docs: function() {
              // NOTE: This fixed issue #7
              // https://github.com/vowstar/gitbook-plugin-uml/issues/7
              // HTML will load after this operation
              // Copy images to output folder every time
              var book = this;
              var output = book.output;
              var rootPath = output.root();
              if (fs.existsSync(ASSET_PATH)) {
                  fs.mkdirs(path.join(rootPath, ASSET_PATH));
                  fs.copySync(ASSET_PATH, path.join(rootPath, ASSET_PATH));
              }
          }