Flutter - Canvas Draw Z Order

Question

Using Flutter's CustomPainter to draw various Image s in a Canvas to then be rendered on screen. My expectation is that the depth order of any drawn overlapping Images to be as they were drawn on the Canvas (eg canvas.drawImage(image1,..); canvas.drawImage(image2,..); canvas.drawImage(image3,..); would render the three images on the screen whith image1 appearing underneath image2 and image3 respectively). What I am finding however is that on certain occasions the order of the rendered images is not entirely preserved.

See example code below:

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;
}

Where: image1 and image3 are: wfw_1.png . image2 is: wnfw_2.png

The rendeder output is as follows on my laptop's Android Emulator device:

rendered output

As can be seen, image2 is rendered on top of the last drawn image ( image3 ) when I was expecting it to be rendered below image2 (ie in between image1 and image3 ).

The only way I have been able to workaround the problem is by either: Adding a dummy drawRect(..) call in between the drawImage(..) calls or, by setting image2 's Offset.dy to offset.dy+0.001 .

I am new to Dart and Flutter in general so I am not sure if it is a) My code that is wrong, b) My understanding of the Flutter rendering engine that is wrong or c) a bug in dart:ui somewhere?

Answer 1

我认为是c），因为在更新为flutter version 0.10.2之后现在保持了顺序。