code: function updateClass (oldVnode, vnode) {
    var el = vnode.elm;
    var ctx = vnode.context;
  
    var data = vnode.data;
    var oldData = oldVnode.data;
    if (!data.staticClass &&
      !data.class &&
      (!oldData || (!oldData.staticClass && !oldData.class))
    ) {
      return
    }
  
    var oldClassList = [];
    // unlike web, weex vnode staticClass is an Array
    var oldStaticClass = oldData.staticClass;
    if (oldStaticClass) {
      oldClassList.push.apply(oldClassList, oldStaticClass);
    }
    if (oldData.class) {
      oldClassList.push.apply(oldClassList, oldData.class);
    }
  
    var classList = [];
    // unlike web, weex vnode staticClass is an Array
    var staticClass = data.staticClass;
    if (staticClass) {
      classList.push.apply(classList, staticClass);
    }
    if (data.class) {
      classList.push.apply(classList, data.class);
    }
  
    if (typeof el.setClassList === 'function') {
      el.setClassList(classList);
    } else {
      var style = getStyle(oldClassList, classList, ctx);
      if (typeof el.setStyles === 'function') {
        el.setStyles(style);
      } else {
        for (var key in style) {
          el.setStyle(key, style[key]);
        }
      }
    }
  }
docs: /*