canvas - Flutter - Canvas 绘制 Z 顺序

Question

使用 Flutter 的 CustomPainter绘制各种Image s 在 Canvas然后在屏幕上呈现。我的期望是任何绘制的重叠图像的深度顺序与它们在 Canvas 上绘制时的深度顺序相同(例如 canvas.drawImage(image1,..); canvas.drawImage(image2,..); canvas.drawImage(image3,..); 将在屏幕上呈现三个图像，其中 image1 分别出现在 image2 和 image3 下方)。然而，我发现在某些情况下，渲染图像的顺序并未完全保留。

参见下面的示例代码:

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

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

class ExampleApp extends StatelessWidget {

  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
      title: 'Ground Example',
      theme: new ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: ExampleScreen(),
    );
  }
}

class ExampleScreen extends StatefulWidget {
  final Offset initPos = Offset(0.0, 0.0);

  @override
  _ExampleScreenState createState() => _ExampleScreenState();
}

class _ExampleScreenState extends State<ExampleScreen> {
  ui.Image _image1;
  ui.Image _image2;
  ui.Image _image3;
  bool _prepDone;

  @override
  void initState() {
    _prepDone = false;
    super.initState();
    //_draggedToOffset = widget.initPos;
  }

  _ExampleScreenState() {
    _prepare();
  }

  _prepare() {
    loadImage('assets/wfw_1.png').then((image1) {
      _image1 = image1;
    }).whenComplete(() {
      loadImage('assets/wnfw_2.png').then((image2) {
        _image2 = image2;
      }).whenComplete(() {
        loadImage('assets/wfw_1.png').then((image3) {
          _image3 = image3;
        }).whenComplete(() {
          _prepDone = true;
          _handle();
        });
      });
    });
  }

  _handle() {
    setState(() {});
  }

  @override
  Widget build(BuildContext context) {
    return new Container(
        constraints: BoxConstraints.expand(),
        color: Colors.grey,
        child: new Stack(children: <Widget>[
          _prepDone ? Container(child: buildPainter()) : Container()
        ]));
  }

  buildPainter() {
    return Container(
        child: CustomPaint(
          painter: ExamplePainter(_image1, _image2, _image3),
        ));
  }
}

class ExamplePainter extends CustomPainter {
  ui.Image _image1;
  ui.Image _image2;
  ui.Image _image3;
  Paint _paint = Paint();

  ExamplePainter(this._image1, this._image2, this._image3);

  @override
  void paint(Canvas canvas, Size size) {
    canvas.drawImage(_image1, Offset(100.0, 35.0), _paint);
    canvas.drawImage(_image2, Offset(70.0, 50.0), _paint);
    canvas.drawImage(_image3, Offset(40.0, 65.0), _paint);
  }

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

Future<ui.Image> loadImage(String assetPath) async {
  ImageStream stream = new AssetImage(assetPath, bundle: rootBundle)
      .resolve(ImageConfiguration.empty);
  Completer<ui.Image> completer = new Completer<ui.Image>();
  void listener(ImageInfo frame, bool synchronousCall) {
    final ui.Image image = frame.image;
    completer.complete(image);
    stream.removeListener(listener);
  }

  stream.addListener(listener);
  return completer.future;
}

地点: image1和 image3是:wfw_1.png . image2是:wnfw_2.png

渲染输出在我的笔记本电脑的 Android Emulator 设备上如下:

rendered output

可以看出，image2呈现在最后绘制的图像( image3 )之上，而我期望它呈现在 image2 下面(即在 image1 和 image3 之间)。

我能够解决该问题的唯一方法是:添加一个虚拟 drawRect(..)在 drawImage(..) 之间打电话调用或通过设置 image2的 Offset.dy至 offset.dy+0.001 .

一般来说，我是 Dart 和 Flutter 的新手，所以我不确定是不是 a) 我的代码有误，b) 我对 Flutter 渲染引擎的理解有误，或者 c) dart:ui 某处的错误？

Answer 1

我认为是 c) 因为在更新到 flutter version 0.10.2 后，订单现在保持不变。