flutter中水印实例: 文本绘制与离屏渲染

在Flutter中，添加文本水印通常涉及到`CustomPaint` widget结合`Skia`图形库，用于自定义绘制。下面是一个基本的例子，展示如何在Flutter中创建一个带有文本的水印并利用离屏渲染提高性能：

import 'package:flutter/material.dart';
import 'dart:ui' as ui;

class WatermarkPainter extends CustomPainter {
  final String watermarkText;
  final Color color;

  WatermarkPainter({required this.watermarkText, required this.color});

  @override
  void paint(Canvas canvas, Size size) {
    // 计算水印的位置和大小
    final textWidth = await文字尺寸计算(watermarkText);
    final textHeight = await 文字高度计算(watermarkText);

    // 离屏渲染，这里假设有一个OffscreenCanvas用于预渲染
    final offscreenCanvas = ui.Canvas(
      width: size.width,
      height: textHeight,
    );
    TextSpan textSpan = TextSpan(text: watermarkText, style: TextStyle(color: color));
    textPainter.draw(in: offscreenCanvas.context, text: textSpan);

    // 将预渲染的结果画到主canvas上
    canvas.drawImage(
      offscreenCanvas.toImage(ui.ImageFormat.png).image,
      Offset(0, size.height - textHeight),
      Paint(),
    );
  }

  @override
  bool shouldRepaint(CustomPainter oldDelegate) => true; // 每次都重绘

  Future<ui.TextMetrics> 文字尺寸计算(String text) async {
    final metrics = await ui.measureText(text, TextStyle(fontSize: 16)); // 调用系统提供的测量函数
    return metrics;
  }

  Future<double> 文字高度计算(String text) async {
    final metrics = await 文字尺寸计算(text);
    return metrics.height;
  }
}

// 使用示例
Widget build(BuildContext context) {
  return Container(
    child: CustomPaint(
      painter: WatermarkPainter(watermarkText: "这是水印", color: Colors.grey),
    ),
  );
}