code: function render(force) {
    var image_eles = document.querySelectorAll('img.placeholder'), img, opts;
    for (var i = 0; i < image_eles.length; i++) {
      img = image_eles[i];
      //图片没有被处理过，立马处理
      if (force || (! _getAttribute(img, rendered_attr, ''))) {
        opts = parseUrlOptions(_getAttribute(img, 'options', ''));
        img.setAttribute('src', placeholder(opts));
        //处理完成之后标记为处理
        img.setAttribute(rendered_attr, '1');
      }
    }
  }
docs: 开始检查图片中