Flutter-如何在文件中的statefulwidget中访问函数，然后在具有另一个有状态类的单独文件中使用它？

Question

我正在尝试使用在文件编号：1中定义的功能到文件编号：2中。我已将文件1导入文件2中，并希望使用文件1类中定义的某些函数。我可以访问类本身，但不能访问变量和函数。我想在受压但不能使用这些功能。

文件1的代码

import 'package:flutter/material.dart';
import 'dart:async';


class TimeDisplay extends StatefulWidget {
  @override
  _TimeDisplayState createState() => _TimeDisplayState();
}

class _TimeDisplayState extends State<TimeDisplay> {
  Stopwatch watch = Stopwatch();
  Timer timer;

  String elapsedTime= '';
  @override
  Widget build(BuildContext context) {
    return Padding(
            padding: EdgeInsets.only(top: 15.0),
            child: Text(
              elapsedTime,
              textAlign: TextAlign.center,
              style: const TextStyle(fontSize: 80.0, color: Colors.white),
            ),
          );
  }
    startWatch(){
    watch.start();
    setTime();
  }

  stopWatch(){
    watch.stop();
    setTime();
  }

  resetWatch(){
    watch.reset();
    setTime();
  }

  setTime(){
    var timeSoFar = watch.elapsedMilliseconds;
    setState(() {
          elapsedTime = transformMilliseconds(timeSoFar);
        }); 
  }

  transformMilliseconds(int milliseconds) {
    int hundreds =(milliseconds / 10).truncate();
    int seconds =(hundreds / 100).truncate();
    int minutes =(seconds / 60).truncate();

    String minutesStr = (minutes % 60).toString().padLeft(2, '0');
    String secondsStr = (seconds % 60).toString().padLeft(2, '0');
    String hundredsStr = (hundreds % 100).toString().padLeft(2, '0');

    return "$minutesStr:$secondsStr";
  }
}

文件2的代码

import 'package:flutter/material.dart';
import 'package:getfit/timedisplay.dart';
import 'dart:async';





class ChangingBtn extends StatefulWidget {
  @override
  _ChangingBtnState createState() => _ChangingBtnState();
}

class _ChangingBtnState extends State<ChangingBtn> {
    bool playing = false;

  @override
  Widget build(BuildContext context) {
    return Column(
      children: <Widget>[
        TimeDisplay(),
        Padding(
          padding: EdgeInsets.only(top: 15.0, bottom: 100.0),
          child: RawMaterialButton(
            shape: CircleBorder(),
            fillColor: Colors.red,
            splashColor: Colors.yellowAccent,
            highlightColor: Colors.orangeAccent.withOpacity(0.5),
            elevation: 10.0,
            highlightElevation: 5.0,
            onPressed: () {

              setState(() {
               playing = !playing;
               });
               playing ?  startWatch
               : stopWatch;
            },

            child: Padding(
              padding: EdgeInsets.all(25.0),

              child: Icon(
               playing ?  Icons.pause
               : Icons.play_arrow,

                color: Colors.white,
                size: 35.0,
              ),
            ),
          ),
        ),
      ],
    );
  }

}

我想我在这里做错了，我不确定

onPressed: () {

              setState(() {
               playing = !playing;
               });
               playing ?  startWatch
               : stopWatch;
            },

如果有人可以解释，我将非常感谢。