//禁止微信h5自带的下拉事件
    var app = document.querySelector(".messageChat");
    console.log(app, 990);
    var touchstartY;
    app.addEventListener(
      "touchstart",
      function (event) {
        console.log(1);
        var events = event.touches[0] || event;
        touchstartY = events.clientY; //获取触摸目标在视口中的y坐标
      },
      false
    );
    app.addEventListener(
      "touchmove",
      function (event) {
        var events = event.touches[0] || event;
        //注意app.scrollTop始终为0
        var scrollTop = app.scrollTop || document.documentElement.scrollTop; //获取滚动部分的高度
        var clientHeight = document.documentElement.clientHeight; //获取手机屏幕高度（可视部分高度）
        var scrollHeight = app.scrollHeight; //所有内容的高度
        if (
          events.clientY > touchstartY &&
          scrollTop === 0 &&
          event.cancelable
        ) {
          event.preventDefault(); //禁止到顶下拉
        } else if (
          scrollTop + clientHeight > scrollHeight &&
          event.cancelable
        ) {
          // event.preventDefault(); //禁止到底上拉
        }
      },
      false
    );