function CFlyAds(objStr, targetStr, startX, startY)
{
    this.objStr = objStr;
    this.target = document.getElementById(targetStr);
    this.target.style.position = "absolute"; //将 position 样式属性设置为 absolute，保证可以飞行
    this.target.style.left = startX + "px";
    this.target.style.top = startY + "px";
    
    this.hd = "right"; //当前在 X 方向上向左还是向右飞行，right 向右飞行，其它值（如 left）向左飞行
    this.vd = "down"; //当前在 Y 方向上向上还是向下飞行，down 向下飞行，其它值（如 up）向上飞行 
    this.stepX = 2; //每次飞行在 X 方向上执行的偏移，单位像素
    this.stepY = 1; //每次飞行在 Y 方向上执行的偏移，单位像素
    this.flyDelay = ((arguments.length>=5)?arguments[4]:30); //每次飞行的时间间隔，单位毫秒
    
    //碰到边缘时暂停并显示另一广告，离开边缘后显示原广告
    this.adsFadeTime = 1000; //广告切换的时间，单位毫秒
    this.flyingAds = ""; //原广告的 HTML 代码
    this.showingAds = ""; //边缘暂停广告的 HTML 代码
    
    this.timer = null;
    
    this.SetFlyProperty = CFlyAds_SetFlyProperty; //设置飞行相关属性
    this.Fly = CFlyAds_Fly; //开始飞行
    
    this.status = "flying"; //状态：flying－飞行中或允许飞行；paused－已经或即将暂停飞行；stopped－已经或即将停止飞行
    this.Pause = CFlyAds_Pause; //飞行 和 暂停 状态之间互相切换，也可以指定一个参数，用于强制指定 status
    this.Clear = CFlyAds_Clear; //清除飞行
    this.ReFly = CFlyAds_ReFly; //继续飞行，仅用于停止状态
}

//设置飞行相关属性
function CFlyAds_SetFlyProperty(hd, vd, stepX, stepY, flyDelay)
{
    this.hd = hd;
    this.vd = vd;
    this.stepX = stepX;
    this.stepY = stepY;
    this.flyDelay = flyDelay;
}   

//开始飞行
function CFlyAds_Fly()
{
    if (this.status == "paused")
    {
        //暂停，不执行后面的飞行代码
        this.timer = setTimeout(this.objStr + ".Fly()", this.flyDelay);
        return;
    }
    else if (this.status == "stopped")
    {
        //已经要求停止，退出
        return;
    }

    //newX 和 newY 分别为预计的新位置
    var newX = parseInt(this.target.style.left);
    newX = (this.hd=="right")?(newX+this.stepX):(newX-this.stepX);
    var newY = parseInt(this.target.style.top);
    newY = (this.vd=="down")?(newY+this.stepY):(newY-this.stepY);
    
    //修正 newX 值，避免飞行物件出界
    if (newX >= document.body.scrollLeft+document.body.clientWidth-this.target.offsetWidth)
    {
        newX = document.body.scrollLeft+document.body.clientWidth-this.target.offsetWidth;
        this.hd = "left";
        this.Pause("paused");
        this.target.innerHTML = this.showingAds;
        setTimeout(this.objStr+".target.innerHTML="+this.objStr+".flyingAds;"+this.objStr+".Pause('flying')", this.adsFadeTime);
    }
    else if (newX < document.body.scrollLeft)
    {
        newX = document.body.scrollLeft;
        this.hd = "right";
        this.Pause("paused");
        this.target.innerHTML = this.showingAds;
        setTimeout(this.objStr+".target.innerHTML="+this.objStr+".flyingAds;"+this.objStr+".Pause('flying')", this.adsFadeTime);
    }
    
    //修正 newY 值，避免飞行物件出界
    if (newY >= document.body.scrollTop+document.body.clientHeight-this.target.offsetHeight)
    {
        newY = document.body.scrollTop+document.body.clientHeight-this.target.offsetHeight;
        this.vd = "up";
        this.Pause("paused");
        this.target.innerHTML = this.showingAds;
        setTimeout(this.objStr+".target.innerHTML="+this.objStr+".flyingAds;"+this.objStr+".Pause('flying')", this.adsFadeTime);
    }
    else if (newY < document.body.scrollTop)
    {
        newY = document.body.scrollTop;
        this.vd = "down";
        this.Pause("paused");
        this.target.innerHTML = this.showingAds;
        setTimeout(this.objStr+".target.innerHTML="+this.objStr+".flyingAds;"+this.objStr+".Pause('flying')", this.adsFadeTime);
    }
    
    //设置飞行物件新位置
    this.target.style.left = newX + "px";
    this.target.style.top = newY + "px";
    
    //准备下一次飞行
    this.timer = setTimeout(this.objStr + ".Fly()", this.flyDelay);
}

//飞行状态切换到暂停状态
//暂停状态切换到飞行状态
function CFlyAds_Pause()
{
    //如果有参数，那么第一个参数就是用于强制设置 status 值
    //否则“反转”status 值
    if (arguments.length>0)
    {
        if (arguments[0]=="flying" || arguments[0]=="paused")
        {
            this.status = arguments[0];
            return;
        }
    }
    
    if (this.status == "flying")
    {
        this.status = "paused";
    }
    else if (this.status == "paused")
    {
        this.status = "flying";
    }
}

//清除飞行
function CFlyAds_Clear()
{
    clearTimeout(this.timer); //必须清除，否则 status 被恢复时，可能（仅仅是可能）会造成两个定时器在工作。
    this.status = "stopped";
    this.target.style.display = "none"
}

//继续飞行，仅用于停止状态
function CFlyAds_ReFly()
{
    if (this.status == "stopped")
    {
        this.status = "flying";
        this.target.style.display = "block";
        this.Fly();
    }
}