flutter 验证码图片算法

Flutter 并没有提供专门的验证码图片算法，但可以使用 Flutter 提供的图形绘制 API 和随机数生成 API 来实现验证码图片算法。

以下是一个简单的示例代码，实现了一个四位数的数字验证码图片生成：

import 'dart:math';
import 'dart:ui';

import 'package:flutter/material.dart';

class CaptchaImage extends StatefulWidget {
  final ValueChanged<String> onCaptchaChanged;

  CaptchaImage({Key key, this.onCaptchaChanged}) : super(key: key);

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

class _CaptchaImageState extends State<CaptchaImage> {
  String captcha;
  final TextEditingController _textEditingController =
      TextEditingController();

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

  void _generateCaptcha() {
    setState(() {
      captcha = _generateRandomString(4); // 生成四个随机数字
      widget.onCaptchaChanged?.call(captcha);
    });
  }

  String _generateRandomString(int length) {
    var random = Random.secure();
    var values = List<int>.generate(length, (i) => random.nextInt(10));
    return values.join();
  }

  @override
  Widget build(BuildContext context) {
    return GestureDetector(
      onTap: _generateCaptcha,
      child: CustomPaint(
        size: Size(100, 40),
        painter: CaptchaPainter(captcha),
      ),
    );
  }
}

class CaptchaPainter extends CustomPainter {
  final String captcha;
  final Paint _paint = Paint()
    ..color = Colors.black
    ..strokeWidth = 1.5
    ..style = PaintingStyle.stroke;

  CaptchaPainter(this.captcha);

  @override
  void paint(Canvas canvas, Size size) {
    // 绘制背景
    canvas.drawRect(Rect.fromLTWH(0, 0, size.width, size.height),
        Paint()..color = Colors.white);

    // 绘制验证码
    var textStyle = TextStyle(fontSize: 24, fontWeight: FontWeight.bold);
    var textSpan = TextSpan(
      text: captcha,
      style: textStyle,
    );
    var textPainter = TextPainter(
      text: textSpan,
      textAlign: TextAlign.center,
      textDirection: TextDirection.ltr,
    );
    textPainter.layout(minWidth: 0, maxWidth: size.width);
    textPainter.paint(canvas, Offset((size.width - textPainter.width) / 2, 8));

    // 绘制干扰线
    var random = Random.secure();
    for (int i = 0; i < 4; i++) {
      var startX = random.nextInt(size.width.toInt());
      var startY = random.nextInt(size.height.toInt());
      var endX = random.nextInt(size.width.toInt());
      var endY = random.nextInt(size.height.toInt());
      canvas.drawLine(Offset(startX.toDouble(), startY.toDouble()),
          Offset(endX.toDouble(), endY.toDouble()), _paint);
    }
  }

  @override
  bool shouldRepaint(CustomPainter oldDelegate) {
    return true;
  }
}

这个示例代码中，`CaptchaImage` 是一个自定义的 `StatefulWidget`，用来生成验证码图片。`CaptchaPainter` 是一个自定义的 `CustomPainter`，用来绘制验证码图片。在 `CaptchaPainter` 中，我们先绘制一个白色的矩形作为背景，然后使用 `TextPainter` 绘制随机生成的四个数字，最后再绘制四条随机干扰线。在 `CaptchaImage` 中，我们通过 `CustomPaint` 将 `CaptchaPainter` 绘制出来，并且通过 `GestureDetector` 来响应用户点击事件，生成新的验证码。