如何计算String的高度？

我试图调整单元格高度调整大小以适应UILabel文本,但它不工作..

var mySize = CGFloat()
    func collectionView(collectionView: UICollectionView, cellForItemAtIndexPath indexPath: NSIndexPath) -> UICollectionViewCell {
        let cell = collectionView.dequeueReusableCellWithReuseIdentifier("cell", forIndexPath: indexPath) as! cellView

        cell.myLabel.text = self.items[indexPath.item]
        cell.myLabel.bounds.size.height = self.mySize

        cell.backgroundColor = UIColor.yellowColor()

        return cell
    }

    func collectionView(collectionView: UICollectionView, didSelectItemAtIndexPath indexPath: NSIndexPath) {
        // handle tap events
        print("You selected cell #\(indexPath.item)!")
    }

    func collectionView(collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAtIndexPath indexPath: NSIndexPath) -> CGSize {

        func heightForLabel(text:String, font:UIFont, width:CGFloat) -> CGFloat
        {
            let label:UILabel = UILabel(frame: CGRectMake(0, 0, width, CGFloat.max))
            label.numberOfLines = 0
            label.lineBreakMode = NSLineBreakMode.ByWordWrapping
            label.font = font
            label.text = items[indexPath.row]

            label.sizeToFit()
            return label.frame.height
        }

        let font = UIFont(name: "Helvetica Neue", size: 30)
        let detailHeight = heightForLabel(items[indexPath.row], font: font!, width: UIScreen.mainScreen().bounds.size.width)

        self.mySize = detailHeight

        return CGSizeMake(UIScreen.mainScreen().bounds.size.width, 358 + detailHeight)
    }

有什么建议在这做什么？我应该用另一种方式吗？请,我需要帮助..问题是UILabel文本设置在cellForItemAtIndexPath,并且items是一个字符串数组.

这是我的项目文件,如果有人注意看看它:http: //www.filedropper.com/test_37