在Flutter中可以通过不同的方式来实现这一点，我将尝试分享其中最简单的一种。在回答之前，值得一提的是以下问题：

• Detect when delete is typed into a TextField #14809

在Flutter中，当TextField为空时，退格键不会发送任何事件（例如，TextField.onChanged不会被调用）。在您的情况下，如果用户位于第三个字段，并且他们按下backspace键返回到第二个字段，那么如果没有在相关问题中讨论的解决方案，就无法捕获该按键。简而言之，您需要添加一个零宽度空格字符（它不会被呈现，但会出现在String中）来检测退格键事件。
我提到这个问题是因为我正在分享一个使用零宽空格字符（简称zwsp）的示例。
在下面的示例中，我只创建了两个列表，其中包含：

• 每个字段为FocusNode
• TextEditingController用于每个字段。

根据索引，您可以通过调用以下命令将焦点置于特定字段：FocusNode.requestFocus().
类似地，您可以通过调用FocusNode.unfocus来移除焦点，也可以通过调用以下命令从任何位置移除任何焦点：FocusScope.of(context).unfocus();（在下面的示例中，在插入最后一个字符后使用它来隐藏键盘）。
话虽如此，这里有一个完整的例子，你可以复制粘贴来尝试一下：

import 'package:flutter/material.dart';

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

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

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

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(title),
      ),
      body: Center(child: CodeField()),
    );
  }
}

/// zero-width space character
///
/// this character can be added to a string to detect backspace.
/// The value, from its name, has a zero-width so it's not rendered
/// in the screen but it'll be present in the String.
///
/// The main reason this value is used because in Flutter mobile,
/// backspace is not detected when there's nothing to delete.
const zwsp = '\u200b';

// the selection is at offset 1 so any character is inserted after it.
const zwspEditingValue = TextEditingValue(text: zwsp, selection: TextSelection(baseOffset: 1, extentOffset: 1));

class CodeField extends StatefulWidget {
  const CodeField({Key key}) : super(key: key);

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

class _CodeFieldState extends State<CodeField> {
  List<String> code = ['', '', '', ''];

  List<TextEditingController> controllers;
  List<FocusNode> focusNodes;

  @override
  void initState() {
    // TODO: implement initState
    super.initState();
    focusNodes = List.generate(4, (index) => FocusNode());
    controllers = List.generate(4, (index) {
      final ctrl = TextEditingController();
      ctrl.value = zwspEditingValue;
      return ctrl;
    });

    WidgetsBinding.instance.addPostFrameCallback((timeStamp) {
      // give the focus to the first node.
      focusNodes[0].requestFocus();
    });
  }

  @override
  void dispose() {
    // TODO: implement dispose
    super.dispose();
    focusNodes.forEach((focusNode) {
      focusNode.dispose();
    });
    controllers.forEach((controller) {
      controller.dispose();
    });
  }

  @override
  Widget build(BuildContext context) {
    return Row(
      mainAxisAlignment: MainAxisAlignment.center,
      children: List.generate(
        4,
        (index) {
          return Container(
            width: 20,
            height: 20,
            margin: const EdgeInsets.all(10),
            decoration: BoxDecoration(
              borderRadius: BorderRadius.circular(10),
            ),
            child: TextField(
              controller: controllers[index],
              focusNode: focusNodes[index],
              maxLength: 2,
              keyboardType: TextInputType.number,
              decoration: InputDecoration(
                counterText: "",
              ),
              onChanged: (value) {
                if (value.length > 1) {
                  // this is a new character event
                  if (index + 1 == focusNodes.length) {
                    // do something after the last character was inserted
                    FocusScope.of(context).unfocus();
                  } else {
                    // move to the next field
                    focusNodes[index + 1].requestFocus();
                  }
                } else {
                  // this is backspace event

                  // reset the controller
                  controllers[index].value = zwspEditingValue;
                  if (index == 0) {
                    // do something if backspace was pressed at the first field

                  } else {
                    // go back to previous field
                    controllers[index - 1].value = zwspEditingValue;
                    focusNodes[index - 1].requestFocus();
                  }
                }
                // make sure to remove the zwsp character
                code[index] = value.replaceAll(zwsp, '');
                print('current code = $code');
              },
            ),
          );
        },
      ),
    );
  }
}

您可能希望在每个TextFormField上使用一个FocusNode，这样，一旦您的用户在TextFormField中输入了文本，您就可以在TextFormField调用myNextTextFieldFocusNode.requestFocus()的回调onChanged中使用

FocusNode textFieldOne = FocusNode();
  FocusNode textFieldTwo = FocusNode();

  // ...

  TextFormField(
        onChanged: (_) {
           textFieldTwo.requestFocus();
        },
        focusNode: textFieldOne,
        controller: textController,
  )

你可以使用onChanged和nodefocus属性。当onChanged被调用时，引用下一个文本域。
初始化焦点节点;

late FocusNode myFocusNode;

  @override
  void initState() {
    super.initState();

    myFocusNode = FocusNode();
  }

  @override
  void dispose() {
    // Clean up the focus node when the Form is disposed.
    myFocusNode.dispose();

    super.dispose();
  }

on已更改的属性;

TextField(
  focusNode: myFocusNode1,
  onChanged: (text) {

   myFocusNode2.requestFocus();// I could not remember the correct usage please check
  },
),

在flutter中，用户在一个块中输入2和4个字符后，如何自动聚焦下一个文本字段