willSet hook in Swift

willset.swift

class Circle {
    var radius: Double
    
    init(radius: Double) {
        self.radius = radius
    }
}

class Square {
    var sideLength: Double
    var numSides: Int
    
    init(sideLength: Double) {
        self.sideLength = sideLength
        numSides = 4
    }
}

class CircleAndSquare {
    var circle: Circle {
        willSet {
            square.sideLength = newValue.radius
        }
    }
    var square: Square {
        willSet {
            circle.radius = newValue.sideLength
        }
    }
    
    init(length: Double) {
        circle = Circle(theRadius length: length)
        square = Square(sideLength: length)
        
    }
}

var circleAndSquare = CircleAndSquare(length: 10)
circleAndSquare.circle.radius                       // 10
circleAndSquare.square.sideLength                   // 10
circleAndSquare.square.sideLength = 20              // Set the square's sideLength to 20
circleAndSquare.circle.radius                       // 20