Drawing.swift

// From:
protocol Drawable {
    func draw()
}

// To:
struct Drawing<Shape> {
    var draw: (Shape) -> ()
}

// 用來畫實心點
let solidPointDrawer = Drawing<CGPoint> { point in
    print("draw solid point: (\(point.x), \(point.y))")
}

// 用來畫空心點
let hollowPointDrawer = Drawing<CGPoint> { point in
    print("draw hollow point: (\(point.x), \(point.y))")
}                      

let point = CGPoint(x: 1, y: 1)

solidPointDrawer.draw(point) // 畫實心點
hollowPointDrawer.draw(point) // 畫空心點