image_network_picker.dart

import 'package:flutter/material.dart';

final Color darkBlue = Color.fromARGB(255, 18, 32, 47);

// store as static reference within app lifecycle
int _devicePixelCategory;

// image samples, e.g. 1x to 4x
// assuming this list is sorted from lowest res
final List<String> _imageOptions = [
  'source/to/image/SD',
  'source/to/image/HD',
  'source/to/image/FHD',
  'source/to/image/UHD',
];

// create util function to be called on every image network request
String imageOptionSelector(List<String> imageOptions) {
  // seems better using switch case
  return imageOptions[_devicePixelCategory - 1];
}

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      theme: ThemeData.dark().copyWith(scaffoldBackgroundColor: darkBlue),
      debugShowCheckedModeBanner: false,
      home: Scaffold(
        body: Center(
          child: MyWidget(),
        ),
      ),
    );
  }
}

class MyWidget extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    // check device pixel once every app start
    // devicePixelRatio (dpr) normally wont go lower than 1
    // if devicePixelRatio is between 2 value, better serve higher res.
    // E.g. devicePixelRatio 1.2 should serve 2x image, 2.1 serve 3x, etc
    _devicePixelCategory ??= MediaQuery.of(context).devicePixelRatio.ceil();

    var width = MediaQuery.of(context).size.width;
    var height = MediaQuery.of(context).size.height;

    return Text(
        'Screen res $width x $height with devicePixelRatio ${MediaQuery.of(context).devicePixelRatio}\n serving ${imageOptionSelector(_imageOptions)}',
        style: Theme.of(context).textTheme.display1);
  }
}