过使用keyPaths与Swift 3相比有多容易.这引出了一个问题......什么是keyPath？说真的,我找不到任何明确的资源.

Answer 1

Objective-C能够动态而不是直接引用属性.这些引用称为keypaths.它们与直接属性访问不同,因为它们实际上不读取或写入值,它们只是将其存放以供使用.

让我们定义一个名为Cavaliers的结构和一个名为Player的结构,然后创建每个结构的一个实例:

// 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()

最后一行创建了一个名为score的goTomaxPoint()方法的引用.问题是,我们不能创建对队长名称属性的引用,但keypath可以做.

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]

请使用Xcode 9或功能快照进行测试.