UITableCell创建前两个的时候UIView.transitionWithView不起作用?

func tableView(tableView: UITableView, cellForRowAtIndexPath indexPath: NSIndexPath) ->  UITableViewCell {

    var bg:UIImage = UIImage(named: "BackgroundImage")!

    var myimg = UIImage(named: "download-loading")

    let link = self.tables[indexPath.row].pics?.first?.string!

    let cell = HomeCell(bgImg: bg, myimg: myimg!, target: self, data: tables[indexPath.row], url: tables[indexPath.row].url!, name: tables[indexPath.row].title!, description: tables[indexPath.row].description!)

    let url = NSURL(string: link!)

    if (self.myCache[indexPath.row]) {

        cell.myImage.sd_setImageWithURL(url, placeholderImage: myimg, options: SDWebImageOptions.LowPriority)

    }else{

        println("load cell")

        cell.myImage.alpha = 0.0

        self.myCache.insert(true, atIndex: indexPath.row)

        cell.myImage.sd_setImageWithURL(url, placeholderImage: myimg, options: SDWebImageOptions.LowPriority)

        cell.myImage.sd_setImageWithURL(url, placeholderImage: myimg!, completed: {

            (img) in

            UIView.transitionWithView(cell.myImage, duration: 0.5, options: UIViewAnimationOptions.TransitionCrossDissolve, animations: {

                cell.myImage.alpha = 1.0

                }, completion: nil)

        })



    }

 return cell

}