state.swift

/*:
🐉 状态（State）
---------

在状态模式中，对象的行为是基于它的内部状态而改变的。
这个模式允许某个类对象在运行时发生改变。

### 示例：
*/
final class Context {
	private var state: State = UnauthorizedState()

    var isAuthorized: Bool {
        get { return state.isAuthorized(context: self) }
    }

    var userId: String? {
        get { return state.userId(context: self) }
    }

	func changeStateToAuthorized(userId: String) {
		state = AuthorizedState(userId: userId)
	}

	func changeStateToUnauthorized() {
		state = UnauthorizedState()
	}
}

protocol State {
	func isAuthorized(context: Context) -> Bool
	func userId(context: Context) -> String?
}

class UnauthorizedState: State {
	func isAuthorized(context: Context) -> Bool { return false }

	func userId(context: Context) -> String? { return nil }
}

class AuthorizedState: State {
	let userId: String

	init(userId: String) { self.userId = userId }

	func isAuthorized(context: Context) -> Bool { return true }

	func userId(context: Context) -> String? { return userId }
}
/*:
### 用法
*/
let userContext = Context()
(userContext.isAuthorized, userContext.userId)
userContext.changeStateToAuthorized(userId: "admin")
(userContext.isAuthorized, userContext.userId) // now logged in as "admin"
userContext.changeStateToUnauthorized()
(userContext.isAuthorized, userContext.userId)