public class ObserverManager {
  private List<Observer> observers = new ArrayList<ObserverManager.Observer>();
  public interface Observer {
    public void update();
    public void updateState();
  }
  public void addObserver(Observer observer) {
    if (observer == null) {
      throw new RuntimeException();
    }
    synchronized (this) {
      if (!observers.contains(observer)) {
        observers.add(observer);
      }
    }
  }
  public void notifyObservers() {
    for (Observer observer : observers) {
      observer.update();
    }
  }
  public void notifyState() {
    for (Observer observer : observers) {
      observer.updateState();
    }
  }
}

public class DetailView extends View implements Observer {
  public DetailView(Context context, AttributeSet attrs, int defStyleAttr) {
    super(context, attrs, defStyleAttr);
  }
  public DetailView(Context context, AttributeSet attrs) {
    super(context, attrs);
  }
  public DetailView(Context context) {
    super(context);
  }
  @Override
  public void update() {
    System.out.println("update");
  }
  @Override
  public void updateState() {
    System.out.println("updateState");
  }
}

mObserverManager.addObserver(new DetailView(this));

public class MainActivity extends Activity {
  private ObserverManager mObserverManager;
  @Override
  protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    mObserverManager = new ObserverManager();
    mObserverManager.addObserver(new DetailView(this));
  }
  public void click(View v) {
    //TODO 比如开始下载
    mObserverManager.notifyUpdate();//
  }
}