如何从Dart / Flutter中的抽象类ui，Image创建子类？

Question

I am trying to get an local asset image loaded into an ui.Image object. But the ui.Image is an abstract class. I basically have this :

    import 'dart:ui' as ui;
    class MyImage implements ui.Image{

    int height;
    int width;

    MyImage(String file){

    }

    @override
    void dispose() {
    // TODO: implement dispose
    super.dispose();
    }

    @override
    String toString() {
    // TODO: implement toString
    return super.toString();
     }
    }

Using your code with the function definition Below. The error is at ' ui.Image image = await loadImage(img); ' await is underlined red and tool tip is 'Undefined name 'await' in function body not marked with async.'

    class TrialApp extends StatefulWidget {
    @override
    _TrialAppState createState() => new _TrialAppState();
    }

    class _TrialAppState extends State<TrialApp> {

        NodeWithSize rootNode;

    @override
    void initState() {
    // TODO: implement initState
        super.initState();
        rootNode = new NodeWithSize(new Size(400.0, 400.0));
    }

    @override
    Widget build(BuildContext context) {
    // define a function that converts the I.Image object into ui.Image 
    //object
        Future<ui.Image> loadImage(I.Image img) async {
        final Completer<ui.Image> imageCompleter = new Completer();
        ui.decodeImageFromList(img.getBytes(), (ui.Image img) {
          imageCompleter.complete(img);
        });

        return imageCompleter.future;
        }

    // Obtain a `I.Image` object from the image file
    I.Image img = I.decodeImage(new io.File('images/tile.png').readAsBytesSync());

    // Obtain the `ui.Image` from the `I.Image` object
    ui.Image image = await loadImage(img);

   Sprite myButton = new Sprite.fromImage(image);
   rootNode.addChild(myButton);
   return new SpriteWidget(rootNode);
    }
   }

Answer 1

First obtain the image from the assetbundle using rootBundle . The convert the obtained ByteData to List<int> . Now you can obtain a ui.Image using the decodeImageFromList method.

Example:

// import statements
import 'dart:async';
import 'dart:typed_data';
import 'dart:ui' as ui;

import 'package:flutter/material.dart';
import 'package:flutter/services.dart';
import 'package:spritewidget/spritewidget.dart';

class TrialApp extends StatefulWidget {
  @override
  _TrialAppState createState() => new _TrialAppState();
}

class _TrialAppState extends State<TrialApp> {
  NodeWithSize rootNode = new NodeWithSize(new Size(400.0, 400.0));

  @override
  void initState() {
    super.initState();
    init();
  }

  Future<Null> init() async {
    rootNode = new NodeWithSize(new Size(400.0, 400.0));

    // Read file from assetbundle
    final ByteData data = await rootBundle.load('images/tile.png');

    // Convert the obtained ByteData into ui.Image
    final ui.Image image = await loadImage(new Uint8List.view(data.buffer)); // Uint8List converts the ByteData into List<int>

    Sprite myButton = new Sprite.fromImage(image);
    rootNode.addChild(myButton);

    // notify to redraw with child
    setState(() {
      rootNode = rootNode;
    });
  }

  // define a function that converts the List<int> into ui.Image object
  Future<ui.Image> loadImage(List<int> img) async {
    final Completer<ui.Image> imageCompleter = new Completer();
    ui.decodeImageFromList(img, (ui.Image img) {
      imageCompleter.complete(img);
    });

    return imageCompleter.future;
  }

  @override
  Widget build(BuildContext context) {
    return new Container(
      color: Colors.white,
      child: new SpriteWidget(rootNode),
    );
  }
}

Hope that helps!

Answer 2

If you are using SpriteWidget you can load images using the ImageMap class to save some code. From the SpriteWidget docuementation:

ImageMap images = new ImageMap(rootBundle);

// Load a single image
ui.Image image = await images.loadImage('assets/my_image.png');

// Load multiple images
await images.load(<String>[
  'assets/image_0.png',
  'assets/image_1.png',
  'assets/image_2.png',
]);

// Access a loaded image from the ImageMap
image = images['assets/image_0.png'];