Did you ever want your app to open the last used document on startup? Well, that’s not too difficult on macOS. In your NSDocumentController subclass you have the array of recentDocumentURLs, and on startup you can try to open the first member if it exists.
But on iOS it’s a little bit more tricky. There is no such array as on macOS. So we have to mimic it by ourself.
This is a review of my post 2 years ago: UIDocumentBrowserViewController and External Storage