Navigator.push中未定义的上下文

Widget build(BuildContext context) {
  return Scaffold(
    appBar: AppBar(
      title: Text('Page 1'),
    ),
    body: Builder(
      builder: (BuildContext context) {
        return RaisedButton(
          child: Text('Go to Page 2'),
          onPressed: () {
            Navigator.push(
              context,
              MaterialPageRoute(builder: (context) => Page2()),
            );
          },
        );
      },
    ),
  );
}

GlobalKey<NavigatorState> navigatorKey = GlobalKey<NavigatorState>();

Widget build(BuildContext context) {
  return MaterialApp(
    navigatorKey: navigatorKey,
    home: Scaffold(
      appBar: AppBar(
        title: Text('Page 1'),
      ),
      body: RaisedButton(
        child: Text('Go to Page 2'),
        onPressed: () {
          Navigator.push(
            navigatorKey.currentContext,
            MaterialPageRoute(builder: (context) => Page2()),
          );
        },
      ),
    ),
  );
}

Widget build(BuildContext context) {
  return Scaffold(
    appBar: AppBar(
      title: Text('Page 1'),
    ),
    body: RaisedButton(
      child: Text('Go to Page 2'),
      onPressed: () {
        var state = context.findAncestorStateOfType<_Page1State>();
        Navigator.push(
          state.context,
          MaterialPageRoute(builder: (context) => Page2()),
        );
      },
    ),
  );
}