兼容firefox的鼠标滚轮事件

function addMouseWheel(obj,fn,preventDefault){
	//添加绑定
	if(window.navigator.userAgent.toLowerCase().indexOf("firefox") != -1){
		obj.addEventListener("DOMMouseScroll",fnWheel,false);
	} else {
		obj.onmousewheel = fnWheel;
	}

	function fnWheel(ev){
		var oEvent = ev || event;
		var bDown = true;//下
		if(oEvent.wheelDelta){//ie chrome
			console.log(oEvent.wheelDelta)
			bDown = oEvent.wheelDelta > 0 ? false:true;
		} else {//ff
			console.log(oEvent.detail)
			bDown = oEvent.detail > 0 ? true:false;
		}
		(typeof fn == "function") && fn(bDown);
		if(preventDefault){
			oEvent.preventDefault && oEvent.preventDefault();
			return false;
		}
	}
}