参考代码：

Flutter - Create Custom Keyboard Examples

本文贴出的代码实现了一个输入十六进制数据的键盘：

（1）支持长按退格键连续删除字符；

（2）可通过退格键删除选中的文字；

（3）监听文本变化（包括粘贴剪切导致的变化）。 

hex_keyboard.dart

import 'dart:async';

import 'package:flutter/material.dart';

class HexKeyboard extends StatefulWidget {
  final TextEditingController controller;
  final void Function() onDone;
  final void Function(String) onTextChanged;

  const HexKeyboard({
    super.key,
    required this.controller,
    required this.onDone,
    required this.onTextChanged,
  });

  @override
  State<HexKeyboard> createState() => _HexKeyboardState();
}

class _HexKeyboardState extends State<HexKeyboard> {
  late TextEditingController _controller;

  final Widget _horizontalPadding = const SizedBox(width: 1.0);
  final Widget _verticalPadding = const SizedBox(height: 1.0);
  Timer? _timer;

  @override
  void initState() {
    super.initState();
    _controller = widget.controller;
  }

  void _handleType(String text) {
    int position = _controller.selection.base.offset;
    var value = _controller.text;

    if (value.isEmpty) {
      _controller.text = text;
    } else {
      _controller.text = value.substring(0, position) +
          text +
          value.substring(position, value.length);
    }

    _controller.selection =
        TextSelection.fromPosition(TextPosition(offset: position + 1));
    widget.onTextChanged(_controller.text);
  }

  void _handleBackspace() {
    final value = _controller.text;

    if (value.isNotEmpty) {
      final start = _controller.selection.start;
      final end = _controller.selection.end;
      print("selection.start=$start, selection.end=$end");
      final int offset;
      if(end > 0) {
        if(start == end) {
          _controller.text = value.substring(0, start - 1) +
              value.substring(start, value.length);
          offset = start - 1;
        } else {
          _controller.text = value.substring(0, start) +
              value.substring(end, value.length);
          offset = start;
        }
        _controller.selection =
            TextSelection.fromPosition(TextPosition(offset: offset));
        widget.onTextChanged(_controller.text);
      }
    }
  }

  Widget _buildButton(String text,
      {VoidCallback? onPressed,
      VoidCallback? onLongPressStart,
      VoidCallback? onLongPressUp}) {
    return Expanded(
      child: Container(
        color: Colors.white,
        child: GestureDetector(
          onLongPressStart: (e) {
            onLongPressStart?.call();
          },
          onLongPressUp: onLongPressUp,
          child: TextButton(
            onPressed: onPressed ?? () => _handleType(text),
            child: Text(
              text,
              style: const TextStyle(color: Colors.black, fontSize: 16),
            ),
          ),
        ),
      ),
    );
  }

  @override
  Widget build(BuildContext context) {
    return _buildButtonKeyboard();
  }

  Widget _buildButtonRow(String key1, String key2, String key3) {
    return Row(
      mainAxisAlignment: MainAxisAlignment.spaceEvenly,
      children: [
        _horizontalPadding,
        _buildButton(key1),
        _horizontalPadding,
        _buildButton(key2),
        _horizontalPadding,
        _buildButton(key3),
        _horizontalPadding,
      ],
    );
  }

  Widget _buildButtonKeyboard() {
    return Container(
      color: const Color(0xffdddddd),
      child: Column(
        children: [
          _verticalPadding,
          _buildButtonRow('A', 'B', 'C'),
          _verticalPadding,
          _buildButtonRow('D', 'E', 'F'),
          _verticalPadding,
          _buildButtonRow('1', '2', '3'),
          _verticalPadding,
          _buildButtonRow('4', '5', '6'),
          _verticalPadding,
          _buildButtonRow('7', '8', '9'),
          _verticalPadding,
          Row(
            children: [
              _horizontalPadding,
              _buildButton(
                '⌫',
                onPressed: _handleBackspace,
                onLongPressStart: () {
                  _timer =
                      Timer.periodic(const Duration(milliseconds: 50), (timer) {
                    _handleBackspace();
                  });
                },
                onLongPressUp: () {
                  _timer?.cancel();
                },
              ),
              _horizontalPadding,
              _buildButton('0'),
              _horizontalPadding,
              _buildButton(
                'Done',
                onPressed: widget.onDone,
              ),
              _horizontalPadding,
            ],
          ),
          _verticalPadding,
        ],
      ),
    );
  }
}

 hex_input_screen.dart

import 'package:flutter/material.dart';

class HexInputScreen extends StatefulWidget {
  final String text;

  const HexInputScreen({super.key, required this.text});

  @override
  State<HexInputScreen> createState() => _HexInputScreenState();
}

class _HexInputScreenState extends State<HexInputScreen> {
  late TextEditingController _controller;
  final FocusNode _focus = FocusNode();
  late ValueNotifier<bool> _focusValueNotifier;
  int _byteCount = 0;

  int _toByteCount(String hex) {
    return hex.length % 2 == 0 ? hex.length ~/ 2 : hex.length ~/ 2 + 1;
  }

  void _onTextChanged(String text) {
    //更新字节数
    setState(() {
      _byteCount = _toByteCount(text);
    });
  }

  @override
  void initState() {
    _controller = TextEditingController(text: widget.text);
    _focus.addListener(_handleFocusChange);
    _focusValueNotifier = ValueNotifier<bool>(_focus.hasFocus);
    _focus.requestFocus();
    setState(() {
      _byteCount = widget.text.length;
    });
    super.initState();
  }

  @override
  void dispose() {
    super.dispose();
    _focus.removeListener(_handleFocusChange);
    _focus.dispose();
  }

  void _handleFocusChange() {
    _focusValueNotifier.value = _focus.hasFocus;
  }

  void _onDone() {
    print(_controller.text);
    Navigator.pop(context, _controller.text);
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: const Text('HEX' /*, style: TextStyle(color: Colors.white)*/),
        // backgroundColor: Colors.black,
      ),
      body: Column(
        mainAxisAlignment: MainAxisAlignment.start,
        children: [
          const SizedBox(height: 10),
          Text('已输入 $_byteCount 字节'),
          Padding(
            padding: const EdgeInsets.all(8.0),
            child: TextField(
              decoration: const InputDecoration(
                border: OutlineInputBorder(
                  borderSide: BorderSide(
                    color: Colors.grey,
                    width: 1,
                  ),
                ),
              ),
              controller: _controller,
              keyboardType: TextInputType.none,
              focusNode: _focus,
              maxLines: 12,
              maxLength: 1024,
              onChanged: _onTextChanged,//这里监听粘贴剪切导致的变化
            ),
          ),
          const Spacer(),
          ListenableBuilder(
            listenable: _focusValueNotifier,
            builder: (BuildContext context, Widget? child) {
              return _focus.hasFocus
                  ? HexKeyboard(
                      controller: _controller,
                      onDone: _onDone,
                      onTextChanged: _onTextChanged,//这里监听自定义键盘导致的变化
                    )
                  : Container();
            },
          ),
        ],
      ),
    );
  }
}