//定义一个协议protocol LogManagerDelegate {    func writeLog()} //用户登录类class UserController {    var delegate : LogManagerDelegate?         func login() {        //查看是否有委托，然后调用它        delegate?.writeLog()    }} //日志管理类class SqliteLogManager : LogManagerDelegate {    func writeLog() {        print("将日志记录到sqlite数据库中")    }}  //使用let userController = UserController()userController.login()  //不做任何事 let sqliteLogManager = SqliteLogManager()userController.delegate = sqliteLogManageruserController.login()  //输出“将日志记录到sqlite数据库中”