iOS swift中取枚举类型原始值的方法

enum Planet: Int {
case Mercury = 1, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune
}

let earthsOrder = Planet.Earth.toRaw()
// earthsOrder is 3

let possiblePlanet = Planet.fromRaw(7)
// possiblePlanet is of type Planet? and equals Planet.Uranus