Add Platform.isWeb

[johnpryan]

The kIsWeb constant is the only way to check if the app is running on the web. It would be great if we could recommend something like Platform.isWeb (alongside isLinux, isIOS, etc.)

If it's not possible to achieve in the framework due to #39998 we could consider adding it to package:platform

Related issues:

• Dart SDK libraries and Flutter for web #39998
• Check if platform is Web #41311
• Add isWeb google/platform.dart#31

[jonahwilliams]

import 'dart:io';
import 'package:flutter/foundation.dart';

extension PlatformHelpers on Platform {
  static bool isWeb => kIsWeb;
}

[jonahwilliams]

Of course, if you make it not constant it won't tree shake

[johnpryan]

Maybe we could add a similar extension somewhere in the SDK? Maybe in https://github.com/flutter/flutter/blob/master/packages/flutter/lib/src/foundation/platform.dart?

extension PlatformHelpers on Platform {
  static const bool isWeb = kIsWeb;
}

[jonahwilliams]

I think @Hixie and @zanderso have some stronger thoughts than me on extension methods in the SDK.

[Hixie]

https://github.com/flutter/flutter/wiki/Style-guide-for-Flutter-repo#avoid-using-extension

But we should totally do this as part of the work for making Platform work on web. That's probably a Dart SDK bug though.
cc @tvolkert who may know more about the dart:io/Platform work.

[jonahwilliams]

The Platform class getters are not constants, so using them is strictly worse than constants in cases where you do have platform specific code.

We don't actually have any built in support for identifiers like kIsLinux, but they could be added by the developer via dart defines. Pushing people to use a runtime interface may be a step backwards instead of forwards.

[johnpryan]

I think it's probably easier for users to keep using Platform.is* than switch to kIs* - but if it's faster to use constants in the implementation that sounds good to me.

I'm not 100% what the ideal API looks like, as discussed in the doc related to #39998 there are different scenarios where it's best to use Theme.of().platform, defaultTargertPlatform, and Platform depending on the use-case. For example, if I'm looking to use a specific layout on the web, I should probably use Theme.of().platform instead. It's also possible that users want to know what OS the browser is running on – isLinux and isMacOS might be useful in a browser context.

[jonahwilliams]

but if it's faster to use constants in the implementation that sounds good to me.

Not that it is faster, but that it allows tree-shaking to work.

[Hixie]

We should make Platform.isFoo const anyway.

FWIW, Theme.of().platform and defaultTargetPlatform will never report web, since web is a metaplatform and not one of the target platforms that is relevant for deciding UI behaviour.

[dumazy]

Concerning kIsWeb, I feel like its implementation is a bit of a code smell.

/// This implementation takes advantage of the fact that JavaScript does not
/// support integers. In this environment, Dart's doubles and ints are
/// backed by the same kind of object. Thus a double `0.0` is identical
/// to an integer `0`. This is not true for Dart code running in AOT or on the
/// VM.
const bool kIsWeb = identical(0, 0.0);

Couldn't conditional imports provide a more robust solution?
It seems like Platform is already using this approach to provide different implementations between VM and Web.

import '_platform_io.dart'
  if (dart.library.html) '_platform_web.dart' as _platform;