如何在flutter应用程序的所有屏幕上添加水印

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('Watermark Demo'),
        ),
        body: Watermark(
          text: 'Watermark',
          color: Colors.grey,
          fontSize: 20.0,
          opacity: 0.5,
        ),
        // 其他页面组件
      ),
    );
  }
}

class Watermark extends StatelessWidget {
  final String text;
  final Color color;
  final double fontSize;
  final double opacity;

  Watermark({
    required this.text,
    this.color = Colors.grey,
    this.fontSize = 20.0,
    this.opacity = 0.5,
  });

  @override
  Widget build(BuildContext context) {
    return CustomPaint(
      foregroundPainter: WatermarkPainter(
        text: text,
        color: color.withOpacity(opacity),
        fontSize: fontSize,
      ),
      child: Container(),
    );
  }
}

class WatermarkPainter extends CustomPainter {
  final String text;
  final Color color;
  final double fontSize;

  WatermarkPainter({
    required this.text,
    required this.color,
    required this.fontSize,
  });

  @override
  void paint(Canvas canvas, Size size) {
    final textStyle = TextStyle(
      color: color,
      fontSize: fontSize,
    );

    final textSpan = TextSpan(
      text: text,
      style: textStyle,
    );

    final textPainter = TextPainter(
      text: textSpan,
      textDirection: TextDirection.ltr,
    );

    textPainter.layout(
      minWidth: 0,
      maxWidth: size.width,
    );

    final x = (size.width - textPainter.width) / 2;
    final y = (size.height - textPainter.height) / 2;

    final offset = Offset(x, y);
    textPainter.paint(canvas, offset);
  }

  @override
  bool shouldRepaint(WatermarkPainter oldDelegate) {
    return oldDelegate.text != text ||
        oldDelegate.color != color ||
        oldDelegate.fontSize != fontSize;
  }
}