Class extention

struct Grade {
    var letter: Character
    var points: Double
    var credits: Double
}

class Person {
    var firstName: String
    var lastName: String
    
    init(firstName: String, lastName: String) {
        self.firstName = firstName
        self.lastName = lastName
    }
    
    func printMyName() {
        print("My name is \(firstName) \(lastName)")
    }
}

class Student: Person {
    var grades: [Grade] = []
}

let jay = Person(firstName: "Jay", lastName: "Lee")
let jason = Student(firstName: "Jason", lastName: "Lee")

jay.firstName   // Jay
jason.firstName // Jason

jay.printMyName()   // Person
jason.printMyName() // Student

let math = Grade(letter: "B", points: 8.5, credits: 3)
let history = Grade(letter: "C", points: 6, credits: 2)
jason.grades.append(math)       // [{letter "B", points 8.5, credits 3}]
jason.grades.append(history)    // [{letter "B", points 8.5, credits 3}, {letter "C", points 6, credits 2}]

jason.grades.count  // 2

// 학생인데 운동선수
class StudentAthelete : Student {
    var minimumTrainingTime: Int = 2
    var trainedTime: Int = 0
    
    func train() {
        trainedTime += 1
    }
}

// 운동선수인데 축구선수
class FootballPlayer: StudentAthelete {
    var footballTeam = "FC Swift"
    
    override func train() {
        trainedTime += 2
    }
}

// Person > Student > Athelete > FootballPlayer

var athelete1 = StudentAthelete(firstName: "Yuna", lastName: "Kim")
var athelete2 = FootballPlayer(firstName: "HeungMin", lastName: "Son")

athelete1.firstName // "Yuna"
athelete2.firstName // "HeungMin"

athelete1.grades.append(math)   // [{letter "B", points 8.5, credits 3}]
athelete2.grades.append(math)   // [{letter "B", points 8.5, credits 3}]

athelete1.minimumTrainingTime   // 2
athelete2.minimumTrainingTime   // 2

//athelete1.footballTeam
athelete2.footballTeam      // "FC Swift"

athelete1.train()
athelete1.trainedTime   // 1

athelete2.train()
athelete2.trainedTime  // 2

athelete1 = athelete2 as StudentAthelete    // as를 사용하여 어퍼캐스팅
athelete1.train()
athelete1.trainedTime   // 4

if let son = athelete1 as? FootballPlayer { // as?를 사용하여 다운캐스팅
    print("--> team:\(son.footballTeam)")   // "--> team:FC Swift\n"
}