Design and Architecture

public class Subject {
    private List<Observer> observers = new ArrayList<>();
    private void register(Observer o) {
        observers.add(o);
    }
    private void notify() {
        for (Observer o : observers)
            o.update();
    }
}

public class MyWidget {...}

public class MyObservableWidget extends MyWidget, Subject {
    ...
}

public class MyObservableWidget extends MyWidget {

   private Subject subject;

   public MyObservableWidget(Subject subject) {
     this.subject = subject;
   }

}