swift - 为什么 '=' 赋值运算符在我的 didSet 属性观察器的 if 语句中不起作用？

Question

struct Circle {
    // Properties

    var radius: Double {
        didSet {
            if oldValue < 0 {
                 radius = 0
            }
        }
    }
    var area:Double {
        get{
            return Double.pi * pow(radius, 2)
        }
    }
    var circumference: Double {
        get {
            return 2 * radius * Double.pi
        }
    }
    init() {
        radius = 0
    }
    init(radius r: Double) {
        radius = r
    }

}

// test circle

var testCircle = Circle()
print ("radius:", testCircle.radius, "area: ", testCircle.area, "circumference: ", testCircle.circumference)

var testCircle2 = Circle(radius: 2.5)
print("radius: ", testCircle2.radius, " area: ", testCircle2.area, "circumference: ", testCircle2.circumference)

var testCircle3 = Circle(radius: 20)
print("radius: ", testCircle3.radius, "area: ", testCircle3.area, "circumference: ", testCircle3.circumference)

var testCircle4 = Circle(radius: -4.5)
print("radius: ", testCircle4.radius, "area: ", testCircle4.area, "circumference: ", testCircle4.circumference)

Answer 1

didSet 属性观察器不会在 init 期间被调用。您可以将赋值包装在 defer { ... } 语句中以强制调用 didSet。

例如:

init(radius r: Double) {
    defer { radius = r }
}

如果您希望默认半径为 0，我建议为 radius 使用默认参数值，如下所示:

init(radius r: Double = 0) {
    defer { radius = r }
}

...这样您就可以避免使用两个 init 方法，并且仍然将 Circle 初始化为 Circle() (radius = 0) 并且作为 Circle(radius: 3) (radius = 3).