How to pass environment variables to String.fromEnvironment() when running dart run build_runner

I'm using the envied_generator and build_runner packages. I want to modify the path within @Envied(path: const String.fromEnvironment('environment')) based on the environment.

I launch my Flutter app through VS Code launch configuration which triggers a prelaunch task (dart run build_runner build --delete-conflicting-outputs). However, const String.fromEnvironment('environment') always returns an empty value. I attempted to pass the environment variable using the --define flag, but it didn’t work.

How can I correctly pass environment variables to String.fromEnvironment() when running build_runner?

UPD:

My launch.json file:

{
    "version": "0.2.0",
    "configurations": [
        {
            "name": "Dev-Debug",
            "request": "launch",
            "type": "dart",
            "flutterMode": "debug",
            "args": ["--flavor", "dev",
            "--dart-define", "environment=dev"],
            "preLaunchTask": "build_runner_dev"
           }
        ]
    }

My tasks.json file:

{
  "version": "2.0.0",
  "tasks": [
      {
          "label": "build_runner_dev",
          "type": "shell",
          "command": "dart run build_runner build -- --define=environment=dev",
          "group": {
              "kind": "build",
              "isDefault": true
          },
      }
  ]
}

Solution

I tried the command option you are showing above:

dart run build_runner build -- --define=environment=dev

and the constant env is always the empty string:

final env = const String.fromEnvironment('environment'); // Empty String

However, build_runner has its own --define option, see How is the configuration for a builder resolved?.

The default options for envied are listed below:

targets:
  $default:
    builders:
      envied_generator|envied:
        options:
          path: .env.custom
          override: true

To set the option path from the command line try:

$ dart run build_runner build --define=envied_generator:envied=path="custom_path" --define=envied_generator:envied=override=true

This should overwrite the default build option set in the build.yaml file.

Updated the command, see Drompai's comment below.