class MyHomePage extends StatefulWidget {
  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  int count = 0;

  GlobalKey<_TextWidgetState> textKey = GlobalKey();

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Demo'),
      ),
      body: Container(
        alignment: Alignment.center,
        child: Column(
          children: <Widget>[
            TextWidget(textKey), //需要更新的Text
            FlatButton(
              child: new Text('按钮 $count'),
              color: Theme.of(context).accentColor,
              onPressed: () {
                count++; // 这里我们只给他值变动，状态刷新交给下面的Key事件
                textKey.currentState.onPressed(count);  //其实这个count值已经改变了 但是没有重绘所以我们看到的只是我们定义的初始值
              },             
            ),
          ],
        ),
      ),
    );
  }
}

//封装的widget
class TextWidget extends StatefulWidget {
  final Key key;

  const TextWidget(this.key);

  @override
  _TextWidgetState createState() => _TextWidgetState();
}

class _TextWidgetState extends State<TextWidget> {
  String text = "0";

  void onPressed(int count) {
    setState((){
      text = count.toString();
    });
  }

  @override
  Widget build(BuildContext context) {
    return new Text(text);
  }
}