code: public final void addData(@NonNull Data<?> data) {
          checkNotNull(data, "data");
          // Clear error message now, in case the caller recently replaced them.
          // If they did, they'd expect the error message to be gone, because the
          // view visibility state is reset.
          if (mDatas.isEmpty()) {
              mErrorMessage = null;
          }
          mDatas.add(data);
          mDataWatcher.setDatas(mDatas);
          updateViews();
      }
docs: Adds a {@link Data} to be observed by this layout.