/**
 * @param warp {DOM || null} 外层容器，当为null时，默认以整个文档结构为外容器
 * @param threshold  {Number} 滚动阀值，即可以设置一个值，当滚动到离地步还有一段距离时，就开始执行callback
 * @param cb {Function} 回掉函数
 */
let scrollToLoad = (warp, threshold, cb) => {
     let scrollTop = 0,
          warpHeight,
          listHeight,
          _threshold_ = threshold || 0;
     if (!warp) {
         // 获取滚动条当前的位置 
         if (document.documentElement && document.documentElement.scrollTop) { 
            scrollTop = document.documentElement.scrollTop; 
         } else if (document.body) { 
            scrollTop = document.body.scrollTop; 
         } 
         // 获取当前可视范围的高度
         if (document.body.clientHeight && document.documentElement.clientHeight) { 
            warpHeight = Math.min(document.body.clientHeight, document.documentElement.clientHeight); 
         } else { 
            warpHeight = Math.max(document.body.clientHeight, document.documentElement.clientHeight); 
         } 

         // 获取list完整的高度
         listHeight = Math.max(document.body.scrollHeight, document.documentElement.scrollHeight);
     } else {
         scrollTop = warp.scrollTop; 
         warpHeight = warp.clientHeight;
         listHeight = warp.scrollHeight;
     }

     if (listHeight <= warpHeight + scrollTop - _threshold_) {
        cb && cb();
     }
}