How to reopen a UIDocument on iCloud using UIDocumentBrowserViewController

I've developed a document based iOS app using the UIDocumentBrowserViewController (its a game). On startup of the app I want to reopen the last used UIDocument automatically. For that I store the url string of the created or opened UIDocument in the user defaults and try to open that on viewDidLoad of my DocumentBrowserViewController. That works fine for documents on local storage but fails on iCloud (or some other foreign) stored documents.

The url string for a locally stored document looks like this:

/private/var/mobile/Containers/Data/Application/F4AF5E0B-C261-47F5-95ED-0B8A1DFEEDE6/Documents/Emsave.emp

The same document on some cloud service has this url string:

/private/var/mobile/Containers/Shared/AppGroup/83CB33FC-5A87-404F-BFB9-8F2910A2192E/File Provider Storage/485172592867551584/Emsave.emp

The viewDidLoad of my DocumentBrowserViewController:

    override func viewDidLoad() {
        super.viewDidLoad()

        delegate = self

        allowsDocumentCreation = true
        allowsPickingMultipleItems = false

        // Do any additional setup after loading the view, typically from a nib.
                                                                // did we already open a game before?
        if let url = UserDefaults.standard.string(forKey: DocumentBrowserViewController.lastGameKey) {
            print("last opened game: \(url)")
            if FileManager.default.fileExists(atPath: url) {    // and is the game still existing?
                presentDocument(at: URL(fileURLWithPath: url))  // open it
            }
        }
    }

The test FileManager.default.fileExists(atPath: url) seems to return always false if the url string specifies a document stored on a cloud service although its there.

Solution

You shouldn't hold onto a url you get from the DocumentPicker or the DocumentBrowser, as the file might have moved, been renamed while your app was off.

You should save a bookmark from the url, and recreate the url from the bookmark next time you need it.

Have a look at this section : Bookmarks and Security Scope in https://developer.apple.com/documentation/foundation/nsurl?language=objc