封装了一个渐隐渐现的轮播图组件，请问如何解决 第一次切换效果不正确？

封装了一个渐隐渐现的轮播图组件，但是只有在第一次点击向左或向右按钮时，切换效果都不是渐隐渐现，其他时候切换效果都正确

轮播图组件代码

//轮播图组件
;(function($){

var Carousel = function(poster){

    var self = this;
    this.poster = poster;
    this.posterMain = poster.find(".slider-list");
    this.posterItem = poster.find(".slider-panel");
    this.posterSize = this.posterItem.length;
    this.prevBtn = poster.find(".slider-page .prev");
    this.nextBtn = poster.find(".slider-page .next");
    this.navSpan = poster.find(".pic-num span");
    this.isanimated = true;
    this.index = 0;
    //默认参数
    this.setting = {
        "autoPlay" : false,
        "speed" : 200,
        "delay" : 3000,
        "hasnav" : false
    };

    $.extend(this.setting,this.getSetting());

    //prevBtn
    this.prevBtn.click(function(){
        if(self.isanimated){
            self.index --;
            if(self.index < 0 ){
                self.index = self.posterSize -1;
            }
            self.showImg(self.index);
        }
    });

    //nextBtn
    this.nextBtn.click(function(){
        if(!self.isanimated){
            return;
        }
        self.index ++;
        if(self.index >= self.posterSize ){
            self.index = 0;
        }
        self.showImg(self.index);
    });

    //判断自动播放
    if(this.setting.autoPlay){
        this.autoPlay();
    };
};

Carousel.prototype = {

    //自动播放
    autoPlay : function(){
        var self = this;
        if(!self.isanimated){
            return;
        }
        self.index ++;
        if(self.index >= self.posterSize ){
            self.index = 0;
        }
        self.showImg(self.index);
        this.timer = window.setInterval(function(){
            self.autoPlay();
        },this.setting.delay);
    },

    //获取设置的参数
    getSetting : function(){
        var setting = this.poster.attr("data-setting");
        if( setting && setting != ""){
            return $.parseJSON(setting);
        }
        return {};
    },

    //showImg
    showImg : function(index){
        var _this = this;
        this.isanimated = false;
        this.posterItem.eq(index).addClass("on").animate({opacity:1,"z-index":0},this.speed,function(){
            _this.isanimated = true;
        }).siblings().removeClass("on").animate({opacity:0,"z-index":-1},this.speed);
        $(".pic-num span").eq(index).addClass("on").siblings(".pic-num span").removeClass("on");
    }
}

//初始化函数
Carousel.init = function(poster){
    var _this = this;
    poster.each(function(){
        new _this($(this));
    });
}

window["Carousel"] = Carousel;

})(jQuery);
html代码

css代码

请问呢该如何解决呢？求大神