如何在Play中实现用户特定的类型通用行为？

Question

Say I wanted to build an application as follows: A user can log in and create some observing mechanisms, that eg lookup a twitter hashtag or check some stocks every hour.

1. You have to have a User object that eg saves email and password for the user to log in.
2. You have to have some Java Code that eg establishes a HttpURLConnection, for both stocks and twitter
3. You have to save user-specific settings for those services: eg userA wants #hashtag1 but user2 wants #anotherTag

For scientific purpose I don't want to have every possible combination of those observers (twitter, stocks, ...) but rather have an abstract class "Observer" that both "TwitterObserver" and "StocksObserver" extend or something.

The problem I'm facing is this:

I am not able to eg select all observers (no matter of type), iterate over them, check if modified and react on user-specific behaviour. To put it another way: I cannot have a generic "Observer"-Class in combination with classes holding user-specific settings for this observer.

Is there any design pattern or best-practice to model this behavior? I can't believe it's all that hard.

I'm thankful for every hint! Thank you guys

Answer 1

In your GenericObserver class create an application job to collect the specific types:

public class Observer {
  // common properties and methods

  public static List<Observer> observers = new ArrayList<Observer>();

  public static void respondUserBehavior() {
    for (Observer observer: observers) {
      // call each observer to respond to user behavior
    }
  }

  @OnApplicationStart
  public static class ObserverCollector extends Job<?> {
    @Override public void doJob() {
      observers.addAll(play.Play.classLoader.getAssignableClasses(Observer.class));
      // or you might want to loop through the list and filter out the abstract classes
    }
  }
}