? 备忘录(行为型模式)
在不破坏封装性的前提下，捕获一个对象的内部状态，并在该对象之外保存这个状态。这样就可以将该对象恢复到原先保存的状态
示例：
typealias Memento = [String: String]发起人（Originator）
protocol MementoConvertible {
var memento: Memento { get }
init?(memento: Memento)
}
struct GameState: MementoConvertible {
private enum Keys {
static let chapter = "com.valve.halflife.chapter"
static let weapon = "com.valve.halflife.weapon"
}
var chapter: String
var weapon: String
init(chapter: String, weapon: String) {
self.chapter = chapter
self.weapon = weapon
}
init?(memento: Memento) {
guard let mementoChapter = memento[Keys.chapter],
let mementoWeapon = memento[Keys.weapon] else {
return nil
}
chapter = mementoChapter
weapon = mementoWeapon
}
var memento: Memento {
return [ Keys.chapter: chapter, Keys.weapon: weapon ]
}
}管理者（Caretaker）
enum CheckPoint {
private static let defaults = UserDefaults.standard
static func save(_ state: MementoConvertible, saveName: String) {
defaults.set(state.memento, forKey: saveName)
defaults.synchronize()
}
static func restore(saveName: String) -> Any? {
return defaults.object(forKey: saveName)
}
}用法
var gameState = GameState(chapter: "Black Mesa Inbound", weapon: "Crowbar")
gameState.chapter = "Anomalous Materials"
gameState.weapon = "Glock 17"
CheckPoint.save(gameState, saveName: "gameState1")
gameState.chapter = "Unforeseen Consequences"
gameState.weapon = "MP5"
CheckPoint.save(gameState, saveName: "gameState2")
gameState.chapter = "Office Complex"
gameState.weapon = "Crossbow"
CheckPoint.save(gameState, saveName: "gameState3")
if let memento = CheckPoint.restore(saveName: "gameState1") as? Memento {
let finalState = GameState(memento: memento)
dump(finalState)
}