我是Flutter的新手。

我有一个带有2个子窗口小部件的应用程序（Android中为2个片段），当我单击WidgetA中的next按钮时，我想将该窗口小部件替换（或推入）WidgetChildA中，就像Android中的push（或替换）片断一样。但是相反，我得到了一个全屏小部件，就像Flutter中的普通屏幕一样。

这是我的代码：

import 'package:flutter/material.dart';

class DemoFragment extends StatefulWidget {
  @override
  State<StatefulWidget> createState() {
    return new DemoFragmentState();
  }
}

class DemoFragmentState extends State<DemoFragment> {
  @override
  Widget build(BuildContext context) {
    print(context.toString() + context.hashCode.toString());
    return new Scaffold(
      appBar: new AppBar(title: new Text("Demo fragment")),
      body: new Column(
        crossAxisAlignment: CrossAxisAlignment.stretch,
        mainAxisAlignment: MainAxisAlignment.spaceEvenly,
        mainAxisSize: MainAxisSize.max,
        children: <Widget>[
          new FragmentA(),
          new FragmentB()
        ],
      ),
    );
  }
}

class FragmentA extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    print(context.toString() + context.hashCode.toString());
    return new Center(
      child: new Column(
        children: <Widget>[
          new Text("Fragment A"),
          new RaisedButton(
              child: new Text("next"),
              onPressed: () {
                print(context.toString() + context.hashCode.toString());
                Navigator.of(context).push(new PageRouteBuilder(
                    opaque: true,
                    transitionDuration: const Duration(milliseconds: 0),
                    pageBuilder: (BuildContext context, _, __) {
                      return new FragmentChildA();
                    }));

                /*showDialog(
                    context: context,
                    builder: (_) => new AlertDialog(
                          title: new Text("Hello world"),
                          content: new Text("this is my content"),
                        ));*/
              })
        ],
      ),
    );
  }
}

class FragmentB extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    print(context.toString() + context.hashCode.toString());
    return new Center(
        child: new Column(
      children: <Widget>[
        new Text("Fragment B"),
        new RaisedButton(
            child: new Text("next"),
            onPressed: () {
              print(context.toString() + context.hashCode.toString());
              Navigator.of(context).push(new PageRouteBuilder(
                  opaque: true,
                  transitionDuration: const Duration(milliseconds: 0),
                  pageBuilder: (BuildContext context, _, __) {
                    return new FragmentChildB();
                  }));
            })
      ],
    ));
  }
}

class FragmentChildA extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return new Scaffold(
        body: new Center(
            child: new Column(
      mainAxisAlignment: MainAxisAlignment.center,
      children: <Widget>[new Text("Fragment Child A")],
    )));
  }
}

class FragmentChildB extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return new Scaffold(
        body: new Center(
            child: new Column(
      mainAxisAlignment: MainAxisAlignment.center,
      children: <Widget>[new Text("Fragment Child B")],
    )));
  }
}

屏幕截图：

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