JS实现图片拖拽交换效果

<div id="photo">
    <ul>
      <li><img src="" alt=""></li>
      <li><img src="" alt=""></li>
      <li><img src="" alt=""></li>
      <li><img src="" alt=""></li>
      <li><img src="" alt=""></li>
      <li><img src="" alt=""></li>
      <li><img src="" alt=""></li>
      <li><img src="" alt=""></li>
      <li><img src="" alt=""></li>
    </ul>
</div>

* {
      margin:0;
      padding: 0;
    }
    body {
      user-select: none;  /*阻止文本选中*/
    }
    #photo {
      width: 600px;
      height: 600px;
      border: 2px solid #000;
      margin: 20px auto;
    }
    #photo ul li {
      list-style:none;
      width: 180px;
      height: 180px;
      margin: 10px;
      float: left;
    }
    #photo ul li:hover {
      background: #c0c;
    }
    #photo ul li img {
      width: 180px;
      height: 180px;
      border: 1px solid #ccc;
    }

var photo = document.getElementById("photo");
    var oUl = photo.getElementsByTagName("ul")[0];
    var aLi = oUl.getElementsByTagName("li");
    var z = 2;

    var arr = [];
    for (var i = 0; i < aLi.length; i++) {
      arr.push([aLi[i].offsetLeft, aLi[i].offsetTop]);
    }

    for (var i = 0; i < aLi.length; i++) {
      aLi[i].style.position = "absolute";
      aLi[i].style.left = arr[i][0] + "px";
      aLi[i].style.top = arr[i][1] + "px";
      aLi[i].style.margin = 0;
    }

    for (var i = 0; i < aLi.length; i++) {
      aLi[i].index = i;
      drag(aLi[i]);
    }

    function drag(obj) {
      obj.onmousedown = function(ev) {
        ev = ev || window.ev;
        var x = ev.clientX;
        var y = ev.clientY;

        var l = obj.offsetLeft;
        var t = obj.offsetTop;

        this.style.zIndex = z++;

        document.onmousemove = function(ev) {
          ev = ev || window.ev;
          var _left = ev.clientX - x + l;
          var _top = ev.clientY - y + t;
          obj.style.left = _left + "px";
          obj.style.top = _top + "px";

          var li = near(obj);
          for (var i = 0; i < aLi.length; i++) {
            aLi[i].style.background = "";
          }
          if (li) {
            li.style.background = "#DF971F";
          }
        }
        document.onmouseup = function() {
          document.onmousemove = null;
          document.onmousedown = null;

          var nearLi = near(obj);
          var tmp = 0;
          if (nearLi) {
            move(nearLi, {left:arr[obj.index][0], top:arr[obj.index][1]});
            move(obj, {left:arr[nearLi.index][0], top:arr[nearLi.index][1]});
            nearLi.style.background = "";

            tmp = obj.index;
            obj.index = nearLi.index;
            nearLi.index = tmp;
          } else {
            move(obj, {left:arr[obj.index][0], top:arr[obj.index][1]});
          }
        }
        return false;
      }
    } 
    function impact(obj1, obj2) {
      var L1 = obj1.offsetLeft;
      var R1 = obj1.offsetLeft + obj1.offsetWidth;
      var T1 = obj1.offsetTop;
      var B1 = obj1.offsetTop + obj1.offsetHeight;

      var L2 = obj2.offsetLeft;
      var R2 = obj2.offsetLeft + obj2.offsetWidth;
      var T2 = obj2.offsetTop;
      var B2 = obj2.offsetTop + obj2.offsetHeight;

      if (L2 > R1 || T2 > B1 || R2 < L1 || B2 < T1) {
        return false;
      } else {
        return true;
      }
    }
    function near(obj) {
      var tmp = 5000;
      var oLi = '';
      for (var i = 0; i < aLi.length; i++) {
        if (impact(obj, aLi[i]) && obj != aLi[i]) {
          var c = disCalc(obj, aLi[i]);

          if (tmp > c) {
            tmp = c;
            oLi = aLi[i];
          }
        }
      }
      return oLi;
    }
    function disCalc(obj1, obj2) {
      var x = obj1.offsetLeft - obj2.offsetLeft; 
      var y = obj1.offsetTop - obj2.offsetTop;
      return Math.sqrt(x * x + y * y);
    }

function move(obj, json, endFn) {
  clearInterval(obj.timer);
  obj.timer = setInterval(function() {
    var bBtn = true;
    for (var attr in json) {
      var iCur = 0;
      if (attr == 'opacity') {
        if (Math.round(parseFloat(getStyle(obj,attr)) * 100) == 0) {
          iCur = Math.round(parseFloat(getStyle(obj,attr)) * 100);
        } else {
          iCur = Math.round(parseFloat(getStyle(obj,attr)) * 100) || 100;
        }  
      } else {
        iCur = parseInt(getStyle(obj,attr)) || 0;
      }

      var iSpeed = (json[attr] - iCur) / 8;
      iSpeed = iSpeed >0 ? Math.ceil(iSpeed) : Math.floor(iSpeed);
      if (iCur != json[attr]) {
        bBtn = false;
      }

      if (attr == 'opacity') {
        obj.style.filter = 'alpha(opacity=' +(iCur + iSpeed)+ ')';
        obj.style.opacity = (iCur + iSpeed) / 100;
      }
      else {
        obj.style[attr] = iCur + iSpeed + 'px';
      }
    }

    if (bBtn) {
      clearInterval(obj.timer);
      if (endFn) {
        endFn.call(obj);
      }
    }  
  }, 30);
}


function getStyle(obj, attr) { 
  if (obj.currentStyle) {
    return obj.currentStyle[attr];
  } else {
    return getComputedStyle(obj, false)[attr];
  }
}