为什么reloadData（）在我的应用程序中不起作用？

I'm trying to download a set of images from Parse.com into a UICollectionView. Everything works except for the reloadData() function. In the app, I can hit the back button and re-enter the UICollectionView and all of the images are there. I just don't know what I'm doing wrong with the reloadData() function.I'm trying to download a set of images from Par