Swift: wait for app execution till file downloads ends-CodePudding

I have a problem, I can't solve by myself. But I think there has to be a very simple solution. So simple I can't see it :-(

I have a function that's downloads a file from a webserver and stores it on the device. this function works:

func downloadFile (fileOnServer: String, fileOnDevice: URL) async  {
  
    if let dataURL = URL(string: fileOnServer) {
        URLSession.shared.downloadTask(with: dataURL) { (tempFileUrl, response, error) in
            
            if let dataTempFileUrl = tempFileUrl {
                do {
                    let urlData = try Data(contentsOf: dataTempFileUrl)

                    try urlData.write(to: fileOnDevice)
                    
              //      checkSavedFile(file: fileOnDevice)
                    
                    print("download successfull")
                } catch {
                    print("Error")
                }
            }
        }.resume()
    }
}

I call this function in a simple SwiftUI view:

truct UpdateView: View {
    
    @State var text = "Loading"
    
    var body: some View {
        VStack{
            
        Text(/*@START_MENU_TOKEN@*/"Hello, World!"/*@END_MENU_TOKEN@*/)
            Text(text)
        }
        .task()
        {
            print ("START downloadFile() ")
            await downloadFile(fileOnServer: htmlLinkForJSON, fileOnDevice: downloadedJSON)
           await downloadFile(fileOnServer: htmlLinkForCategories, fileOnDevice: downloadedCategories)
            print ("END downloadFile()")
            print ("---------")
            print ("Start checkSavedFile()")
            checkSavedFile(file: downloadedJSON)
            checkSavedFile(file: downloadedCategories)
            print ("END checkSavedFile()")
            print ("---------")
        }
    }      
}

Now my problem: As you can see on the console output the checkSavedFile() starts before the download ends. How can I pause my code till the downloads ends? Do you know what I mean?

START downloadFile() 
END downloadFile()
---------
Start checkSavedFile()
checkSavedFile gestartet
file:///Users/Library/Developer/CoreSimulator/Devices/AB464055-A554-4B4B-9AB3-BF7FAD670A81/data/Containers/Data/Application/7C940BA4-99EB-4250-83CB-BA6D2AA9669B/Documents/downloadedJSON.txt available!
checkSavedFile gestartet
file:///Users/Library/Developer/CoreSimulator/Devices/AB464055-A554-4B4B-9AB3-BF7FAD670A81/data/Containers/Data/Application/7C940BA4-99EB-4250-83CB-BA6D2AA9669B/Documents/downloadedCategories.txt available!
END checkSavedFile()
---------
2022-01-04 20:27:17.573376 0100 Blindzeln_Prototyp[4296:186653] [boringssl] boringssl_metrics_log_metric_block_invoke(151) Failed to log metrics
download successfull
download successfull

CodePudding user response：

You have to make downloadFile really async by using the appropriate async URLSession API. The completion handler API doesn't wait.

enum URLError : Error {
    case badURL
}

func downloadFile (fileOnServer: String, fileOnDevice: URL) async throws {
    guard let dataURL = URL(string: fileOnServer) else { throw URLError.badURL }
    let (dataTempFileUrl, _) = try await URLSession.shared.download(from: dataURL)
    try FileManager.default.moveItem(at: dataTempFileUrl, to: fileOnDevice)
}

In the task you have add a do - catch block to handle the error(s)

.task()
    {
        do {
            print ("START downloadFile() ")
            try await downloadFile(fileOnServer: htmlLinkForJSON, fileOnDevice: downloadedJSON)
            try await downloadFile(fileOnServer: htmlLinkForCategories, fileOnDevice: downloadedCategories)
            print ("END downloadFile()")
            print ("---------")
            print ("Start checkSavedFile()")
            checkSavedFile(file: downloadedJSON)
            checkSavedFile(file: downloadedCategories)
            print ("END checkSavedFile()")
            print ("---------")
       } catch {
            print(error)
       }    
    }