Avoid using web-only libraries outside Flutter web plugin packages

Question

I'm building a Flutter app that I am trying to make work on the web. Part of it contains some web specific code:

import 'dart:html' as html;
import 'package:flutter/foundation.dart';

class DownloadViewModel extends ChangeNotifier {
  static const String url = 'https://example.com/api/v1/app/myapp_1.0.0.apk';
  void onAndroidDownloadPressed() {
    html.window.open(url, 'AndroidApp');
  }
}

However the dart:html import gives the following error:

Avoid using web-only libraries outside Flutter web plugin packages

The longer version of the warning looks like this:

Avoid using web libraries, dart:html, dart:js and dart:js_util in Flutter packages that are not web plugins. These libraries are not supported outside a web context; functionality that depends on them will fail at runtime in Flutter mobile, and their use is generally discouraged in Flutter web.

Web library access is allowed in:

plugin packages that declare web as a supported context

otherwise, imports of dart:html, dart:js and dart:js_util are disallowed.

And it's not just a warning. This actually prevents building an Android or iOS app (even though this method isn't accessible from non-Web Flutter apps).

The only solution I've figured out is to comment out the import when I am building for Android and iOS and then uncomment it when I am building for the web. Is there a better solution?

Solution

Use the universal_html package. It supports the browser, Dart VM, and Flutter and is a stand-in replacement for dart:html and other web related libraries.

dependencies:
  universal_html: ^1.2.1

Then instead of using import 'dart:html' as html; you can use the following import:

import 'package:universal_html/html.dart' as html;

For those who came to this page for other related web import problems (like dart:js), this plugin also supports the following imports:

import 'package:universal_html/driver.dart';
import 'package:universal_html/html.dart';
import 'package:universal_html/indexed_db.dart';
import 'package:universal_html/js.dart';
import 'package:universal_html/js_util.dart';
import 'package:universal_html/prefer_sdk/html.dart';
import 'package:universal_html/prefer_sdk/indexed_db.dart';
import 'package:universal_html/prefer_sdk/js.dart';
import 'package:universal_html/prefer_sdk/js_util.dart';
import 'package:universal_html/prefer_sdk/svg.dart';
import 'package:universal_html/prefer_sdk/web_gl.dart';
import 'package:universal_html/prefer_universal/html.dart';
import 'package:universal_html/prefer_universal/indexed_db.dart';
import 'package:universal_html/prefer_universal/js.dart';
import 'package:universal_html/prefer_universal/js_util.dart';
import 'package:universal_html/prefer_universal/svg.dart';
import 'package:universal_html/prefer_universal/web_gl.dart';
import 'package:universal_html/svg.dart';
import 'package:universal_html/web_gl.dart';

Comments

[Stack Underflow]

universal_html: ^1.2.2 wouldn't work for me, but universal_html: ^1.2.1 as shown in the linked documentation did.

[Suragch]

@StackUnderflow. OK, thanks for the notification. Maybe it is a bug in their versioning number. Anyway, I temporarily updated my answer to use 1.2.1.

[Bensal]

I was using FunctionStringCallback and it was causing this error. Try changing it to a final variable.

[Shan Mk]

universal_html is unidentified, is there any alternative?