This project is going to give you some practice with collection views, the image picker and GCD, but at the same time introduce you to a new technology called the multipeer connectivity framework. This is a way to let users form impromptu connections to each other and send data, rather like BitTorrent.
The app we're going to make will show photos of your choosing in a collection view. That much is easy enough, because we did pretty much that already in project 10. But this time there's a subtle difference: when you add a photo it's going to automatically send it to any other devices you are currently connected to, and any photos they select will appear for you.
Create a new Single View App project in Xcode, naming it Project25. Please note: the nature of peer-to-peer apps is that you need to have at least two copies of your app running, one to send and one to receive. There are a few ways of setting this up, but I recommend using one physical device talking to the iOS simulator – that’s more than enough.