原文地址

Step.1 Canvas转换为图片

首先，你要了解flutter如何将canvas转换成图片，这里有一个非常好的例子分享给大家flutter_canvas_to_image，这里简单讲解一下思路：

1.首先创建一个 ui.PictureRecorder（记录仪）, 然后作为载体创建一个canvas对象

// 创建一个记录仪
final recorder = new ui.PictureRecorder();
final canvas = new Canvas(
    recorder,
    new Rect.fromPoints(
        new Offset(0.0, 0.0), new Offset(200.0, 200.0)));

2.在canvas上进行绘制

final stroke = new Paint()
  ..color = Colors.grey
  ..style = PaintingStyle.stroke;

canvas.drawRect(
    new Rect.fromLTWH(0.0, 0.0, 200.0, 200.0), stroke);

final paint = new Paint()
  ..color = color
  ..style = PaintingStyle.fill;

canvas.drawCircle(
    new Offset(
      widget.rd.nextDouble() * 200.0,
      widget.rd.nextDouble() * 200.0,
    ),
    20.0,
    paint);

3. 转换和显示

// 关闭记录仪
final picture = recorder.endRecording();
final img = picture.toImage(200, 200);
final pngBytes = await img.toByteData(format: new ui.EncodingFormat.png());
// 显示图片
new Image.memory(new Uint8List.view(imgBytes.buffer));

Step.2 Canvas截取

当我们懂得如何将canvas转为图片之后，我们简单看下canvas clip的代码（这并不是本文主要讲述的内容，所以不做详细说明），我们以canvas.clipPath截取图片为例

// 绘制一个三角形的path
Path _path = Path()
      ..moveTo(100, 50)
      ..lineTo(50, 150)
      ..lineTo(150, 150)
      ..lineTo(100, 50);

canvas.clipPath(_path);

canvasClip.drawImageRect(
      _image, // ui.Image
      Rect.fromLTWH(0, 0, _image.width.toDouble(), _image.height.toDouble()),, 
      Rect.fromLTWH(0, 0, 200, 200), // 画布Rect
      Paint()
    );

image

Step.3 获取截取之后的图形

我们已经将图片截取了，这个时候获取的图片大小是200*200的，但是我们只想要那个三角形怎么办那？思路如下:

1. 获取path围成的矩形
2. 将这个矩形移到画布的左上角
3. 转成目标大小的图片

思路有了，我们看实际解决办法，首先针对第一点，flutter Path 为我们提供了一个现成的方法 getBounds(), 看下官网的描述

getBounds() → Rect
Computes the bounding rectangle for this path. 

然后看下第二点，我们可以将之前生成的图片通过位移画在一个新的canvas的左上角，第三步在Step.1 已经充分get了，看下代码：

// 获取Path围成的矩形
Rect _bounds = _path.getBounds();
// 上一步生成的图片
ui.Image img = await picture.toImage(200, 200);
// 新建一个新的记录仪和canvas
final recorder2 = ui.PictureRecorder();
final canvasClip = Canvas(recorder2, Rect.fromLTWH(0, 0, size.width, size.height));

canvasClip.drawImageRect(
      img,
      _bound, // _bound 中已经包含左上角的offset，可以直接拿过来用
      Rect.fromLTWH(0, 0, _bound.width, _bound.height),
      Paint()
 );
// 停止录制 生成image
final picture2 = recorder2.endRecording();
ui.Image img2 = await picture2.toImage(size.width.toInt(), size.height.toInt());

final pngBytes = await img2.toByteData(format: ui.ImageByteFormat.png);

image

看起来没什么变化？我们可以打印下输出

image

very nice，大佬们如果有更好的方法麻烦告诉下，非常感谢

Step.4 献上完整代码

import 'dart:typed_data';
import 'dart:ui' as ui;
import 'package:myapp/utils/image.dart';
import 'package:flutter/material.dart';

void main() => runApp(App());

const kCanvasSize = 200.0;

class App extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        body: ImageGenerator(),
      ),
      debugShowCheckedModeBanner: false,
    );
  }
}

class ImageGenerator extends StatefulWidget {
  @override
  _ImageGeneratorState createState() => _ImageGeneratorState();
}

class _ImageGeneratorState extends State<ImageGenerator> {
  ByteData imgBytes;
  ui.Image _image;

  @override
  void initState() {
    super.initState();
    loadImage('assets/images/face.jpg').then((image) {
      setState(() {
        _image = image;
      });
    });
  }

  @override
  Widget build(BuildContext context) {
    return Center(
      child: Column(
        mainAxisSize: MainAxisSize.max,
        mainAxisAlignment: MainAxisAlignment.spaceEvenly,
        crossAxisAlignment: CrossAxisAlignment.center,
        children: <Widget>[
          Padding(
            padding: const EdgeInsets.all(12.0),
            child: RaisedButton(
                child: Text('Generate image'), onPressed: generateImage),
          ),
          imgBytes != null
              ? Container(
                  child: Image.memory(
                  Uint8List.view(imgBytes.buffer),
                  width: kCanvasSize,
                  height: kCanvasSize,
                ))
              : Container()
        ],
      ),
    );
  }

  void generateImage() async {
    final recorder = ui.PictureRecorder();
    final canvas = Canvas(recorder,
        Rect.fromPoints(Offset(0.0, 0.0), Offset(kCanvasSize, kCanvasSize)));

    final stroke = new Paint()
      ..color = Colors.grey
      ..style = PaintingStyle.stroke;

    canvas.drawRect(Rect.fromLTWH(0.0, 0.0, kCanvasSize, kCanvasSize), stroke);

    Path _path = Path()
      ..moveTo(100, 50)
      ..lineTo(50, 150)
      ..lineTo(150, 150)
      ..lineTo(100, 50);

    canvas.clipPath(_path);
    Rect _bound = _path.getBounds();

    canvas.drawImageRect(
        _image,
        Rect.fromLTWH(0, 0, _image.width.toDouble(), _image.height.toDouble()),
        Rect.fromLTWH(0, 0, 200, 200),
        Paint());

    final picture = recorder.endRecording();
    ui.Image img = await picture.toImage(200, 200);

    print('img的尺寸: $img');

    final recorder2 = ui.PictureRecorder();
    final canvasClip =
        Canvas(recorder2, Rect.fromLTWH(0, 0, _bound.width, _bound.height));

    canvasClip.drawImageRect(
        img, _bound, Rect.fromLTWH(0, 0, _bound.width, _bound.height), Paint());

    final picture2 = recorder2.endRecording();
    ui.Image img2 =
        await picture2.toImage(_bound.width.toInt(), _bound.height.toInt());

    print('img2的尺寸: $img2');

    final pngBytes = await img2.toByteData(format: ui.ImageByteFormat.png);

    setState(() {
      imgBytes = pngBytes;
    });
  }
}