Step 1 - Types

enum SenderKind { email, sms, push }

abstract class NotificationSender {
  void send(String to, String body);
}

class EmailSender implements NotificationSender {
  @override
  void send(String to, String body) {
    print('email -> $to: $body');
  }
}

class SmsSender implements NotificationSender {
  @override
  void send(String to, String body) {
    print('sms -> $to: $body');
  }
}

class PushSender implements NotificationSender {
  @override
  void send(String to, String body) {
    print('push -> $to: $body');
  }
}

Step 2 - Factory

class SenderFactory {
  static NotificationSender create(SenderKind kind) {
    return switch (kind) {
      SenderKind.email => EmailSender(),
      SenderKind.sms => SmsSender(),
      SenderKind.push => PushSender(),
    };
  }
}

void main() {
  final sender = SenderFactory.create(SenderKind.sms);
  sender.send('+1000', 'hello');
}

When this fits

• Construction rules grow (config flags, A/B tests, environment).
• Call sites should not list every new variant.

Practice tasks

• Add SenderKind.console that prints a tagged line for local dev.
• Throw ArgumentError for unsupported combinations when you add a second enum (for example Region).
• Replace static create with a top-level function NotificationSender senderFor(SenderKind k) and note readability tradeoffs.