class ViewController: UICollectionViewController {

    let refreshControl = UIRefreshControl()

    override func viewDidLoad() {
        super.viewDidLoad()
  
        //注意这里，collectionView 的 alwaysBounceVertical 必须打开
        //高度没有超过屏幕时不会开启滑动，当然也就无法触发下拉刷新
        self.collectionView?.alwaysBounceVertical = true
       
        refreshControl.tintColor = UIColor.grayColor()

        //下拉刷新
        //Selector 语法为 Swift 2.2
        refreshControl.addTarget(self, action: #selector(FlickrPhotosViewController.refresh), forControlEvents: UIControlEvents.ValueChanged)

        self.collectionView?.addSubview(refreshControl)
    }

    func refresh() {

        print("下拉刷新")
        refreshControl.endRefreshing()
    }

  //数据源 代理相关方法省略....
}