如何在flutter中将焦点切换到另一个文本表单字段

import 'package:flutter/material.dart';

TextEditingController _textFieldController1 = TextEditingController();
TextEditingController _textFieldController2 = TextEditingController();

TextField textField1 = TextField(
  controller: _textFieldController1,
  decoration: InputDecoration(
    labelText: '文本字段1',
  ),
);

TextField textField2 = TextField(
  controller: _textFieldController2,
  decoration: InputDecoration(
    labelText: '文本字段2',
  ),
);

FocusNode _focusNode1 = FocusNode();
FocusNode _focusNode2 = FocusNode();

TextField textField1 = TextField(
  controller: _textFieldController1,
  focusNode: _focusNode1,
  decoration: InputDecoration(
    labelText: '文本字段1',
  ),
);

TextField textField2 = TextField(
  controller: _textFieldController2,
  focusNode: _focusNode2,
  decoration: InputDecoration(
    labelText: '文本字段2',
  ),
);

FocusScope.of(context).requestFocus(_focusNode2);

import 'package:flutter/material.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(
          title: Text('焦点切换示例'),
        ),
        body: Center(
          child: Column(
            mainAxisAlignment: MainAxisAlignment.center,
            children: <Widget>[
              TextField(
                controller: _textFieldController1,
                focusNode: _focusNode1,
                decoration: InputDecoration(
                  labelText: '文本字段1',
                ),
              ),
              TextField(
                controller: _textFieldController2,
                focusNode: _focusNode2,
                decoration: InputDecoration(
                  labelText: '文本字段2',
                ),
              ),
              RaisedButton(
                child: Text('切换焦点'),
                onPressed: () {
                  FocusScope.of(context).requestFocus(_focusNode2);
                },
              ),
            ],
          ),
        ),
      ),
    );
  }
}

TextEditingController _textFieldController1 = TextEditingController();
TextEditingController _textFieldController2 = TextEditingController();

FocusNode _focusNode1 = FocusNode();
FocusNode _focusNode2 = FocusNode();