code: @NonNull
      public ObservableDataBuilder<T> loading(@Nullable Observable<Boolean> loading) {
          mLoading = loading;
          return this;
      }
docs: The {@link Data#isLoading()} property will match the emissions of this observable, starting with {@code false}
  until the first emission.
  <p>
  If not specified, the resulting data considers itself in a loading state until the first emission of any of the
  content observables.
  @see #contents(Observable)
  @see #prepends(Observable)
  @see #appends(Observable)