Observer Pattern

1)

  public interface ObserverListener {
    void register(Observer observer);
    void unregister(Observer observer);
    void notifyObserver();
  }

2)

  public interface Observer {
    void update();
  }

3) MainActivity

   public class MainActivity implements Observer {
          ......
    TestFragment fragment = new TestFragment();
    fragment.register(this); // add Observer
          ......
    @Override
    public void update() {
        // update  , unregister
    }
    
   }

   
4) TestFragment 
   public class TestFragment implements ObserverListener{
          ......
    private List mObservers = new ArrayList<>();

    @Override
    public void onStart() {
        super.onStart();
        notifyObserver();
    }

    @Override
    public void register(Observer observer) {
        if(mObservers != null && !mObservers.contains(observer))
            mObservers.add(observer);
    }

    @Override
    public void unregister(Observer observer) {
        if (mObservers != null)
            mObservers.remove(observer);
    }

    @Override
    public void notifyObserver() {
        for(Observer observer : mObservers){
            observer.update();
        }
    }
    
   }

https://stackoverflow.com/questions/42983101/using-observable-in-android

Using Observable in Android