swift - 如何在 swift 中更改子类中的属性值？

Question

根据swift language guide的Inheritance章节，我尝试在子类中编写计算的peoperty。当我将 newValue 设置为类实例的属性时，setter 似乎起作用，而属性值并未转换为 newValue。

class Vehicle {
    var currentSpeed = 1.0
    var description: String {
        return "The current speed is \(currentSpeed) miles per hour"
    }
    func makeNoise() {
    }
}
class Car: Vehicle {
    var gear = 0
    override var description: String {
        return super.description + " in gear \(gear)"
    }
}
class AutomaticCar: Car {
    override var currentSpeed: Double {
        get {
            return super.currentSpeed
        }
        set {
            gear = Int(newValue/10) + 1
        }
    }
}

let automaticCar = AutomaticCar()
automaticCar.currentSpeed = 12.0
print(automaticCar.currentSpeed)//It prints "1.0"
print(automaticCar.description)//It prints "The current speed is 1.0 miles per hour in gear 2"

automaticCar.currentSpeed 的属性值仍然是“1.0”而不是“12.0”，而实例的 gear 属性似乎有效。我已经搜索过了，没有找到答案，是什么原理导致了这种情况？

进一步的问题:

class A {
    var test1 = 1
    var test2 = 2
    var sum: Int {
        get {
            return test1 + test2
        }
        set {
            test1 = newValue - test2
        }
    }
}
var a = A()
print(a.sum)
a.sum = 4
print(a.sum)//It ptints "4"
print(a.test1)//It prints "2"

在这种情况下，我不需要刻意设置新的 sum 属性的值，这两种情况有什么区别？

Answer 1

class AutomaticCar: Car {
    override var currentSpeed: Double {
        get {
            return super.currentSpeed
        }
        set {
            super.currentSpeed = newValue
            gear = Int(newValue/10) + 1
        }
    }
}

您刚刚忘记在覆盖的 setter 中设置 currentSpeed 的值。