SessionManager

SessionManager 作為管理類，只做配置相關的設置，定義上傳、下載等功能的入口函數，具體功能的實現，數據的處理則下沉到其他相關類處理，使用非常簡潔，方便，容易理解

初始化

  public static let `default`: SessionManager = {
        let configuration = URLSessionConfiguration.default
        configuration.httpAdditionalHeaders = SessionManager.defaultHTTPHeaders
        return SessionManager(configuration: configuration)
    }()

public init(
        configuration: URLSessionConfiguration = URLSessionConfiguration.default,
        delegate: SessionDelegate = SessionDelegate(),
        serverTrustPolicyManager: ServerTrustPolicyManager? = nil)
    {
        self.delegate = delegate
        self.session = URLSession(configuration: configuration, delegate: delegate, delegateQueue: nil)
        commonInit(serverTrustPolicyManager: serverTrustPolicyManager)
    }

• 自定義URLSessionConfiguration的httpAdditionalHeaders信息，有助于后臺阻止非法請求
• URLSession的代理移交給SessionDelegate，達到任務分離的目的

后臺下載
后臺下載需要我們定義一個用于后臺下載的sessionManager，這里我們包裝成一個單例

struct BackgroundManager {
    static let shared = BackgroundManager()
    let manager: SessionManager = {
        let config = URLSessionConfiguration.background(withIdentifier: "com.alamofire.background")
        config.httpAdditionalHeaders = SessionManager.defaultHTTPHeaders
        config.sharedContainerIdentifier = "group.com.alamofire"
        return SessionManager(configuration: config)
    }()
}

下載代碼：

        BackgroundManager.shared.manager.download(url) { (url, res) -> (destinationURL: URL, options: DownloadRequest.DownloadOptions) in
            let documentUrl = FileManager.default.urls(for: .documentationDirectory, in: .userDomainMask).first
            let fileUrl = documentUrl?.appendingPathComponent(res.suggestedFilename!)
            return (fileUrl!, [.removePreviousFile, .createIntermediateDirectories])
            }
            .downloadProgress { (progress) in
                print("------\(progress)")
            }
            .response { (response) in
                print("完成")
        }

從下面的SessionManager.init方法可以看到，sessionDelegate實現了閉包sessionDidFinishEventsForBackgroundURLSession

private func commonInit(serverTrustPolicyManager: ServerTrustPolicyManager?) {
        session.serverTrustPolicyManager = serverTrustPolicyManager
        delegate.sessionManager = self
        delegate.sessionDidFinishEventsForBackgroundURLSession = { [weak self] session in
            guard let strongSelf = self else { return }
            DispatchQueue.main.async { strongSelf.backgroundCompletionHandler?() }
        }
    }

//sessionDelegate 后臺下載完成代理方法
open func urlSessionDidFinishEvents(forBackgroundURLSession session: URLSession) {
        sessionDidFinishEventsForBackgroundURLSession?(session)
    }

delegate.sessionDidFinishEventsForBackgroundURLSession會在session后臺下載完成后執行，故而我們只要strongSelf.backgroundCompletionHandler保存AppDelegate授權后臺執行的閉包completionHandler就可以了,即

 func application(_ application: UIApplication, handleEventsForBackgroundURLSession identifier: String, completionHandler: @escaping () -> Void) {
        BackgroundManager.shared.manager.backgroundCompletionHandler = completionHandler
    }

如此，當后臺下載完成后，最終會回調執行系統的閉包completionHandler
如果最后不執行系統的completionHandler，會有以下影響：

• 造成界面刷新卡頓
• 打印臺會輸出警告

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

執行結果：
當下載過程中，進入后臺，則會停止進度打印

進入后臺

當過一會回到前臺時，直接輸出完成，說明在后臺是在下載的

回到前臺