移动端如何用下拉刷新的方式实现上拉加载

<div class="main-box" id="box1">
  <div class="popup-box">
  </div>
</div>
<div class="main-box" id="box2">
  <div class="popup-box">
  </div>
</div>

    * {
      margin: 0;
      padding: 0;
    }
    .main-box {
      background: skyblue;
      width: 100%;
      height: 300px;
      overflow: hidden;
    }
    .popup-box {
      width: 100%;
    }
    .item {
      width: 100%;
      line-height: 40px;
      text-align: center;
      padding: 20px;
      box-sizing: border-box;
    }
    .tips{
      text-align: center;
    }
    #box2 {
      margin-top: 50px;
    }

  /*下拉加载*/
  function tDscroll(obj) {
    this.key = true;         //防止重复的请求
    this.dom = obj.dom;        //传入的dom
    this.fn = obj.fn;         //回调函数
    this.outDom = this.dom.querySelector(".popup-box"); //获取内容盒子
    this.showHeight = dom.offsetHeight;         //显示的高度
    this.actualHeight = this.outDom.offsetHeight;       //获取实际高度的内容
    this.startY = 0;                //起始点击位置
    this.changedY = 0;               //手指移动的距离
    this.originY = 0;               //偏移量
    var that = this;
    this.dom.addEventListener("touchstart",function (ev) {
      that.onStart(ev);
    });
    this.dom.addEventListener("touchmove",function (ev) {
      that.onMove(ev);
    });
    this.dom.addEventListener("touchend",function (ev) {
      that.onEnd(ev);
    });
    this.fn.call(this,this.outDom);
  };

  tDscroll.prototype.onStart = function (ev) {
    this.startY = ev.targetTouches[0].clientY;
    var tempArr = window.getComputedStyle(this.outDom).transform.split(",");
    if (tempArr.length > 2) {
      this.originY = parseInt(tempArr[tempArr.length - 1]) || 0;
    }
  };

  tDscroll.prototype.onMove = function (ev) {
    this.changedY = ev.touches[0].clientY - this.startY;
    var changNum = (this.originY + this.changedY);
    var scrollHeight = -changNum + this.showHeight;
    if (changNum > 50)return;
    if (scrollHeight > this.actualHeight + 50)return;
    if (scrollHeight > this.actualHeight - 50 && this.key) {
      this.fn.call(this,this.outDom);
    }
    this.outDom.style.cssText = "transform: translateY(" + changNum + "px);";
  };

  tDscroll.prototype.onEnd = function() {
    if ((this.originY + this.changedY) > 50 ) {
      this.outDom.style.cssText = "transform: translateY(0px);transition:all .3s";
    }
    if (-(this.originY + this.changedY) + this.showHeight > this.actualHeight + 50) {
      this.outDom.style.cssText = "transform: translateY(-"+(this.actualHeight - this.showHeight)+"px);transition:all .3s";
    }
  };


  var dom = document.querySelector("#box1");  //获取dom
  var dom2 = document.querySelector("#box2");  //获取dom
  var obj = {
    dom : dom,
    fn : add
  };
  var obj2 = {
    dom : dom2,
    fn : add
  };
  new tDscroll(obj);
  new tDscroll(obj2);
  var page = 0;          //当前的页数(模拟用)

  // 模拟ajax
  function add(outDom) {
    var that = this;
    this.key = false;
    var str = "";
    for (var i = 1;i < 11;i++) {
      str+="<div class='item'>"+(i+((page)*10))+"</div>"
    }
    page++;
    setTimeout(function () {
      var tips = outDom.querySelector(".tips");       //获取提升
      tips && outDom.removeChild(tips); //如果不是第一次 添加
      str += "<div class='tips'>加载更多</div>";
      outDom.innerHTML += str;
      that.actualHeight = outDom.offsetHeight;
      that.key = true;
    },2000)
  }