draw network image in CustomPainter and save content to file

customPainter.dart

import 'dart:io';
import 'dart:ui' as ui;

import 'package:flutter/material.dart';
import 'package:path_provider/path_provider.dart';

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

String FILENAME = 'timg.jpeg';

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      home: PainterPage(), // PosterPage(),
    );
  }
}

class PainterPage extends StatefulWidget {
  @override
  _PainterPageState createState() {
    return _PainterPageState();
  }
}

class _PainterPageState extends State<PainterPage> {
  ImageInfo _imageInfo;
  GlobalKey _containerKey = GlobalKey();

  @override
  void initState() {
    // load Image
    var imgUrl =
        'https://img.tuguaishou.com/ips_templ_preview/0b/66/10/lg_2034979_1566193207_5d5a363796f4e.jpg!w384?auth_key=2199888000-0-0-80d642dfdc7b206f09b718c79547acc8&v=1554825701';
    var img = NetworkImage(imgUrl);
    img.load(img).addListener(
        ImageStreamListener((ImageInfo imageInfo, bool synchronousCall) {
      this.setState(() {
        _imageInfo = imageInfo;
      });
    }));
  }

  _handleSavePressed() async {
    ui.PictureRecorder recorder = ui.PictureRecorder();
    Canvas canvas = Canvas(recorder);
    var painter = _PosterPainter(_imageInfo);
    var size = _containerKey.currentContext.size;

    painter.paint(canvas, size);
    ui.Image renderedImage = await recorder
        .endRecording()
        .toImage(size.width.floor(), size.height.floor());

    var pngBytes =
        await renderedImage.toByteData(format: ui.ImageByteFormat.png);

    Directory saveDir = await getApplicationDocumentsDirectory();
    File saveFile = File('${saveDir.path}/$FILENAME');

    if (!saveFile.existsSync()) {
      saveFile.createSync(recursive: true);
    }
    saveFile.writeAsBytesSync(pngBytes.buffer.asUint8List(), flush: true);
    Navigator.push(
      context,
      MaterialPageRoute(builder: (context) => ShowcasePage()),
    );
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('制作'),
      ),
      floatingActionButton: FloatingActionButton(
        child: Text('save'),
        onPressed: _handleSavePressed,
      ),
      body: SafeArea(
        child: Column(
          children: <Widget>[
            Expanded(
              child: Container(
                key: _containerKey,
                child: CustomPaint(
                  painter: _PosterPainter(_imageInfo),
                  child: Container(),
                ),
              ),
            ),
          ],
        ),
      ),
    );
  }
}

class _PosterPainter extends CustomPainter {
  ImageInfo imageInfo;

  _PosterPainter(this.imageInfo);

  paint(Canvas canvas, Size size) {
    canvas.save();
    canvas.drawRect(Rect.fromLTWH(0, 0, size.width, size.height),
        Paint()..color = Colors.blueGrey);

    canvas.restore();
    if (imageInfo != null) {
      canvas.save();
      canvas.drawImageRect(
        imageInfo.image,
        Rect.fromLTWH(
          0,
          0,
          imageInfo.image.width.toDouble(),
          imageInfo.image.height.toDouble(),
        ),
        Rect.fromLTWH(
          0,
          0,
          size.width,
          imageInfo.image.height.toDouble() /
              (imageInfo.image.width.toDouble() / size.width),
        ),
        Paint(),
      );
      canvas.restore();
    }
  }

  @override
  bool shouldRepaint(CustomPainter oldDelegate) => true;
}

class ShowcasePage extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('预览'),
      ),
      body: SafeArea(
          child: FutureBuilder(
        future: getApplicationDocumentsDirectory(),
        builder: (BuildContext context, snapshot) {
          if (snapshot.connectionState == ConnectionState.done) {
            return Image.file(File('${snapshot.data.path}/$FILENAME'));
          } else {
            return Container();
          }
        },
      )),
    );
  }
}