是否可以使用 Dart/Flutter 绘制图像？

Question

我正在寻找从颤振应用程序中生成图像（jpeg 或 png）的路径。图像将由圆圈、线条、文本等组成。

似乎确实有一种方法可以使用画布 (https://docs.flutter.io/flutter/dart-ui/Canvas/Canvas.html) 在屏幕上绘图，但似乎没有等效的方法可以创建可以在应用程序内部呈现或发送/使用的图像.

是否有任何可用于绘制图像的 dart 库？考虑到底层的skia框架，这似乎是可能的。在 Dart-html 包中有一个 CanvasRenderingContext2D。

编辑：获得类似以下的工作（根据理查德的建议）将是一个开始：

import 'package:flutter/material.dart';
import 'package:path_provider/path_provider.dart';
import 'dart:ui';
import 'dart:typed_data';
import 'dart:async';
import 'dart:io';

void main() => runApp(new MyApp());

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
      title: 'Flutter Demo',
      theme: new ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: new MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);
  final String title;

  @override
  _MyHomePageState createState() => new _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  Image _image;

  @override
  void initState() {
    super.initState();
    _image = new Image.network(
      'https://www.google.com/images/branding/googlelogo/2x/googlelogo_color_120x44dp.png',
    );
  }

  Future<String> get _localPath async {
    final directory =
        await getApplicationDocumentsDirectory(); //From path_provider package
    return directory.path;
  }

  Future<File> get _localFile async {
    final path = await _localPath;
    return new File('$path/tempImage.png');
  }

  Future<File> writeImage(ByteData pngBytes) async {
    final file = await _localFile;
    // Write the file
    file.writeAsBytes(pngBytes.buffer.asUint8List());
    return file;
  }

  _generateImage() {
    _generate().then((val) => setState(() {
          _image = val;
        }));
  }

  Future<Image> _generate() async {
    PictureRecorder recorder = new PictureRecorder();
    Canvas c = new Canvas(recorder);
    var rect = new Rect.fromLTWH(0.0, 0.0, 100.0, 100.0);
    c.clipRect(rect);

    final paint = new Paint();
    paint.strokeWidth = 2.0;
    paint.color = const Color(0xFF333333);
    paint.style = PaintingStyle.fill;

    final offset = new Offset(50.0, 50.0);
    c.drawCircle(offset, 40.0, paint);
    var picture = recorder.endRecording();

    final pngBytes =
        await picture.toImage(100, 100).toByteData(format: ImageByteFormat.png);

    //Aim #1. Upade _image with generated image.
    var image = Image.memory(pngBytes.buffer.asUint8List());
    return image;

    //new Image.memory(pngBytes.buffer.asUint8List());
    // _image = new Image.network(
    //   'https://github.com/flutter/website/blob/master/_includes/code/layout/lakes/images/lake.jpg?raw=true',
    // );

    //Aim #2. Write image to file system.
    //writeImage(pngBytes);
    //Make a temporary file (see elsewhere on SO) and writeAsBytes(pngBytes.buffer.asUInt8List())
  }

  @override
  Widget build(BuildContext context) {
    return new Scaffold(
      appBar: new AppBar(
        title: new Text(widget.title),
      ),
      body: new Center(
        child: new Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            _image,
          ],
        ),
      ),
      floatingActionButton: new FloatingActionButton(
        onPressed: _generateImage,
        tooltip: 'Generate',
        child: new Icon(Icons.add),
      ),
    );
  }
}

Answer 1

PictureRecorder 允许您创建 Canvas，使用 Canvas 绘图方法并提供返回 Picture 的 endRecording()。您可以将此图片绘制到其他场景或画布上，或使用.toImage(width, height).toByteData(format) 将其转换为 PNG（或 raw - 不支持 jpeg）。

例如：

import 'dart:ui';
import 'dart:typed_data';
....
  PictureRecorder recorder = new PictureRecorder();
  Canvas c = new Canvas(recorder);
  c.drawPaint(paint); // etc
  Picture p = recorder.endRecording();
  ByteData pngBytes =
      await p.toImage(100, 100).toByteData(format: ImageByteFormat.png);

确保您使用的是 Flutter 0.4.4，否则您可能没有可用的 format 参数。

不过，看到您的编辑后，我怀疑您真的在寻找CustomPainter，其中一个小部件为您提供了一个可以在其上绘制的画布。这是来自类似问题的example。