Command line variables on Flutter using --dart-define.

This post explains how to send command line variables and use them with flutter.

Why send variables via command line?

Being able to set a variable through command line comes in very handy whenever you want to change the variable depending on the environment/client it’s being built for. Without having to change code manually every time.

A few common use cases would be

• to configure your app’s base URL during build time.
• to use separate API keys or client ids for development and production.

–dart-define to the rescue

--dart-define helps send key-value pairs to flutter at build time.

All you have to do is run flutter run with --dart-define=<KEY>=<VALUE> argument.
If you want to send multiple key-value pairs, just send --dart-define multiple times.

Example: flutter run --dart-define=BASE_URL=https://dev.example.com --dart-define=ENABLE_SECRET_FEATURE=true

note: --dart-define works starting from Flutter 1.17

Accessing the key-value pairs in code

You can access the data sent via --dart-define by using
const <DataType>.fromEnvironment(<Key>,{defaultValue:<Default value>}).

Example:

• to get String value
  const String.fromEnvironment("BASE_URL")
• to get bool value
  const bool.fromEnvironment("ENABLE_SECRET_FEATURE", defaultValue: false)

note: const is important. You wouldn’t get the value otherwise

Usage

class BuildConfig {
    static const String BASE_URL = const String.fromEnvironment("BASE_URL");
    static const bool ENABLE_SECRET_FEATURE = 
        const bool.fromEnvironment("ENABLE_SECRET_FEATURE",defaultValue: false);
    static const int ORG_NO = const int.fromEnvironment("ORG_NO");
}

class MyWidget extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Column(
      children: [
        Text("Base Url: ${BuildConfig.BASE_URL}"),
        Text("Enable Feature: ${BuildConfig.ENABLE_SECRET_FEATURE}"),
        Text("Org No: ${BuildConfig.ORG_NO}"),
      ],
    );
  }
}