override func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
    .
    .
    .

    //按下cell後的動作--> 將畫面切換到navigation controller
        cell.buttonPressedAction = {
            //performSeuge在傳送之前會先觸發prepare，performSegue是透過sender傳送資料
            self.performSegue(withIdentifier: "idFirstSegue", sender: indexPath.row)
        }
 return cell
 }

     //使用prepare傳遞資料到SelectedPhotoViewController
    override func prepare(for segue: UIStoryboardSegue, sender: Any?) {

        //使用此方法將資料穿過navigation controller送到 SelectedPhotoViewController
        let navigationController = segue.destination as! SelectedNavigationController
        //first代表在collection中的第一個東西(是root view controller，viewController是一個陣列)
        //順帶一提，back view controller是在陣列的 n-2，top controller在 n-1，n為array中item的數量
        let SelectedPhotoController = navigationController.viewControllers.first as? SelectedPhotoViewController

        //由 performSegue 觸發的 prepare function 後面的 sender 代表要傳遞的東西
        //而若是直接從 storyboard 點選所觸發的 segue ，這邊的 sender 則是代表點選的元件

    }