拖拽实现步骤：

鼠标按下 开始拖拽

鼠标移动 拖拽过程

鼠标松开 结束

然后在移动过程中，将偏移值赋值给拖拽的对象

var params = {
		left: 0,
		top: 0,
		currentX: 0, //开始时的x坐标
		currentY: 0, //开始时的y坐标
		flag: false  //标记鼠标按下或松开
	};
	//获取相关CSS属性
	//o是移动对象
	var getCss = function (o, key) {
		//兼容ie的currentStyle和其他浏览器支持的getComputedStyle方法
		return o.currentStyle ? o.currentStyle[key] : document.defaultView.getComputedStyle(o, false)[key];
	};
	//拖拽的实现
	var startDrag = function (bar, target, callback) {
		if (getCss(target, "left") !== "auto") {
			params.left = getCss(target, "left");
		}
		if (getCss(target, "top") !== "auto") {
			params.top = getCss(target, "top");
		}
		//触发拖拽的对象发生鼠标按下事件
		bar.onmousedown = function (event) {
			params.flag = true;
			//兼容ie
			if (!event) {
				event = window.event;
				//防止IE文字选中 
				bar.onselectstart = function () {
					return false;
				}
			}
			var e = event;
			//传入初始坐标
			params.currentX = e.clientX;
			params.currentY = e.clientY;
		};
		document.onmouseup = function () {
			params.flag = false;
			if (getCss(target, "left") !== "auto") {
				//获取移动后的x
				params.left = getCss(target, "left");
			}
			if (getCss(target, "top") !== "auto") {
				//获取移动后的y
				params.top = getCss(target, "top");
			}
		};
		document.onmousemove = function (event) {
			var e = event ? event : window.event;
			//鼠标还在按下状态
			if (params.flag) {
				//获取现在的坐标
				var nowX = e.clientX,
					nowY = e.clientY;
				//鼠标移动路径
				var disX = nowX,
					disY = nowY;
				//将移动后的坐标传给target
				target.style.left = disX + "px";
				target.style.top = disY + "px";
				if (event.preventDefault) {
					event.preventDefault();
				}
				return false;
			}
			
			if (typeof callback == "function") {
				callback(parseInt(params.left) + disX, parseInt(params.top) + disY);
			}
		}
	};