code: protected int getCheckedTagIndex() {
          final int count = getChildCount();
          for (int i = 0; i < count; i++) {
              final TagView tag = getTagAt(i);
              if (tag.isChecked) {
                  return i;
              }
          }
          return -1;
      }
docs: Return the checked tag index.
  
  @return the checked tag index, or -1 if not exists.