A list,

Starting with iOS7, apple introduced URLSession, called NSURLSession in OC, instead of URLConnection. Since the underlying layer of Alamofire is also a encapsulated URLSession, let’s first understand the native URLSession, and then learn Alamofire more thoroughly. If you already have a good grasp of URLSession, you can skip this article.

URLSession request

1. Basic process of request

Configurations ➡️ Creates a session ➡️ creates a task ➡️ Resume () Starts asking ➡️ to listen for callbacks

URLSession.shared.dataTask(with: url) { (data, response, error) in
    if error == nil {
        print("Request successful"\(String(describing: response))" )
    }
}.resume()
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The above is a basic request format for URLSession, where urlsession.shared provides us with a singleton session object that provides the default configuration for creating tasks. Configurations are omitted, and the above code can be expanded in detail as follows:

let configure = URLSessionConfiguration.default
let session = URLSession.init(configuration: configure)
session.dataTask(with: url) { (data, response, error) in
    if error == nil {
        print("Request successful"\(String(describing: response))" )
    }
}.resume()
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URLSession Instances can be created using the URLSessionConfiguration. Why is it possible to omit configure in the first part of the code?

URLSessionConfiguration:

The reason is that there are three types of configure:

  • .default: is the default value, which is the same as the session created by urlsession.shared, but can be extended more than this singleton.
  • .ephemeral: ephemeral 和 URLSession.sharedIt’s very similar, except it doesn’t writecaches, cookies 或 credentials(Certificate) to disk.
  • Background: background session, which can be uploaded and downloaded when the program cuts to the background, crashes, or does not run.

URLSessionTask:

URLSessionTask is an abstract class that represents a task object. A session creates a task. Tasks here refer to fetching data, downloading or uploading files. There are four types of tasks:

  • URLSessionDataTask: Handles getting data from the server into memory from HTTP GET requests.
  • URLSessionUploadTask: uploads a file from a hard disk to the server in HTTP POST or PUT mode
  • URLSessionDownloadTask: Downloads a file from a remote server to a temporary file location.
  • NSURLSessionStreamTask: A stream-based URL session task that provides a TCP/IP connection interface created through NSURLSession.

Task operation

Note: Our network task is suspend by default, and you need to use the resume() function after retrieving the dataTask to resume or start the request.

  • resume(): Start a task
  • suspend(): Pause a task
  • cancel(): Cancel the task

2. Background download

Since the introduction of URLSession, iOS has really implemented background downloads. Below is a simple background download implementation.

1. Initialize the configuration of a background mode

let configuration = URLSessionConfiguration.background(withIdentifier: self.createID())
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2. Initialize the network download session through configuration, set the related proxy, and call back the data signal response.

let session = URLSession.init(configuration: configuration, delegate: self, delegateQueue: OperationQueue.main)
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3. Session Creates a downloadTask. – Resume starts

session.downloadTask(with: url).resume()
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4. Initiate a connection – send a related request – callback proxy response

  • Download the progress of the agent callback urlSession (_ session: downloadTask: didWriteData bytesWritten: totalBytesWritten: totalBytesExpectedToWrite: ) to monitor download progress:
func urlSession(_ session: URLSession, downloadTask: URLSessionDownloadTask, didWriteData bytesWritten: Int64, totalBytesWritten: Int64, totalBytesExpectedToWrite: Int64) {
    print(" bytesWritten \(bytesWritten)\n totalBytesWritten \(totalBytesWritten)\n totalBytesExpectedToWrite \(totalBytesExpectedToWrite)")
    print("Download progress:\(Double(totalBytesWritten)/Double(totalBytesExpectedToWrite))\n")}Copy the code

Due to HTTP shard transport, download progress is called back at regular intervals.

  • The didFinishDownloadingTo agent callback transfers the data in the temporary file to the appropriate sandbox after downloading
func urlSession(_ session: URLSession, downloadTask: URLSessionDownloadTask, didFinishDownloadingTo location: URL) {
    // Download complete - Start sandbox migration
    print("Download complete -\(location)")
    let locationPath = location.path
    // Copy to user directory (filename named after timestamp)
    let documnets = NSHomeDirectory() + "/Documents/" + self.lgCurrentDataTurnString() + ".mp4"
    print("Mobile address:\(documnets)")
    // Create file manager
    let fileManager = FileManager.default
    try! fileManager.moveItem(atPath: locationPath, toPath: documnets)
}
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5. Enable background download permission

In addition to these, want to realize the background download must also be implemented in the AppDelegate handleEventsForBackgroundURLSession, open the background download permissions:

class AppDelegate: UIResponder.UIApplicationDelegate {
    var window: UIWindow?
    // Used to save the completionHandler for the background download
    var backgroundSessionCompletionHandler: (() -> Void)?
    func application(_ application: UIApplication, handleEventsForBackgroundURLSession identifier: String, completionHandler: @escaping (a) -> Void) {
        self.backgroundSessionCompletionHandler = completionHandler
    }
}
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6, in urlSessionDidFinishEvents callback system callback

You also need to call the system callback in the main thread to tell the system to update the screen in time

func urlSessionDidFinishEvents(forBackgroundURLSession session: URLSession) {
    print("Background task download back")
    DispatchQueue.main.async {
        guard let appDelegate = UIApplication.shared.delegate as? AppDelegate.let backgroundHandle = appDelegate.backgroundSessionCompletionHandler else { return }
        backgroundHandle()
    }
}
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⚠️ If you do not implement the callback function in the agent, you can still implement the background download, but there will be a lag problem, the console will also appear warning.

Warning: Application delegate received call to – application:handleEventsForBackgroundURLSession:completionHandler: but the completion handler was never called.

So much for the basics of URLSession, let’s take a look at Alamofire in the next article!

The above summary refers to and partly extracts the following articles, thanks very much to the following authors for sharing! :

1. “Basic Use of URLSession” by Shengzhi _HY

2. Alamofire- essential skills of URLSession by Cooci

Reprint please note the original source, shall not be used for commercial communication – any more