flutter - 无法在 Flutter Canvas 上绘制图像

Question

我目前正在尝试在 flutter canvas 上绘制图像，然后在图像上绘制矩形以覆盖特定数据。我遇到了一个问题，矩形绘制得很好，但图像没有被绘制在后面。它看起来好像根本没有被绘制。

我正在将图像从字节加载到 Dart UI Image 对象中。加载图像后，我创建一个 PictureRecorder 并将其传递到 Canvas 对象中。然后我尝试先绘制图像对象，然后在顶部绘制矩形。完整代码如下:

import 'dart:async';
import 'dart:typed_data';

import 'dart:ui' as ui;

import 'package:common/ocr/scraped_card_field.dart';
import 'package:flutter/material.dart';
import 'package:flutter/services.dart';

class ObfuscatedCardRenderer {
  ui.Image _backgroundImage;

  ///
  ///
  ///
  Future<ByteData> renderImage(
      Uint8List imageBytes, List<ScrapedCardField> fields) async {
    await _loadImageBackground(imageBytes);
    var obfuscatedImage = await _obfuscateImage(fields);

    var byteD =
        await obfuscatedImage.toByteData(format: ui.ImageByteFormat.png);

    return byteD;
  }

  ///
  ///
  ///
  Future<ui.Image> _obfuscateImage(List<ScrapedCardField> fields) async {
    ui.PictureRecorder recorder = ui.PictureRecorder();
    ui.Canvas c = ui.Canvas(recorder);

    var paint = ui.Paint();

    c.drawImage(_backgroundImage, ui.Offset(0.0, 0.0), new Paint());
    for (ScrapedCardField field in fields) {
      paint.color = Colors.white;
      c.drawRect(field.boundingBox, paint);
    }

    var picture = recorder.endRecording();
    return picture.toImage(_backgroundImage.width, _backgroundImage.height);
  }

  ///
  ///
  ///
  Future<void> _loadImageBackground(Uint8List imageBytes) async {
    if (imageBytes != null) {
      _backgroundImage = await _getImageFromBytes(imageBytes);
    } else {
      return null;
    }
  }

  ///
  ///
  ///
  Future<ui.Image> _getImageFromBytes(Uint8List imageBytes) async {
    var completer = Completer<ui.Image>();
    ui.decodeImageFromList(imageBytes, (image) {
      completer.complete(image);
    });

    return completer.future;
  }
}

结果如下:

如您所见，矩形绘制得非常好，但是背景中的图像不可见。我还应该做些什么来绘制图像，还是有其他方法可以做到这一点？

谢谢!

Answer 1

我在下面发布了我的应用程序的工作代码，该代码加载图像并将其显示在 flutter Canvas 中。希望对以后的读者有所帮助。

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

class ImageInsideCanvasPage extends StatefulWidget {
  @override
  _ImageInsideCanvasPageState createState() => _ImageInsideCanvasPageState();
}

class _ImageInsideCanvasPageState extends State<ImageInsideCanvasPage> {
  ui.Image? image;

  @override
  void initState() {
    // Add your own asset image link
    _load('assets/img.png');
    super.initState();
  }

  void _load(String path) async {
    var bytes = await rootBundle.load(path);
    image = await decodeImageFromList(bytes.buffer.asUint8List());
    setState(() {});
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Center(
        child: Container(
          child: CustomPaint(
            painter: ImageInsideCanvas(image: image),
            child: SizedBox.expand(),
          ),
        ),
      ),
    );
  }
}

class ImageInsideCanvas extends CustomPainter {
  ImageInsideCanvas({required this.image});
  ui.Image? image;

  @override
  void paint(Canvas canvas, Size size) async {
    Paint greenBrush = Paint()..color = Colors.greenAccent;
    canvas.save();
    canvas.drawImage(image!, Offset(0, 0), greenBrush);
    canvas.restore();
  }

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