Downloading a file, getting error in DropboxTransportClient.swift, line 200

Hi all,

I've checked the forum and didn't find an answer.

Overview: my app works with a SQLite db. I have an option to store the DB in DropBox (only store it, not working on it).

The user download the DB, I create a file "device name".usr to say to other users the DB is in use right now.

I've created this procedure (sorry, comments are in Italian):

    func copiaDB() {
        let databaseURL = try! FileManager.default
            .url(for: .applicationSupportDirectory, in: .userDomainMask, appropriateFor: nil, create: true)
        .appendingPathComponent("db.sqlite")
        let completeUrl = URL(fileURLWithPath: databaseURL.path)
        let destination: (URL, HTTPURLResponse) -> URL = { temporaryURL, response in
                return completeUrl
        }
        
        let semo = DispatchSemaphore.init(value: 0)
        let downQueue = OperationQueue.init()
        downQueue.maxConcurrentOperationCount = 1
        // operation 1
        downQueue.addOperation {

            // detach database o errore sqlite
                dbQueue = try? AppDatabase.openDatabase(atPath: "")
                // copio file
            self.client?.files.download(path: "/db.sqlite", overwrite: true, destination: destination)
        }
        // operation 2
        downQueue.addOperation {
            // avverto che qualcuno sta lavorando sul file
            
            let fileData = "Database in uso".data(using: String.Encoding.utf8, allowLossyConversion: false)!

            self.client?.files.upload(path: "/\(UIDevice.current.name).usr", input: fileData)
        }
        // operation 3
        downQueue.addOperation {
            semo.wait()
            dbQueue = try? AppDatabase.openDatabase(atPath: databaseURL.path)
        }
    }

 It work's fine, detach the DB, download it, create the usr file, attach the DB to queue so I can work on it locally.

Here there is a little problem, not reguaridng DropBox. I need a way to advise user the download is completed, but i can't create an alert or change a label text because the download is async with the main queue.

I need the procedure to work with OperationQueue because

dbQueue = try? AppDatabase.openDatabase(atPath: databaseURL.path)

needs to be exeguted after the download is finished, or i risk to compromise the db.

 

Then I created the procedure to check if the db is available in DropBox: 

   

    func checkDB() {
        
        client?.files.search(path: "", query: ".usr", mode: .filename).response {
            response, error in
            if let response = response {
                if response.matches.count == 0 {
                    self.copiaDB()
                } else {
                    self.inUso(tipo: (response.matches.first?.metadata.name)!)
                }
            } else if let _ = error {
                
            }
        }
    }

self.inUso simply create an alert saying the device name is using the DB. This works, but, calling the first procedure copiaDB() I now got this error:

Fatal error: Unexpectedly found nil while implicitly unwrapping an Optional value: file /Users/marcopirola/Desktop/Progetto Swift/Gestione Affitti/SwiftyDropbox/Source/SwiftyDropbox/Shared/Handwritten/DropboxTransportClient.swift, line 200

2019-11-10 11:10:02.238294+0100 Gestione Affitti[11658:6548246] Fatal error: Unexpectedly found nil while implicitly unwrapping an Optional value: file /Users/marcopirola/Desktop/Progetto Swift/Gestione Affitti/SwiftyDropbox/Source/SwiftyDropbox/Shared/Handwritten/DropboxTransportClient.swift, line 200

 I tried to debug it, it actually download the db and create the .usr file, but then somehow it recalls the download procedure with a nil command string.

I removed checkDB() from the procedure calling directly copiaDB() and it works again. Any idea?

Thanks, Marco "Chetral" Pirola