How to Set/Update State of StatefulWidget from other StatefulWidget in Flutter?

問題：

1. For Example in the below code plus button works and able to update the text but the minus button does not.例如，在下面的代碼中，加號按鈕起作用並且能夠更新文本，但減號按鈕不起作用。
2. But if we press FloatingActionButton then the State is refreshed .但是如果我們按下 FloatingActionButton 則狀態會被刷新。
3. The minus button is changing the value of the variable but not updating the state of parent widget .減號按鈕正在更改變量的值但不更新父小部件的狀態。

here is code .....這是代碼.....

import 'package:flutter/material.dart';

void main() => runApp(new MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
      title: 'Flutter Demo',
      theme: new ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: new MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

int number;

EdgeInsets globalMargin = const EdgeInsets.symmetric(horizontal: 20.0, vertical: 20.0);
TextStyle textStyle = const TextStyle(
  fontSize: 100.0,
  color: Colors.black,
);

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);

  final String title;

  @override
  _MyHomePageState createState() => new _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  @override
  void initState() {
    super.initState();
    number = number ?? 0;
  }

  @override
  Widget build(BuildContext context) {
    return new Scaffold(
      appBar: new AppBar(
        title: new Text(widget.title),
      ),
      body: new Column(
        children: <Widget>[
          new Text(
            number.toString(),
            style: textStyle,
          ),
          new GridView.count(
            crossAxisCount: 2,
            shrinkWrap: true,
            scrollDirection: Axis.vertical,
            children: <Widget>[
              new InkResponse(
                child: new Container(
                    margin: globalMargin,
                    color: Colors.green,
                    child: new Center(
                      child: new Text(
                        "+",
                        style: textStyle,
                      ),
                    )),
                onTap: () {
                  setState(() {
                    number = number + 1;
                  });
                },
              ),
              new Sub(),
            ],
          ),
        ],
      ),
      floatingActionButton: new FloatingActionButton(
        onPressed: () {
          setState(() {});
        },
        child: new Icon(Icons.update),
      ),
    );
  }
}

class Sub extends StatefulWidget {
  @override
  _SubState createState() => new _SubState();
}

class _SubState extends State<Sub> {
  @override
  Widget build(BuildContext context) {
    return new InkResponse(
      child: new Container(
          margin: globalMargin,
          color: Colors.red,
          child: new Center(
            child: new Text(
              "-",
              style: textStyle,
            ),
          )),
      onTap: () {
        setState(() {
          number = number - 1;
        });
      },
    );
  }
}

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解決方案：

參考： https://stackoom.com/en/question/3HQH8