How to achieve precompiler directive like functionality

Issue

I’m developing an angular app, and it’s recommended to use generated code for a lot of things running in production, namely template caches, expression caches, and a static DI injector. There’s currently no nice way to switch between different build configurations, so I’m using the pattern recommended here:

In lib/main.dart you can see initializer-prod.dart file being imported, which has initializer-dev.dart counterpart. Switching between those two file will allow you to switch between prod and dev modes. You will need to run the generator script before using the prod mode.

This results in the following import:

//import 'initializer_prod.dart' as init; // Use in prod/test.
import 'initializer_dev.dart' as init; // Use in dev.

As you can see, switching the import is a manual process. Is there a better, more automatic way to achieve this?

Solution

I see two possibilities (haven’t tried any of these myself yet)

• one is to use a transformer (see also Pass custom parameters to a dart application when using pub serve to run it)

or

• Compile-time dead code elimination with dart2js
  Recently a feature should have been added that pub build allows adding an environment variables using a command line option (like dart2js’s -d)

log(String msg) {
  if (const String.fromEnvironment('DEBUG') != null) {
    print('debug: $msg');
  }
}

main() {
  log('In production, I do not exist');
}

Some links about transformers:

• Can We Build It? Yes, We Can!
• Assets and Transformers
• Day 992: Search and Replace Dart Transformer to Hide from Polymer
• Dart Transformers for Polymer Cleanup
• Pub transformers
• dart2js_dransformer.dart
• Document user-defined transformers

EDIT
I was able to configure dart2js options in pubspec.yaml like

transformers:
- $dart2js:
    commandLineOptions: [-DDEBUG=true]
    environment:
      DEBUG: "true"
    suppressWarnings: true
    terse: true

They are validate and pub build fails if an unknown option is provided or if it’s not the expected format (yaml list for commandLineOptions, yaml map form environment)
BUT String.fromEnvironment() didn’t get a value

According to this issue, this is supported:
Passing in arguments to dart2js during pub build

I filed a bug How to pass options to dart2js from pubspec.yaml

EDIT-2

I tried it and it is working now:

transformers: # or dev_transformers
- $dart2js:
  environment: { PROD: "true" }

access it from the code like

String.fromEnvironment()

main() {
  print('PROD: ${const String.fromEnvironment('PROD')}'); 
  // works in the browser
  // prints 'PROD: null' in Dartium
  // prints 'PROD: true' in Chrome
}

see also Configuring the Built-in dart2js Transformer

EDIT-3

Another way is to use assert to set variables.
assert is ignored in production.

Answered By – Günter Zöchbauer

Answer Checked By – Mary Flores (FlutterFixes Volunteer)