Anot.directive('text', {
  update: function(val) {
    var elem = this.element
    val = val == null ? '' : val //不在页面上显示undefined null
    if (elem.nodeType === 3) {
      //绑定在文本节点上
      try {
        //IE对游离于DOM树外的节点赋值会报错
        elem.data = val
      } catch (e) {}
    } else {
      //绑定在特性节点上
      elem.textContent = val
    }
  }
})