docs: boolean setObserver(Observer observer) { // F16043
          boolean success;
          Observer oldObserver;
  
          synchronized (this) {
              oldObserver = ivObserver;
              success = oldObserver != null || !isDone();
              ivObserver = success ? observer : null;
          }
  
          if (oldObserver != null) {
              oldObserver.update(null, observer);
          }
          return success;
      }