Flutter: Build web app with file other than 'lib/main.dart' - flutter

Question

I use more than one dart main file in my lib folder for passing different app configuration depending on server each will use.
Example of main_dev.dart
void main() async {
FlavorConfig(
flavorName: 'test',
values: {
'BASE_URL': 'http://$MY_IP:9002/graphql',
'WS_URL': 'ws://$MY_IP:9002/graphql',
},
);
await runApp(ProviderApp());
}
For now I have main_test.dart and main_live.dart and everything works fine when building android app.
Example I use the following command to build test app.
flutter build apk --obfuscate --split-debug-info=./build/app/outputs/symbols lib/main_test.dart
Note the last part lib/main_test.dart I specify which file I intend to use and it works really well for building android app.
But I tried similar thing for web and I get the following error as it seems to me building web app needs main.dart.
Target dart2js failed: Exception:
.dart_tool/flutter_build/11c1feed0a867bb072ab6c7b64967a31/main.dart:8:8:
Error: Error when reading 'lib/main.dart': Error reading
'lib/main.dart' (The system cannot find the file specified. ) import
'package:my_amazing_app/main.dart' as entrypoint;
^ .dart_tool/flutter_build/11c1feed0a867bb072ab6c7b64967a31/main.dart:13:3:
Error: Method not found: 'main'. entrypoint.main(); ^^^^ Error:
Compilation failed.
Compiling lib\main.dart for the Web...
20.3s Exception: Failed to compile application for the Web.
How can can specify the file I want in command for web too?.
I have tried the following without success flutter build web lib/main_test.dart

Answer 1

Rather than using different entrypoints, you can use compile-time constants. When running flutter build, you can pass in --dart-define=foo=bar to set a compile-time constant, and you can then use it from your code using:
const foo = String.fromEnvironment('foo');
print(foo); // prints "bar"
const fooExists = bool.hasEnvironment('foo');
print(fooExists); // prints "true"
Note that this is a const value, so the compiler can still use this information for tree shaking.
In your case, assuming you have 2 environments: prod and test, you could do something like this:
// lib/main.dart
void main() {
const env = String.fromEnvironment('env');
if (env == 'test') {
runApp(TestApp());
} else if (env == 'prod') {
runApp(ProdApp());
} else {
throw 'unknown environment: $env';
}
}
Then when you build your app, you can use:
flutter build web --dart-define=env=prod <other-args>