factory Timer.periodic(Duration duration, void callback(Timer timer))

import 'package:flutter/material.dart';
import 'dart:async';
class CounterDownPage extends StatefulWidget {
  @override
  _CounterDownPageState createState() => _CounterDownPageState();
}
class _CounterDownPageState extends State<CounterDownPage> {
  // 用來在布局中顯示相應(yīng)的剩余時間
  String remainTimeStr = '';
  Timer _timer;
   //倒計(jì)時 
  void startCountDown(int time) {
    // 重新計(jì)時的時候要把之前的清除掉
    if (_timer != null) {
      if (_timer.isActive) {
        _timer.cancel();
        _timer = null;
      }
    }
    if (time <= 0) {
      return;
    }
    var countTime = time;
    const repeatPeriod = const Duration(seconds: 1);
    _timer = Timer.periodic(repeatPeriod, (timer) { 
      if (countTime <= 0) {
        timer.cancel();
        timer = null;
        //待付款倒計(jì)時結(jié)束，可以在這里做相應(yīng)的操作
        return;
      }
      countTime--;
    //外面?zhèn)鬟M(jìn)來的單位是秒，所以需要根據(jù)總秒數(shù)，計(jì)算小時，分鐘，秒
    int hour = (countTime ~/ 3600) % 24;//如果不止24小時的就不用%24
    int minute = countTime % 3600 ~/60;
    int second = countTime % 60;
    var str = '';
    if (hour > 0) {
      str = str + hour.toString()+':';
    }
    if (minute / 10 < 1) {//當(dāng)只有個位數(shù)時，給前面加“0”，實(shí)現(xiàn)效果：“:01”,":02"
      str = str + '0' + minute.toString() + ":";
    } else {
      str = str + minute.toString() + ":";
    }
    if (second / 10 < 1) {
      str = str + '0' + second.toString();
    } else {
      str = str + second.toString();
    }
    setState(() {
      remainTimeStr = str;
    });
    });
  }
 @override
  void initState() {
    super.initState();
    //開始倒計(jì)時，這里傳入的是秒數(shù)
     startCountDown(5000);
  }
@override
  void dispose() {
    super.dispose();
    if (_timer != null) {
      if (_timer.isActive) {
        _timer.cancel();
        _timer = null;
      }
    }
  }
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text("倒計(jì)時"),
      ),
      body: Center(
        child: Row(
       mainAxisAlignment: MainAxisAlignment.center,
       children: [
         Text("剩余", style: TextStyle(
           fontSize: 18,
           color: Color.fromRGBO(255, 111, 50, 1),
           fontWeight: FontWeight.bold
         ),),
          Text(remainTimeStr.length > 0 ? remainTimeStr: "--", style: TextStyle(
           fontSize: 18,
           color: Color.fromRGBO(255, 111, 50, 1),
           fontWeight: FontWeight.bold
         ),),
       ],
      ),
      ),
    );
  }
}