code: public final boolean isEmpty() {
          if (mDatas.isEmpty()) {
              return true;
          }
          for (int i = 0; i < mDatas.size(); i++) {
              if (!mDatas.get(i).isEmpty()) {
                  return false;
              }
          }
          return true;
      }
docs: Returns if all of the observed {@link Data} instances are empty, or if none are being observed.