Flutter 完美的验证码输入框（2 种方法）

import 'package:flutter/material.dart';
import 'package:flutter/services.dart';
 
class VerificationBox extends StatefulWidget {
  final ValueChanged<String> onChanged;
  final int length;
  final String autoFillChar;
 
  const VerificationBox({
    Key? key,
    required this.onChanged,
    this.length = 6,
    this.autoFillChar = '●',
  }) : super(key: key);
 
  @override
  _VerificationBoxState createState() => _VerificationBoxState();
}
 
class _VerificationBoxState extends State<VerificationBox> {
  final TextEditingController _controller = TextEditingController();
  final FocusNode _focusNode = FocusNode();
  String _pinCode = '';
 
  @override
  void initState() {
    super.initState();
    _controller.text = '';
    _controller.addListener(() {
      setState(() {
        _pinCode = _controller.text;
        widget.onChanged(_pinCode);
      });
    });
    _focusNode.addListener(() {
      if (!_focusNode.hasFocus) {
        _controller.text = _pinCode;
      }
    });
  }
 
  @override
  void dispose() {
    _controller.dispose();
    _focusNode.dispose();
    super.dispose();
  }
 
  @override
  Widget build(BuildContext context) {
    return TextField(
      controller: _controller,
      focusNode: _focusNode,
      inputFormatters: [
        LengthLimitingTextInputFormatter(widget.length),
        PinCodeTextInputFormatter(widget.length, widget.autoFillChar),
      ],
      keyboardType: TextInputType.number,
      textAlign: TextAlign.center,
      decoration: InputDecoration.collapsed(hintText: ''),
      style: const TextStyle(fontSize: 20.0),
      onChanged: (value) {},
    );
  }
}
 
class PinCodeTextInputFormatter extends TextInputFormatter {
  PinCodeTextInputFormatter(this.expectedLength, this.character)
      : assert(expectedLength == null || expectedLength > 0);
 
  final int? expectedLength;
  final String character;
 
  @override
  TextEditingValue formatEditUpdate(TextEditingValue oldValue, TextEditingValue newValue) {
    final newText = StringBuffer();
    int textLength = 0;
    for (int i = 0; i < newValue.text.length; i++) {
      if (newValue.text[i] != character) {
        newText.write(newValue.text[i]);
        text