In this lesson, we'll be implementing <b>didSelectItem</b> in our <b>UICollectionView</b> to handling the selection of our photos in the list. In order to show the selected photo, we will setup a custom header that holds reference to a <b>UIImageView</b> that we'll set using our selected image. In addition to this, we will analyze the behavior of efficiency and UI hanging when trying to optimize for the best user experience.