swift4 kvc无论结构体还是类都可以使用这个方法[keyPath: ReferenceWritableKeyPat

问题

在swift4中使用kvc:

class Person: NSObject {
    var dog: Dog?
}
class Dog: NSObject {
    var name: String
    init(name: String) {
        self.name = name
    }
}

let whiteDog = Dog(name: "white")
let person = Person()
person.setValue(whiteDog, forKey: "dog")
let myDog: Dog = person.value(forKeyPath: "dog") as! Dog
print(myDog.name)

然后会发生crash得到如下信息：

Terminating app due to uncaught exception 'NSUnknownKeyException', reason: '[<RXLearn.Person 0x1c4002610> setValue:forUndefinedKey:]: this class is not key value coding-compliant for the key dog.'

原因

swift为了性能优化，需要标记为@objc的属性才能使用kvc，所以这样:

class Person: NSObject {
    @objc var dog: Dog?
}

swift4 新功能

无论结构体还是类都可以使用这个方法[keyPath: ReferenceWritableKeyPath<Root, Value>]进行取赋值。

class Person {
    var dog: Dog
    init(dog: Dog) {
        self.dog = dog
    }
}
class Dog {
    var name: String
    init(name: String) {
        self.name = name
    }
}
//
let blackDog = Dog(name: "black")
let whiteDog = Dog(name: "white")
let person = Person(dog: blackDog)
person[keyPath: \Person.dog] = whiteDog
        
let mydog = person[keyPath: name]
print(mydog.name) // 打印white

将person改为结构体依然可以：

struct Person {
    var dog: Dog
    init(dog: Dog) {
        self.dog = dog
    }
}