Swift实现自动计算collectionView的item的大

实现步骤：
1）遵守代理UICollectionViewDelegateFlowLayout
2）实现UICollectionViewDelegateFlowLayout代理
在下面的代理方法中实现自动计算文本宽度即可！！！

    func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAt indexPath: IndexPath) -> CGSize {

        let str = titleArr[indexPath.row]
        let dic = [NSAttributedStringKey.font:UIFont.systemFont(ofSize: 13.0)]
        let size = CGSize(width: CGFloat(MAXFLOAT), height: 30)
        let width = str.boundingRect(with: size, options: .usesLineFragmentOrigin, attributes: dic, context: nil).size.width
        return CGSize(width: width+20, height: 30)
    }