import 'dart:async';

class _LoginPageState extends State<LoginPage> {
 ...
 Timer _timer;
 int _countdownTime = 0;
 ...
}

GestureDetector(
 onTap: () {
  if (_countdownTime == 0 && validateMobile()) {
   //Http请求发送验证码
   ...
   setState(() {
    _countdownTime = 60;
   });
   //开始倒计时
   startCountdownTimer();
  }
 },
 child: Text(
  _countdownTime > 0 ? '$_countdownTime后重新获取' : '获取验证码',
  style: TextStyle(
   fontSize: 14,
   color: _countdownTime > 0
     ? Color.fromARGB(255, 183, 184, 195)
     : Color.fromARGB(255, 17, 132, 255),
  ),
 ),
)

void startCountdownTimer() {
 const oneSec = const Duration(seconds: 1);
 
 var callback = (timer) => {
  setState(() {
   if (_countdownTime < 1) {
    _timer.cancel();
   } else {
    _countdownTime = _countdownTime - 1;
   }
  })
 };
 
 _timer = Timer.periodic(oneSec, callback);
}

@override
void dispose() {
 super.dispose();
 if (_timer != null) {
  _timer.cancel();
 }
}