<section id="banner" class="carousel slide" data-ride="carousel">
        <!--小圆点 -->
        <ol class="carousel-indicators">
          <li data-target="#banner" data-slide-to="0" class="active"></li>
          <li data-target="#banner" data-slide-to="1"></li>
          <li data-target="#banner" data-slide-to="2"></li>
          <li data-target="#banner" data-slide-to="3"></li>
        </ol>
        <!-- 轮播图片 -->
        <div class="carousel-inner" role="listbox">
          <div class="item active" data-img-lg="img/slide_01_2000x410.jpg" data-img-xs="img/slide_01_640x340.jpg">
          </div>
          <div class="item" data-img-lg="img/slide_02_2000x410.jpg" data-img-xs="img/slide_02_640x340.jpg">
          </div>
          <div class="item" data-img-lg="img/slide_03_2000x410.jpg" data-img-xs="img/slide_03_640x340.jpg">
          </div>
          <div class="item" data-img-lg="img/slide_04_2000x410.jpg" data-img-xs="img/slide_04_640x340.jpg">
          </div>
        </div>
        <!-- 左右轮播 -->
        <a class="left carousel-control" href="#banner" role="button" data-slide="prev">
          <span class="glyphicon glyphicon-chevron-left" aria-hidden="true"></span>
          <span class="sr-only">Previous</span>
        </a>
        <a class="right carousel-control" href="#banner" role="button" data-slide="next">
          <span class="glyphicon glyphicon-chevron-right" aria-hidden="true"></span>
          <span class="sr-only">Next</span>
        </a>
    </section>

<script type="text/javascript">
      $(function() {
        //获取屏幕宽度
//        var screenWidth=$(window).width();
        //判断是不是手机屏幕
//        if(screenWidth<768){//手机屏幕
//          $("#banner .carousel-inner .item").each(function(index,item){
//          var $item=$(item);//因为传递的item是dom对象，要把dom对象改为jQuery对象
//          $item.css("background-image","url("+$item.data('img-xs')+")");
//          });
//        }else{//大屏幕
//          $("#banner .carousel-inner .item").each(function(index,item){
//          var $item=$(item);//因为传递的item是dom对象，要把dom对象改为jQuery对象
//          $item.css("background-image",'url('+$item.data('img-lg')+')');
//          });
//        }
        //三元式
//        var isPhone=screenWidth<768;
//        $("#banner .carousel-inner .item").each(function(index,item){
//          var $item = $(item);//因为传递的item是dom对象，要把dom对象改为jQuery对象
//          $item.css("background-image","url("+$item.data(isPhone?'img-xs':'img-lg')+")");
//        });
        function selectImg(){
          var screenWidth=$(window).width();
          var isPhone=screenWidth<768;
          $("#banner .carousel-inner .item").each(function(index,item){
            var $item = $(item);//因为传递的item是dom对象，要把dom对象改为jQuery对象
            $item.css("background-image",'url('+$item.data(isPhone?'img-xs':'img-lg')+')');
            //我们需要小图的时候等比例变化，所以我们需要img标签
            if(isPhone){
              $item.html("<img src='"+$item.data('img-xs')+"' alt='' />");
            }else {
              $item.empty();
            }
          });
        }
//        $(window).on("resize",selectImg);//这样是没有效果的，必须屏幕尺寸变化一下才会加载图片
        $(window).on('resize',selectImg).trigger('resize');//触发的是resize事件，不是事件处理程序selectImg
      });
    </script>