Alamofire Download in Background Session

AlamoFire Download in Background Session

Update

Based on this amazing tutorial, I have put together an example project available on GitHub. It has an example for background session management.

According to Apple's URL Loading System Programming Guide:

In both iOS and OS X, when the user relaunches your app, your app
should immediately create background configuration objects with the
same identifiers as any sessions that had outstanding tasks when your
app was last running, then create a session for each of those
configuration objects. These new sessions are similarly automatically
reassociated with ongoing background activity.

So apparently by using the appropriate background session configuration instances, your downloads will never be "in flux".

I have also found this answer really helpful.

Original answer

From Alamofire's GitHub page:

Applications can create managers for background and ephemeral
sessions, as well as new managers that customize the default session
configuration, such as for default headers (HTTPAdditionalHeaders) or
timeout interval (timeoutIntervalForRequest).

By default, top level methods use a shared Manager instance with default session configuration. You can however create a manager with background session configuration like so:

let configuration = NSURLSessionConfiguration.backgroundSessionConfigurationWithIdentifier("com.example.app.background")
let manager = Alamofire.Manager(configuration: configuration)

You can then make requests using this Manager instance.

manager.startRequestsImmediately = true
let request = NSURLRequest(URL: NSURL(string: "your.url.here")!)
manager.request(request)

By looking at its implementation, it also has a property called backgroundCompletionHandler, so you can add a completion block:

manager.backgroundCompletionHandler = {
        // do something when the request has finished
    }

Video download using Alamofire is freezing the UI in Swift iOS

Try using below code:

func download(url: String,fileName: String,progressUpdate: ((_ percent: Double) -> Void)? = nil, completion:@escaping (_ success: Bool, _ error: Error?,_ fileUrl: URL?) -> Void) {
        let utilityQueue = DispatchQueue.global(qos: .utility)
        Alamofire.download(url)
            .downloadProgress(queue: utilityQueue) { progress in
                DispatchQueue.main.async {
                    progressUpdate?(progress.fractionCompleted)
                }
            }
            .responseData { response in
                if let data = response.result.value {
                    let fileURL = try! FileManager.default.url(for: .documentDirectory, in: .userDomainMask, appropriateFor: nil, create: false).appendingPathComponent(fileName)
                    do {
                        try data.write(to: fileURL, options: .atomic)
                        completion(true,nil,fileURL)
                    } catch {
                        completion(false,error,nil)
                    }
                }
        }

    }

How to use it:

download(url: "", fileName: "filName", progressUpdate: { (progress) in
            print("Progress \(progress)")
        }) { (success, error, filePath) in
            print("success \(success) error \(error?.localizedDescription ?? "nil") path \(filePath?.absoluteString ?? "nil")")
        }

Alamofire requests while device is sleeping

Yes, background sessions only permit upload and download tasks, not data tasks. They also only permit delegate-based requests, not completion handler-based requests. This answer outlines many of the considerations when doing this in conjunction with Alamofire.

But this begs the question as to whether you really want to use a background session at all. When your app is awaken for background fetch, if you’re able to finish you request within a reasonable amount of time (e.g. 30 seconds), you should probably consider a standard session, not a background session. It’s a lot simpler.

Do not conflate an app running in the “background” with a “background” URLSessionConfiguration: They’re completely different patterns. Just because your app is running in the background, that doesn’t mean you have to use background URLSessionConfiguration. If your app is running (whether in foreground or in the background), then a standard session is fine. You only need background session if you want it to continue after the app is suspended (or is terminated) and you’re willing to encumber yourself with all the extra overhead that background sessions entail.

Background sessions are not intended for requests performed while the app is running in the background. They’re intended for requests that will continue after your app is suspended (and even if it eventually is terminated in the course of its natural lifecycle). That means that background sessions are ideal for slow requests that cannot be completed in a reasonable amount of time, e.g., downloading video asset, downloading many large image assets or documents, etc.

But if you’re just performing a routine GET/POST request that will complete in a reasonable amount of time, consider not using background URLSessionConfiguration, but just do normal request and call the background fetch completion handler when your request is done (i.e., in your network request’s completion handler).

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