RxSwift实现倒计时效果（可规避前后台切换，页面切换）

func countDownAfterSendMsg() {
         /// 项目真实开发可以设置let codeBtnEnable=BehaviorRelay<Bool>(value: true)接收绑定btn的可点击状态
         ///  codeBtn.isEnable = false 在网络请求时设置可以禁止重复点击
         let originalData = NSDate().timeIntervalSince1970
         codeBtn.setTitle("60s", for: .selected)
        let mapResult: Observable<Double> =  Observable<Int>.interval(1, scheduler: MainScheduler.instance).map { (_) -> Double in
            let currentData = NSDate().timeIntervalSince1970
            return currentData - originalData
        }
        mapResult.takeWhile {
            $0 <= 60
        }.subscribe(onNext: { [weak self] (second) in
            let number = Int(floor(60 - second))
            if number > 0 {
                self?.codeBtn.setTitle("\(number)s", for: .selected)
            } else {
                self?.codeBtn.setTitle("发送验证码", for: .normal)
            }
        }, onCompleted: {
            DispatchQueue.main.async {
                self.codeBtn.isEnable = true
            }
        }).disposed(by: self.disposeBag)
}