func collectionView(collectionView: UICollectionView, didSelectItemAtIndexPath indexPath: NSIndexPath) {
    let person = people[indexPath.item]

    let ac = UIAlertController(title: "Rename person", message: nil, preferredStyle: .Alert)
    ac.addTextFieldWithConfigurationHandler(nil)

    ac.addAction(UIAlertAction(title: "Cancel", style: .Cancel, handler: nil))

    ac.addAction(UIAlertAction(title: "OK", style: .Default) { [unowned self, ac] _ in
        let newName = ac.textFields![0]
        person.name = newName.text!

        self.collectionView.reloadData()
    })

    presentViewController(ac, animated: true, completion: nil)
}

在上面这段代码中为什么需要[unowned self]