Dart extension functions not found

Question

I have a project on flutter and I want to use extension methods there very much. I've upgraded pubspec.yaml file to use sdk: ">=2.6.0 <3.0.0". I create a file list_extensions.dart with content

import 'dart:math';

extension ListExtension<T> on List<T> {

  T randomElement() => this.elementAt(Random().nextInt(this.length));

}

When I try to use this extension method in other files like this

String getRandomText(List<String> texts) => texts.randomElement();

I see a compiler error with text: The method 'randomElement' isn't defined for the class 'List'.

But when I try to use this extension method inside list_extensions.dart file - compiler is ok with that.

The thing is that my other project can found all extension methods that I declare.

flutter doctor says:

Doctor summary (to see all details, run flutter doctor -v):
[✓] Flutter (Channel master, v1.10.17-pre.74, on Mac OS X 10.15 19A603, locale en-RU)
[!] Android toolchain - develop for Android devices (Android SDK version 28.0.3)
    ✗ Android license status unknown.
      Try re-installing or updating your Android SDK Manager.
      See https://developer.android.com/studio/#downloads or visit https://flutter.dev/setup/#android-setup for detailed instructions.
[✓] Xcode - develop for iOS and macOS (Xcode 10.2)
[✓] Android Studio (version 3.5)
[✓] Android Studio (version 3.4)
[✓] VS Code (version 1.39.2)
[✓] Connected device (1 available)

flutter --version says:

Flutter 1.10.17-pre.74 • channel master • https://github.com/flutter/flutter.git
Framework • revision bcc93bca23 (6 days ago) • 2019-11-13 11:31:20 -0800
Engine • revision 31cd2dfca2
Tools • Dart 2.7.0

What can I do to force the compiler to see my extension methods?

Answer 1

Extension methods were introduced in Dart 2.7, so you can change your sdk version: ">=2.7.0 <3.0.0"

Answer 2

It was not your case, but for me, it worked after updating the Android Studio, Flutter plugin and Dart plugin. Maybe it'll help someone else.

Version that extension methods are working:

• Android Studio 3.6.2
• Flutter Plugin 192.7761
• Dart Plugin 45.1.1

And another important point is that extension must be named. If it's not named it can't be used in other files:

Private extension:

extension on Banana {
  // ...
}

Public extension:

extension BananaExtension on Banana {
  // ...
}

I found it in a comment on Dart extension announcement on YouTube