【flutter】イメージの描画

イメージはアプリケーション内の「assets」というフォルダに配置するのが一般的
flutter_app/assets/images/image.jpg

pubspec.yamlにassetsを追加する

flutter:
　assets:
  - assets/images/image.jpg

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

class _MyHomePageState extends State<MyHomePage>{

  static ui.Image? _img = null;
  static bool _flg = false;

  Future<void> loadAssetImage(String fname) async {
    final bd = await rootBundle.load("images/$fname");
    final Uint8List u8lst = await Uint8List.view(bd.buffer);
    final codec = await ui.instantiateImageCodec(u8lst);
    final frameInfo = await codec.getNextFrame();
    _img = frameInfo.image;
    setState(()=> _flg = true);
  }

  @override
  Widget build(BuildContext context) {
    loadAssetImage('image.jpg');

    return Scaffold(
      backgroundColor: Color.fromARGB(255, 255,255,255),
      appBar: AppBar(
        title: Text('App Name', style: TextStyle(fontSize: 30.0),),
      ),
      body:Container(
        child: CustomPaint(
          painter: MyPainter(_img),
        ),
      ),
    );
  }
}

class MyPainter extends CustomPainter {
  ui.Image? _img = null;

  MyPainter(this._img);

  @override
  void paint(Canvas canvas, Size size){
    Paint p = Paint();

    Offset off = Offset(50.0, 50.0);
    if(_img != null){
      canvas.drawImage(_img!, off, p);
    }
  }

  @override
  bool shouldRepaint(CustomPainter oldDelegate) => true;
}