Dart: fluent 1

main.dart

void main() {
  final url = Auth0
    .webAuth('elkdanger.eu.auth0.com', '123')
    .audience('test')
    .buildUrl();

  print(url);
}

class WebAuth {
  final String domain;
  final String clientId;
  String? _audience;

  WebAuth(this.domain, this.clientId);

  audience(String audience) {
    _audience = audience;
    return this;
  }

  buildUrl() {
    var base = 'https://$domain/authorize';
    
    // Explicit `true` check as nullable expression can't be used as
    // a condition.
    if (_audience?.isNotEmpty == true) {
      base += '?audience=$_audience';
    }

    return base;
  }
}

class Auth0 {
  static webAuth(String domain, String clientId) => WebAuth(domain, clientId);
}