javascript 实现简单拖拽(鼠标事件 mousedown mousemove mouseup)

* {
  margin: 0;
  padding: 0;
}

#box {
  margin: 10px;
  position: relative;
  width: 400px;
  height: 400px;
  background: #ccc;
  border: 1px solid #333;
}

#dragBox {
  width: 50px;
  height: 50px;
  text-align: center;
  line-height: 50px;
  position: absolute;
  background: blue;
  cursor: move;
  font-size: 12px;
  color: #fff;
}

<div id="box">
  <div id="dragBox">拖我</div>
</div>

var VVG = {};  //命名空间
VVG.DOM = {
  $: function (id) { //创建方便的选择符
    return typeof id == "string" ? document.getElementById(id) : id;
  },
  bindEvent: function (node, type, func) { //事件绑定方法
    if (node.addEventListener) {
      node.addEventListener(type, func, false);
    } else if (node.attachEvent) {
      node.attachEvent("on" + type, func);
    } else {
      node["on" + type] = func;
    }
  },
  getEvent: function (event) { //获取事件
    return event ? event : window.event;
  },
  getTarget: function (event) { //获取事件目标
    return event.target || event.srcElement;
  }
}
var DragDrop = function () { //新建一个返回对象的函数
  var box = VVG.DOM.$("box"); //获取外围BOX
  var dragBox = VVG.DOM.$("dragBox");//获取需要拖动的BOX
  var dragging = null; //初始化对象
  function drag(event) { //事件执行函数
    event = VVG.DOM.getEvent(event);
    var target = VVG.DOM.getTarget(event);
    switch (event.type) {//判断事件类型
      case "mousedown":
        if (target.id == "dragBox") { //当事件对象的ID等于要拖动的BOX的ID时
          dragging = target; //赋值到拖动目标
        }
        break;
      case "mousemove":
        if (dragging) { //当有拖动目标时执行
          sTop = document.documentElement.scrollTop || document.body.scrollTop; //当有滚动条的时候卷去页面的高度
          dragging.style.left = (event.clientX - box.offsetLeft - dragBox.offsetWidth / 2) + "px";
          dragging.style.top = (event.clientY + sTop - box.offsetTop - dragBox.offsetHeight / 2) + "px";
          var left = parseInt(dragging.style.left);
          var top = parseInt(dragging.style.top);
          //console.log("left:"+left+"-----top:"+top + "-------sTop:"+sTop);
          // 比较坐标是否超出外围的BOX
          if (left < 0) {
            dragging.style.left = 0 + "px";
          }
          if (top < 0) {
            dragging.style.top = 0 + "px";
          }
          if (left > box.offsetWidth - dragBox.offsetWidth) {
            dragging.style.left = (box.offsetWidth - dragBox.offsetWidth - 2) + "px";
          }
          if (top > box.offsetHeight - dragBox.offsetHeight) {
            dragging.style.top = (box.offsetHeight - dragBox.offsetHeight - 2) + "px";
          }
        }
        break;
      case "mouseup":
        // 清空拖动目标
        dragging = null;
        break;
    }
  };
  return {
    dragStart: function () {
      // 绑定事件
      VVG.DOM.bindEvent(document, "mousedown", drag);
      VVG.DOM.bindEvent(document, "mousemove", drag);
      VVG.DOM.bindEvent(document, "mouseup", drag);
    }
  }
}();
DragDrop.dragStart();