Flutter override environment variables in unit tests

Question

In my production code I set environment variables using the --DART-DEFINE flag and then retrieve them by using String.fromEnvironment.

How can I override the value returned by String.fromEnvironment in my unit tests, to test how my production code responds to different environment variable values?

Answer 1

Wrap your environment variables in a class and use your preferred dependency injection or service locator package (eg. get_it) to access your injected class from within your app.

Your environment_config.dart:

class EnvironmentConfig {

    final String someValue;

    EnvironmentConfig({
      required this.someValue,
    });

    factory EnvironmentConfig.init() {
      const someValue = String.fromEnvironment('someEnvVar');
      return EnvironmentConfig(someValue: someValue);
    }
  
}

main.dart

void main() {
    // instance can now be accessed from any class/widget within your app
    GetIt.I.registerSingleton<EnvironmentConfig>(EnvironmentConfig.init());

    runApp(MyApp());
}

Then within unit tests, you can inject your own instance of EnvironmentConfig with your own values:

void main() {
    setUp(() {
      final config = EnvironmentConfig(someValue: 'myCustomValue');
      GetIt.I.registerSingleton<EnvironmentConfig>(config);
    });

    tearDown(() {
      GetIt.I.reset();
    });

    test('Verify env vars', () {
      expect(GetIt.I<EnvironmentConfig>().someValue, equals('myCustomValue');
    });
}

EDIT: Given this is now the top result when searching for an answer to this, it's worth noting that the trade-off to the above approach is that it doesn't cater for tree-shaking, as the EnvironmentConfig class is not a compile-time constant when using the factory constructor.