暂时不太懂的swift下的get,set,willSet,didSet

class EquilateralTriangle: NamedShape {
var sideLength: Double = 0.0

init(sideLength: Double, name: String) {
    self.sideLength = sideLength
    super.init(name: name)
    numberOfSides = 3
}

var perimeter: Double {
    get {
        return 3.0 * sideLength
    }
    set {
        sideLength = newValue / 3.0
    }
}

override func simpleDescription() -> String {
    return "An equilateral triangle with sides of length \(sideLength)."
}
}
var triangle = EquilateralTriangle(sideLength: 3.1, name: "a triangle")
print(triangle.perimeter)
triangle.perimeter = 9.9
print(triangle.sideLength)
在 perimeter的 setter 中，新值被隐式地命名为 newValue。你可以提供一个显式的名字放在 set 后边的圆括号里。

注意 EquilateralTriangle类的初始化器有三个不同的步骤：

设定子类声明的属性的值；
调用父类的初始化器；
改变父类定义的属性中的值，以及其他任何使用方法，getter 或者 setter 等需要在这时候完成的内容。
 如果你不需要计算属性但仍然需要在设置一个新值的前后执行代码，使用 willSet和 didSet。比如说，下面的类确保三角形的边长始终和正方形的边长相同。

class TriangleAndSquare {
    var triangle: EquilateralTriangle {
        willSet {
            square.sideLength = newValue.sideLength
        }
}
var square: Square {
    willSet {
        triangle.sideLength = newValue.sideLength
    }
}
init(size: Double, name: String) {
    square = Square(sideLength: size, name: name)
    triangle = EquilateralTriangle(sideLength: size, name: name)
    }
}
var triangleAndSquare = TriangleAndSquare(size: 10, name: "another test shape")
print(triangleAndSquare.square.sideLength)
print(triangleAndSquare.triangle.sideLength)
triangleAndSquare.square = Square(sideLength: 50, name: "larger square")
print(triangleAndSquare.triangle.sideLength)

class TriangleAndSquare {
    var triangle: EquilateralTriangle {
        willSet {
            square.sideLength = newValue.sideLength
        }
    }
    var square: Square {
        willSet {
            triangle.sideLength = newValue.sideLength
        }
    }
    init(size: Double, name: String) {
        square = Square(sideLength: size, name: name)
        triangle = EquilateralTriangle(sideLength: size, name: name)
    }
}
var triangleAndSquare = TriangleAndSquare(size: 10, name: "another test shape")
print(triangleAndSquare.square.sideLength)
print(triangleAndSquare.triangle.sideLength)
triangleAndSquare.square = Square(sideLength: 50, name: "larger square")
print(triangleAndSquare.triangle.sideLength)
//当你操作可选项的值的时候，你可以在可选项前边使用 ?比如方法，属性和下标脚本。如果 ?前的值是 nil，那 ?后的所有内容都会被忽略并且整个表达式的值都是 nil。否则，可选项的值将被展开，然后 ?后边的代码根据展开的值执行。在这两种情况当中，表达式的值是一个可选的值。

let optionalSquare: Square? = Square(sideLength: 2.5, name: "optional square")
let sideLength = optionalSquare?.sideLength

let optionalSquare: Square? = Square(sideLength: 2.5, name: "optional square")
let sideLength = optionalSquare?.sideLength

原链接：(https://www.cnswift.org/a-swift-tour)