window.onscroll = function() {
                //变量scrollTop是滚动条滚动时，距离顶部的距离
                  var scrollTop =
                    document.documentElement.scrollTop || document.body.scrollTop;
                //变量windowHeight是可视区的高度
                  var windowHeight =
                    document.documentElement.clientHeight || document.body.clientHeight; 
                //变量scrollHeight是滚动条的总高度
                  var scrollHeight =
                    document.documentElement.scrollHeight || document.body.scrollHeight; 
                //滚动条到底部的条件
                  if (scrollTop + windowHeight == scrollHeight) {
                    //写后台加载数据的函数
                    // _this.onLoad();
                    console.log(
                      "距顶部" +
                        scrollTop +
                        "可视区高度" +
                        windowHeight +
                        "滚动条总高度" +
                        scrollHeight
                    );
                  }
                };