code: @Optional
    @InputFiles
    public FileCollection getExtraFilesDirectoriesAsInputFiles() {
      if (extraFilesDirectories == null) {
        return null;
      }
      FileCollection files = project.files();
      for (File directory : extraFilesDirectories) {
        files = files.plus(project.fileTree(directory));
      }
      return files;
    }
docs: This method is purely for incremental build calculations.