使用 PictureRecorder 保存 Canvas 的图片会导致图像为空

Question

首先，该程序的目标是允许用户使用手机或平板电脑签署官方文件。 程序必须将图像保存为 png。

我使用 Flutter（和 dart）和 VS Code 来开发这个应用程序。

什么有效：

-The user can draw on the canvas.

什么不起作用：

-the image can't be saved as a png

我发现了什么：

-The **Picture** get by ending the **PictureRecoder** of the canvas is empty (i tried to display it but no success)
-I tried to save it as a PNG using **PictureRecorder.EndRecording().toImage(100.0,100.0).toByteDate(EncodingFormat.png())** but the size is really small, and it can't be displayed.

如果你们中的一些人能给我一些关于问题出在哪里的提示，那就太好了。

仅供参考：Flutter 在开发频道上是最新版本

这是完整的代码：

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

///This class is use just to check if the Image returned by
///the PictureRecorder of the first Canvas is not empty.
///FYI : The image is not displayed.
class CanvasImageDraw extends CustomPainter {

    ui.Picture picture;

    CanvasImageDraw(this.picture);

    @override
    void paint(ui.Canvas canvas, ui.Size size) {
      canvas.drawPicture(picture);
    }

    @override
    bool shouldRepaint(CustomPainter oldDelegate) {
      return true;
    }
}

///Class used to display the second canvas
class SecondCanvasView extends StatelessWidget {

    ui.Picture picture;

    var canvas;
    var pictureRecorder= new ui.PictureRecorder();

    SecondCanvasView(this.picture) {
      canvas = new Canvas(pictureRecorder);
    }

    @override
    Widget build(BuildContext context) {
      return new Scaffold(
        appBar: new AppBar(
          title: new Text('Image Debug'),
        ),
        body: new Column(
          children: <Widget>[
            new Text('Top'),
            CustomPaint(
              painter: new CanvasImageDraw(picture),
            ),
            new Text('Bottom'),
          ],
        ));
    }
}

///This is the CustomPainter of the first Canvas which is used
///to draw to display the users draw/sign.
class SignaturePainter extends CustomPainter {
  final List<Offset> points;
  Canvas canvas;
  ui.PictureRecorder pictureRecorder;

  SignaturePainter(this.points, this.canvas, this.pictureRecorder);

  void paint(canvas, Size size) {
    Paint paint = new Paint()
      ..color = Colors.black
      ..strokeCap = StrokeCap.round
      ..strokeWidth = 5.0;
    for (int i = 0; i < points.length - 1; i++) {
      if (points[i] != null && points[i + 1] != null)
        canvas.drawLine(points[i], points[i + 1], paint);
    }
  }

  bool shouldRepaint(SignaturePainter other) => other.points != points;
}

///Classes used to get the user draw/sign, send it to the first canvas, 
///then display it.
///'points' list of position returned by the GestureDetector
class Signature extends StatefulWidget {
  ui.PictureRecorder pictureRecorder = new ui.PictureRecorder();
  Canvas canvas;
  List<Offset> points = [];

  Signature() {
    canvas = new Canvas(pictureRecorder);
  }

  SignatureState createState() => new SignatureState(canvas, points);
}

class SignatureState extends State<Signature> {
  List<Offset> points = <Offset>[];
  Canvas canvas;

  SignatureState(this.canvas, this.points);

  Widget build(BuildContext context) {
    return new Stack(
      children: [
        GestureDetector(
          onPanUpdate: (DragUpdateDetails details) {
            RenderBox referenceBox = context.findRenderObject();
            Offset localPosition =
            referenceBox.globalToLocal(details.globalPosition);

            setState(() {
              points = new List.from(points)..add(localPosition);
            });
          },
          onPanEnd: (DragEndDetails details) => points.add(null),
        ),
        new Text('data'),
        CustomPaint(
            painter:
                new SignaturePainter(points, canvas, widget.pictureRecorder)),
        new Text('data')
      ],
    );
  }
}


///The main class which display the first Canvas, Drawing/Signig area
///
///Floating action button used to stop the PictureRecorder's recording,
///then send the Picture to the next view/Canvas which should display it
class DemoApp extends StatelessWidget {
  Signature sign = new Signature();
  Widget build(BuildContext context) => new Scaffold(
        body: sign,
        floatingActionButton: new FloatingActionButton(
          child: new Icon(Icons.save),
          onPressed: () async {
            ui.Picture picture = sign.pictureRecorder.endRecording();
            Navigator.push(context, new MaterialPageRoute(builder: (context) => new SecondCanvasView(picture)));
          },
        ),
  );
}

void main() => runApp(new MaterialApp(home: new DemoApp()));

Answer 1

首先，感谢您提出有趣的问题和独立的答案。这令人耳目一新，而且更容易提供帮助！

您的代码存在一些问题。首先是您不应该将画布和点从 Signature 传递到 SignatureState；这是 Flutter 中的反模式。

这里的代码有效。我已经做了一些对答案来说不必要的小事情来清理它，对此感到抱歉=D。

import 'dart:ui' as ui;

import 'package:flutter/material.dart';

///This class is use just to check if the Image returned by
///the PictureRecorder of the first Canvas is not empty.
class CanvasImageDraw extends CustomPainter {
  ui.Image image;

  CanvasImageDraw(this.image);

  @override
  void paint(ui.Canvas canvas, ui.Size size) {
    // simple aspect fit for the image
    var hr = size.height / image.height;
    var wr = size.width / image.width;

    double ratio;
    double translateX;
    double translateY;
    if (hr < wr) {
      ratio = hr;
      translateX = (size.width - (ratio * image.width)) / 2;
      translateY = 0.0;
    } else {
      ratio = wr;
      translateX = 0.0;
      translateY = (size.height - (ratio * image.height)) / 2;
    }

    canvas.translate(translateX, translateY);
    canvas.scale(ratio, ratio);
    canvas.drawImage(image, new Offset(0.0, 0.0), new Paint());
  }

  @override
  bool shouldRepaint(CanvasImageDraw oldDelegate) {
    return image != oldDelegate.image;
  }
}

///Class used to display the second canvas
class SecondView extends StatelessWidget {
  ui.Image image;

  var pictureRecorder = new ui.PictureRecorder();

  SecondView({this.image});

  @override
  Widget build(BuildContext context) {
    return new Scaffold(
        appBar: new AppBar(
          title: new Text('Image Debug'),
        ),
        body: new Column(
          children: <Widget>[
            new Text('Top'),
            CustomPaint(
              painter: new CanvasImageDraw(image),
              size: new Size(200.0, 200.0),
            ),
            new Text('Bottom'),
          ],
        ));
  }
}

///This is the CustomPainter of the first Canvas which is used
///to draw to display the users draw/sign.
class SignaturePainter extends CustomPainter {
  final List<Offset> points;
  final int revision;

  SignaturePainter(this.points, [this.revision = 0]);

  void paint(canvas, Size size) {
    if (points.length < 2) return;

    Paint paint = new Paint()
      ..color = Colors.black
      ..strokeCap = StrokeCap.round
      ..strokeWidth = 5.0;

    for (int i = 0; i < points.length - 1; i++) {
      if (points[i] != null && points[i + 1] != null)
        canvas.drawLine(points[i], points[i + 1], paint);
    }
  }

  // simplified this, but if you ever modify points instead of changing length you'll
  // have to revise.
  bool shouldRepaint(SignaturePainter other) => other.revision != revision;
}

///Classes used to get the user draw/sign, send it to the first canvas,
///then display it.
///'points' list of position returned by the GestureDetector
class Signature extends StatefulWidget {
  Signature({Key key}): super(key: key);

  @override
  State<StatefulWidget> createState()=> new SignatureState();
}

class SignatureState extends State<Signature> {
  List<Offset> _points = <Offset>[];
  int _revision = 0;

  ui.Image get rendered {
    var pictureRecorder = new ui.PictureRecorder();
    Canvas canvas = new Canvas(pictureRecorder);
    SignaturePainter painter = new SignaturePainter(_points);

    var size = context.size;
    // if you pass a smaller size here, it cuts off the lines
    painter.paint(canvas, size);
    // if you use a smaller size for toImage, it also cuts off the lines - so I've
    // done that in here as well, as this is the only place it's easy to get the width & height.
    return pictureRecorder.endRecording().toImage(size.width.floor(), size.height.floor());
  }

  void _addToPoints(Offset position) {
    _revision++;
    _points.add(position);
  }

  Widget build(BuildContext context) {
    return new Stack(
      children: [
        GestureDetector(
          onPanStart: (DragStartDetails details) {
            RenderBox referenceBox = context.findRenderObject();
            Offset localPosition = referenceBox.globalToLocal(details.globalPosition);
            setState(() {
              _addToPoints(localPosition);
            });
          },
          onPanUpdate: (DragUpdateDetails details) {
            RenderBox referenceBox = context.findRenderObject();
            Offset localPosition = referenceBox.globalToLocal(details.globalPosition);
            setState(() {
              _addToPoints(localPosition);
            });
          },
          onPanEnd: (DragEndDetails details) => setState(() => _addToPoints(null)),
        ),
        CustomPaint(painter: new SignaturePainter(_points, _revision)),
      ],
    );
  }
}

///The main class which display the first Canvas, Drawing/Signing area
///
///Floating action button used to stop the PictureRecorder's recording,
///then send the Picture to the next view/Canvas which should display it
class DemoApp extends StatelessWidget {
  GlobalKey<SignatureState> signatureKey = new GlobalKey();

  Widget build(BuildContext context) => new Scaffold(
        body: new Signature(key: signatureKey),
        floatingActionButton: new FloatingActionButton(
          child: new Icon(Icons.save),
          onPressed: () async {
            var image = signatureKey.currentState.rendered;
            Navigator.push(context, new MaterialPageRoute(builder: (context) => new SecondView(image: image)));
          },
        ),
      );
}

void main() => runApp(new MaterialApp(home: new DemoApp()));

您遇到的最大问题是您将自己的 canvas 对象到处都是 - 这不是您真正应该做的事情。 CustomPainter 提供了自己的画布。当你在画画的时候，我不认为它实际上是在正确的地方画画。

此外，每次添加点时，您都​​会重新创建列表。我将假设这是因为other.points != points 导致您的画布无法绘制。我已将其更改为采用一个修订 int，每次将某些内容添加到列表中时它都会递增。更好的是使用您自己的列表子类，它自己执行此操作，但是对于此示例 =D 来说有点多。如果您确定永远不会修改列表中的任何元素，您也可以只使用 other.points.length != points.length。

考虑到图像的大小，还有一些事情要做。我走了最简单的路线，使画布与其父级的大小相同，因此渲染更容易，因为它可以再次使用相同的大小（因此渲染一个手机屏幕大小的图像）。如果您不想这样做而是渲染自己的大小，则可以这样做。您必须向 SignaturePainter 传递一些内容，以便它知道如何缩放点以使其适合新大小（如果您愿意，可以在 CanvasImageDraw 中调整方面适合代码）。

如果您对我所做的事情有任何其他问题，请随时提出。