34  Dart Define

Q: How do I get the environment variable set in the --dart-define in Dart?

In Dart/Flutter, you can access environment variables that were set using --dart-define through the String.fromEnvironment() constructor. Here’s how:

34.1 Basic Usage

// Access a dart-define variable
const String apiKey = String.fromEnvironment('API_KEY');
const String environment = String.fromEnvironment('ENVIRONMENT', defaultValue: 'development');

34.2 Common Pattern for Different Data Types

// String values
const String apiUrl = String.fromEnvironment('API_URL', defaultValue: 'https://api.example.com');

// Boolean values
const bool isDebugMode = bool.fromEnvironment('DEBUG_MODE', defaultValue: false);

// Integer values
const int timeout = int.fromEnvironment('TIMEOUT_SECONDS', defaultValue: 30);

34.3 Practical Example

Let’s say you run your Flutter app like this:

flutter run --dart-define=API_KEY=abc123 --dart-define=ENVIRONMENT=staging --dart-define=DEBUG_MODE=true

You can access these in your Dart code:

class AppConfig {
  static const String apiKey = String.fromEnvironment('API_KEY');
  static const String environment = String.fromEnvironment('ENVIRONMENT', defaultValue: 'development');
  static const bool debugMode = bool.fromEnvironment('DEBUG_MODE', defaultValue: false);
  
  static void printConfig() {
    print('API Key: $apiKey');
    print('Environment: $environment');
    print('Debug Mode: $debugMode');
  }
}

34.4 Best Practices

Create a dedicated config class to centralize your environment variables:

class EnvConfig {
  // API Configuration
  static const String apiBaseUrl = String.fromEnvironment('API_BASE_URL', 
    defaultValue: 'https://api.development.com');
  static const String apiKey = String.fromEnvironment('API_KEY', 
    defaultValue: '');
  
  // App Configuration  
  static const String appName = String.fromEnvironment('APP_NAME', 
    defaultValue: 'My App');
  static const bool enableLogging = bool.fromEnvironment('ENABLE_LOGGING', 
    defaultValue: true);
  
  // Validation
  static bool get isApiKeyValid => apiKey.isNotEmpty;
  
  static void validateConfig() {
    assert(isApiKeyValid, 'API_KEY must be provided');
  }
}

The key points:

• Use String.fromEnvironment(), bool.fromEnvironment(), or int.fromEnvironment()
• Always provide defaultValue for safer fallbacks
• These are compile-time constants, so they’re resolved during build
• Perfect for API keys, environment settings, feature flags, etc.

This approach keeps your sensitive configuration out of your source code while making it easy to switch between different environments (development, staging, production).