利用URLSession實(shí)現(xiàn)后臺(tái)下載用法比較簡(jiǎn)單：

    func backgroundDownloadTest(){
        
        //url實(shí)例
        let url: URL = URL.init(string: "http://pic1.win4000.com/pic/5/9e/4933293953.jpg")!
        
        //會(huì)話配置
        let config = URLSessionConfiguration.background(withIdentifier: "backgroundID")
        
        //會(huì)話對(duì)象
        let session = URLSession(configuration: config, delegate: self, delegateQueue: .main)
        
        //啟動(dòng)會(huì)話
        session.downloadTask(with: url).resume()
        
    }

以上是初始化并啟動(dòng)會(huì)話的步驟尊勿。接下來需要在會(huì)話代理類中實(shí)現(xiàn)URLSessionDownloadDelegate中的方法：

urlSession(_:downloadTask:didFinishDownloadingTo:)

下載完成之后就回調(diào)
    func urlSession(_ session: URLSession, downloadTask: URLSessionDownloadTask, didFinishDownloadingTo location: URL) {
        /*
         下載完成 開始沙盒遷移
         */
        print("下載完成--\(location.path)")
        let locationPath = location.path
        
        //拷貝到用戶目錄 文件名以時(shí)間戳命名
        let documents = NSHomeDirectory()+"/Documents/" + self.getTimestampStr() + ".jpg"
        
        print("遷移地址--\(documents)")
        
        let fileManager = FileManager.default
        
        try! fileManager.moveItem(atPath: locationPath, toPath: documents)
 
    }

    func getTimestampStr() -> String {
        
        let date = NSDate()
        
        let dateformatter = DateFormatter()
        dateformatter.dateFormat = "yyyy年MM月dd日 HH:mm:ss"
        print("當(dāng)前時(shí)間-\(dateformatter.string(from: date as Date))")
        
        let timeInteval: TimeInterval = date.timeIntervalSince1970
        let timeStamp = Int(timeInteval)
        
        print("當(dāng)前時(shí)間時(shí)間戳-\(timeStamp)")

        return String(timeStamp)
    }

urlSession(_:downloadTask:didWriteData:totalBytesWritten:totalBytesExpectedToWrite:)

    //下載進(jìn)度監(jiān)控
    func urlSession(_ session: URLSession, downloadTask: URLSessionDownloadTask, didWriteData bytesWritten: Int64, totalBytesWritten: Int64, totalBytesExpectedToWrite: Int64) {
        
        print("bytesWritten \(bytesWritten), totalBytesWritten \(totalBytesWritten), totalBytesExpectedToWrite \(totalBytesExpectedToWrite)")
        
        print("下載進(jìn)度：\(Double(totalBytesWritten)/Double(totalBytesExpectedToWrite))\n")

    }

只是完成以上工作姻锁，還不能完成后臺(tái)下載任務(wù)紊册，還需要以下步驟:
AppDelegate中

    //用于保存后臺(tái)下載的completionHandler
    var backgroundSessionCompletionHandler: (() -> Void)?
    
    func application(_ application: UIApplication, handleEventsForBackgroundURLSession identifier: String, completionHandler: @escaping () -> Void) {
        self.backgroundSessionCompletionHandler = completionHandler
    }

最后會(huì)話代理實(shí)現(xiàn)：urlSessionDidFinishEvents(forBackgroundURLSession:)

    // 告訴委托排隊(duì)等待會(huì)話的所有消息都已傳遞。 (Tells the delegate that all messages enqueued for a session have been delivered.)
    func urlSessionDidFinishEvents(forBackgroundURLSession session: URLSession) {
        print("后臺(tái)任務(wù)下載回來")
        DispatchQueue.main.async {
            guard let appDelegate = UIApplication.shared.delegate as? AppDelegate, let backgroundHandle = appDelegate.backgroundSessionCompletionHandler else { return }
            backgroundHandle()
        }
    }

到此骨稿，后臺(tái)下載就可以順利完成了碉咆。