监听dom节点变化api

      let observer = new MutationObserver(callback);

      observer.observe(this.ue.body, {
        attributes: true, //属性变动
        characterData: true, //内容变动
        childList: true, //子节点
        subtree: true, //所有后代节点
        attributeOldValue: true, //属性变动前旧值
        characterDataOldValue: true, //内容变动前旧值
        attributeFilter:['class','src'] //观察特定属性
      });


function callback (mutations, observer) {
    let mutation = mutations[0]; //只取dom变化数组的第一个
    for(let i = 0, len = mutation.removedNodes.length; i < len; i++){
        if(mutation.removedNodes[i].className == "poll_placeholder"){
            console.log(mutation.removedNodes[i]);
			...                   
        }
    }
}