code: @CallSuper
      public void unregisterDataObserver(@NonNull DataObserver dataObserver) {
          checkNotNull(dataObserver, "dataObserver");
          mDataObservable.unregisterObserver(dataObserver);
          if (mDataObservable.getObserverCount() == 0) {
              onLastObserverUnregistered();
          }
      }
docs: De-registers an observer from this adapter.
  <p>
  Note that it is an error to unregister the same observer twice, and doing so may result in an
  {@link IllegalStateException}.
  @param dataObserver The observer to be unregistered.
  @see #registerDataObserver(DataObserver)