js实现打开页面弹出下载，加载背景图实现

前端代码：  

<div class="container">

   <img src="img_tg/tx_01.jpg"/>
   <img src="img_tg/tx_02.jpg"/>
   <img src="img_tg/tx_03.jpg"/>
   <img src="img_tg/tx_04.jpg"/>
   <img src="img_tg/tx_05.jpg"/>
   <div class="xfdiv" >
   <a href="javascript:void(0)"  id="down_app">
   </a>
   </div>

   </div>

js代码：

 <script>

              var imgReady = (function () {
                  var list = [],
            intervalId = null,
                  // 用来执行队列
            tick = function () {
                var i = 0;
                for (; i < list.length; i++) {
                    list[i].end ? list.splice(i--, 1) : list[i]();
                };
                !list.length && stop();
            },
                  // 停止所有定时器队列
            stop = function () {
                clearInterval(intervalId);
                intervalId = null;
            };
                  return function (url, ready, load, error) {
                      var onready,
                width,
                height,
                newWidth,
                newHeight,
                img = new Image();
                      img.src = url;
                      // 如果图片被缓存，则直接返回缓存数据
                      if (img.complete) {
                          ready.call(img); // ready 准备好的回调
                          load && load.call(img); // load 加载完的回调
                          return;
                      };
                      width = img.width; // 0 或 complete了不是0了
                      height = img.height;
                      // 加载错误后的事件
                      img.onerror = function () {
                          error && error.call(img);
                          onready.end = true;
                          img = img.onload = img.onerror = null;
                      };
                      // 图片尺寸就绪
                      onready = function () { // 执行到这里说明没有complete
                          newWidth = img.width;
                          newHeight = img.height;
                          console.log(newWidth, width, newHeight, height, newWidth * newHeight)
                          if (newWidth !== width || newHeight !== height ||
                          // 如果图片已经在其他地方加载可使用面积检测
                        newWidth * newHeight > 1024
                    ) {
                              ready.call(img);
                              onready.end = true;
                          };
                      };
                      onready();
                      // 完全加载完毕的事件
                      img.onload = function () {
                          // onload在定时器时间差范围内可能比onready快
                          // 这里进行检查并保证onready优先执行
                          !onready.end && onready();
                          load && load.call(img);
                          // IE gif动画会循环执行onload，置空onload即可
                          img = img.onload = img.onerror = null;
                      };
                      // 加入队列中定期执行
                      if (!onready.end) {
                          list.push(onready);
                          // 无论何时只允许出现一个定时器，减少浏览器性能损耗
                          // 模拟img.onready
                          if (intervalId === null) intervalId = setInterval(tick, 40);
                      };
                  };
              })();
              var $img = $('img')
              var len = $img.length
              var count = 0

              $img.each(function () {
                  imgReady(this.src, function () {
                      count++
                      console.log(count, '---------')
                      if (count >= len) {
                        //要执行的操作
                      }
                  })
              })
      
      
  </script>