Configuring apps with compilation environment declarations

You can specify compilation environment declarations when building or running a Dart application. Compilation environment declarations specify configuration options as key-value pairs that are accessed and evaluated at compile time.

Your app can use the values of environment declarations to change its functionality or behavior. Dart compilers can eliminate the code made unreachable due to control flow using the environment declaration values.

You might define and use environment declarations to:

• Add functionality during debugging, such as enabling logging.
• Create separate flavors of your application.
• Configure application behavior, such as the port of an HTTP server.
• Enable an experimental mode of your application for testing.
• Switch between testing and production backends.

To specify an environment declaration when running or compiling a Dart application, use the --define option or its abbreviation, -D. Specify the declaration key-value pair using a <NAME>=<VALUE> format:

$ dart run --define=DEBUG=true -DFLAVOR=free

To learn how to set these declarations with other tools, check out the specifying environment declarations section in this guide. That section explains the declaration syntax and how to specify them on the command line and in IDEs and editors.

To access specified environment declaration values, use one of the fromEnvironment constructors with const or within a constant context. Use bool.fromEnvironment for true or false values, int.fromEnvironment for integer values, and String.fromEnvironment for anything else.

Each of the fromEnvironment constructors require the name or key of the environment declaration. They also accept an optional defaultValue named argument to override the default fallback value. The default fallback value is used when a declaration isn't defined or the specified value cannot be parsed as the expected type.

For example, if you want to print log messages only when the environment declaration DEBUG is set to true:

void log(String message) {
  // Log the debug message if the environment declaration 'DEBUG' is `true`.
  // If there was no value specified, do not log.
  if (const bool.fromEnvironment('DEBUG', defaultValue: false)) {
    print('Debug: $message');
  }
}

In this snippet, if DEBUG is set to false during compilation, or not specified at all, production compilers can completely remove the condition and its body.

The fromEnvironment constructors fallback to a default value when the declaration isn't specified or the specified value cannot be parsed. Therefore, to specifically check whether an environment declaration has been specified, use the bool.hasEnvironment constructor:

if (const bool.hasEnvironment('DEBUG')) {
  print('Debug behavior was configured!');
}

Both dart run and the dart compile subcommands accept any number of the -D or --define options to specify environment declaration values.

$ dart run --define=DEBUG=true -DFLAVOR=free main.dart
$ dart compile exe --define=DEBUG=true -DFLAVOR=free main.dart
$ dart compile js --define=DEBUG=true -DFLAVOR=free main.dart
$ dart compile aot-snapshot --define=DEBUG=true -DFLAVOR=free main.dart
$ dart compile jit-snapshot --define=DEBUG=true -DFLAVOR=free main.dart
$ dart compile kernel --define=DEBUG=true -DFLAVOR=free main.dart

To learn about configuring webdev to pass environment declarations to both the development and production web compilers, check out the webdev configuration documentation.

In your launch configuration (launch.json) under configurations, add a new toolArgs key containing the your desired environment declarations:

"configurations": [
    {
        "name": "Dart",
        "request": "launch",
        "type": "dart",
        "toolArgs": [
          "--define=DEBUG=true"
        ]
    }
]

To learn more, check out the documentation for VS Code launch configurations.

In the Run/Debug Configurations for your project, add your desired environment declarations to VM options:

To learn more, check out JetBrains' documentation for Dart Run/Debug Configurations.

To specify environment declarations to the Flutter tool, use the --dart-define option instead:

$ flutter run --dart-define=DEBUG=true

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