a custom countdown button(一个自定义的倒计时按钮)
//倒计时已经开始
@objc optional func countdownButtonDidStarted(countdownButton: RNCountDownButton)
//倒计时已经结束
@objc optional func countdownButtonDidStarted(countdownButton: RNCountDownButton)
//秒数已经更新
@objc optional func countdownButton(countdownButton: RNCountDownButton, didUpdatedWith second: Int)
//总的倒计时时间,默认为60s
var RNCountDownNumber = 60