While porting my Flutter mobile app to Web, I ran into this exception:

Caught error: Unsupported operation: Platform._version

It originated from dart-sdk/lib/_http/http.dart, more precisely as soon as I instantiate dart io's HttpClient.

Starting from dart io's HttpClient

After reading a few StackOverflow posts, it turns out that there are two ways to make HTTP requests. One with dart io's HttpClient, and one with the http package. I was using the former. It turns out dart io is for non Web apps only:

File, socket, HTTP, and other I/O support for non-web applications.

Important: Browser-based apps can't use this library.

dart:io library - Dart API

dart:io library API docs, for the Dart programming language.

Dart API

It's interesting that it works for all platforms (server, mobile, desktop) but web.

My code using HttpClient looked roughly like so:

import 'dart:io';

client = new HttpClient();

Future<String> downloadResponse(HttpClient client, Uri url) {
  return client.getUrl(url).then((HttpClientRequest request) {
    request.headers
        ..add('accept', 'text/html,application/xhtml+xml,application/xml')
        ..add('accept-language', 'en-US,en');
    return request.close();
  }).then((HttpClientResponse response) async {
    try {
      final textResponse = await readResponse(response);
      return textResponse;
    } catch (e) {
      print(e);
      return '';
    }
  }).catchError((e) {
    print(e);
    return '';
  });
}

// From https://stackoverflow.com/questions/27808848/retrieving-the-response-body-from-an-httpclientresponse
Future<String> readResponse(HttpClientResponse response) {
  var completer = new Completer<String>();
  var contents = new StringBuffer();
  Converter<List<int>, String> decoder = utf8.decoder;
  response.transform(decoder).listen(
      (String data) {
        contents.write(data);
      },
      onDone: () => completer.complete(contents.toString()),
      cancelOnError: true,
      onError: (e) {
        print(e);
        completer.completeError(Exception("Bad encoding"));
      });
  return completer.future;
}

To be honest, I have no idea why the code was this complicated, using a decoder, etc.

http package

http supports all platforms, web included.

Usage seems straightforward too. Btw, it supports two modes. One-off requests, and initializing a client, then reusing repeatedly:

If you're making multiple requests to the same server, you can keep open a persistent connection

So here's how the code above should look like using http:

get method - Client class - http library - Dart API

API docs for the get method from the Client class, for the Dart programming language.

Dart API

import 'package:http/http.dart';

client = Client();

Future<String> downloadResponse(Client client, Uri url) async {
  final headers = {
      'accept': 'text/html,application/xhtml+xml,application/xml',
      'accept-language': 'en-US,en'
  }
  final response = await client.get(url, headers: headers);
  return response.body;
}

I am surprised at how concise the code is, and how I was able to keep the same signatures. As a result, migrating unit tests was a breeze as well.