ProxyImageProvider to lazily get the image dimensions

detect_image_size.dart

// Test at:
//   https://dartpad.dev/?id=a461c845871455e6d785248ded5a7982

import 'dart:async';
import 'dart:math';

import 'package:flutter/material.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      debugShowCheckedModeBanner: false,
      home: Scaffold(
        body: Center(
          child: MyWidget(),
        ),
      ),
    );
  }
}

class MyWidget extends StatefulWidget {
  @override
  State<MyWidget> createState() => _MyWidgetState();
}

class _MyWidgetState extends State<MyWidget> {
  // Initial dimensions, can be anything
  double width = 0;
  double height = 0;

  @override
  Widget build(BuildContext context) {
    return Stack(
      children: [
        Image(
          width: width,
          height: height,
          image: ProxyImageProvider(
            const NetworkImage('https://i.imgur.com/54Njk5Y.jpeg'),
            (width, height) {
              print('Image dimensions: $width x $height');
              setState(() {
                this.width = min(400.0, width.toDouble());
                this.height = height * this.width / width;
              });
              print('Computed dimensions: ${this.width} x ${this.height}');
            },
            onError: (exception, stackTrace) {
              print('Unable to load the image: $exception');
            },
          ),
        ),
        SizedBox(
          width: width,
          height: height,
          child: const FlutterLogo(),
        ),
      ],
    );
  }
}

class ProxyImageProvider<T extends ImageProvider<S>, S extends Object>
    extends ImageProvider<S> {
  ProxyImageProvider(
    this.imageProvider,
    this.onSizeAvailable, {
    this.onError,
  });

  final T imageProvider;

  final void Function(int width, int height) onSizeAvailable;

  final ImageErrorListener? onError;

  @override
  Future<S> obtainKey(ImageConfiguration configuration) {
    return imageProvider.obtainKey(configuration);
  }

  @override
  ImageStreamCompleter load(S key, DecoderCallback decode) {
    final isc = imageProvider.load(key, decode);
    late final ImageStreamListener listener;
    listener = ImageStreamListener(
      (image, synchronousCall) {
        onSizeAvailable(image.image.width, image.image.height);
        isc.removeListener(listener);
      },
      onError: (exception, stackTrace) {
        isc.removeListener(listener);
        onError?.call(exception, stackTrace);
      },
    );
    isc.addListener(listener);
    return isc;
  }

  @override
  bool operator ==(dynamic other) {
    return imageProvider == other;
  }

  @override
  int get hashCode => imageProvider.hashCode;
}