## Methods Structures and enumerations are value types. By default, the properties of a value type cannot be modified from within its instance methods. ~~~ struct Point { var x = 0.0, y = 0.0 mutating func moveByX(deltaX: Double, y deltaY: Double) { x += deltaX y += deltaY } } var somePoint = Point(x: 1.0, y: 1.0) somePoint.moveByX(2.0, y: 3.0) println("The point is now at (\(somePoint.x), \(somePoint.y))") // prints "The point is now at (3.0, 4.0)" ~~~ ~~~ let fixedPoint = Point(x: 3.0, y: 3.0) fixedPoint.moveByX(2.0, y: 3.0) // this will report an error ~~~ ~~~ enum TriStateSwitch { case Off, Low, High mutating func next() { switch self { case Off: self = Low case Low: self = High case High: self = Off } } } var ovenLight = TriStateSwitch.Low ovenLight.next() // ovenLight is now equal to .High ovenLight.next() // ovenLight is now equal to .Off ~~~ ~~~ struct LevelTracker { static var highestUnlockedLevel = 1 static func unlockLevel(level: Int) { if level > highestUnlockedLevel { highestUnlockedLevel = level } } static func levelIsUnlocked(level: Int) -> Bool { return level <= highestUnlockedLevel } var currentLevel = 1 mutating func advanceToLevel(level: Int) -> Bool { if LevelTracker.levelIsUnlocked(level) { currentLevel = level return true } else { return false } } } class Player { var tracker = LevelTracker() let playerName: String func completedLevel(level: Int) { LevelTracker.unlockLevel(level + 1) tracker.advanceToLevel(level + 1) } init(name: String) { playerName = name } } var player = Player(name: "Argyrios") player.completedLevel(1) println("highest unlocked level is now \(LevelTracker.highestUnlockedLevel)") // prints "highest unlocked level is now 2” player = Player(name: "Beto") if player.tracker.advanceToLevel(6) { println("player is now on level 6") } else { println("level 6 has not yet been unlocked") } // prints "level 6 has not yet been unlocked" ~~~