AppConfig.fromEnv constructor

AppConfig.fromEnv({ String hostVariable = 'HOST', String portVariable = 'PORT', String defaultHost = '0.0.0.0', int defaultPort = 8080, String? prefix = _defaultPrefix, int workers = 1, int backlog = 0, Env? env, })

Takes host and port from the environment at startup.

@App()
final class MyApp extends AppConfig {
  MyApp() : super.fromEnv();
}

Note this constructor is not const — it reads the process environment, which is only knowable at runtime — so the app class cannot have a const constructor either. That is the whole point: a container image is built once and told what it is by its environment, so a port baked in at compile time is a port that cannot be changed by the platform running it.

Two defaults differ from AppConfig.defaultApp deliberately:

• Host is 0.0.0.0, not localhost. A server bound to localhost inside a container accepts only connections originating in that same container, so every request from outside is refused — with the process looking perfectly healthy.
• Port comes from PORT. Cloud Run, Heroku, Render and Fly all assign a port this way and route to it; ignoring it means listening where nothing is being sent.

A PORT that is set but not a number throws rather than falling back: someone set it on purpose, and quietly listening somewhere else is worse than not starting.

Implementation

AppConfig.fromEnv({
  String hostVariable = 'HOST',
  String portVariable = 'PORT',
  String defaultHost = '0.0.0.0',
  int defaultPort = 8080,
  this.prefix = _defaultPrefix,
  this.workers = 1,
  this.backlog = 0,
  Env? env,
})  : host = (env ?? Env.current).string(hostVariable, orElse: defaultHost),
      port = (env ?? Env.current).integer(portVariable, orElse: defaultPort),
      securityContext = null,
      requestClientCertificate = false,
      assert(workers >= 1, 'workers must be >= 1'),
      assert(backlog >= 0, 'backlog must be >= 0');