系统后台下载

• config三种模式
  default
ephemeral
  本文主角background

• 使用分片下载

// 1：初始化configuration
let configuration = URLSessionConfiguration.background(withIdentifier:id)
// 2：初始化网络下载会话
let session = URLSession.init(configuration: configuration, delegate: self, delegateQueue: OperationQueue.main)
// 3：启动任务
session.downloadTask(with: url).resume()

//MARK: - session代理
extension ViewController:URLSessionDownloadDelegate{
    func urlSession(_ session: URLSession, downloadTask: URLSessionDownloadTask, didFinishDownloadingTo location: URL) {
        // 下载完成 
    }
    
    func urlSession(_ session: URLSession, downloadTask: URLSessionDownloadTask, didWriteData bytesWritten: Int64, totalBytesWritten: Int64, totalBytesExpectedToWrite: Int64) {
       // 下载进度
}

官方提示，需要在适当的时候调用handler

You should call the completionHandler as soon as you're finished handling the callbacks.

So

//用于保存后台下载的completionHandler
    var backgroundSessionCompletionHandler: (() -> Void)?

class AppDelegate: UIResponder, UIApplicationDelegate {
    var window: UIWindow?
    //用于保存后台下载的completionHandler
    var backgroundSessionCompletionHandler: (() -> Void)?
    func application(_ application: UIApplication, handleEventsForBackgroundURLSession identifier: String, completionHandler: @escaping () -> Void) {
        self.backgroundSessionCompletionHandler = completionHandler
    }
}

最后实现代理

extension ViewController:URLSessionDownloadDelegate{
    func urlSession(_ session: URLSession, downloadTask: URLSessionDownloadTask, didFinishDownloadingTo location: URL) {
    }
    
    func urlSession(_ session: URLSession, downloadTask: URLSessionDownloadTask, didWriteData bytesWritten: Int64, totalBytesWritten: Int64, totalBytesExpectedToWrite: Int64) {}
    
    func urlSessionDidFinishEvents(forBackgroundURLSession session: URLSession) {
    //实现调用
        DispatchQueue.main.async {
            guard let appDelegate = UIApplication.shared.delegate as? AppDelegate, let backgroundHandle = appDelegate.backgroundSessionCompletionHandler else { return }
            backgroundHandle()
        }
    }
    
}

• 代码分散
• 不易于管理
• 不实现代理会使界面刷新卡顿 Warning: Application delegate received call to - application:handleEventsForBackgroundURLSession:completionHandler: but the completion handler was never called.

Alamofire下载

• 创建单例管理config
• 所有delegate内部封装
• 所有操作内部管理做好主线程callBack不需要外界操作
• 链式调用
• 不用在乎调用顺序

struct XXBackgroundManger {
    
    static let shared = XXBackgroundManger()
    
    let manager: SessionManager = {
        let configuration = URLSessionConfiguration.background(withIdentifier: "xxxx.demo")
        configuration.httpAdditionalHeaders = SessionManager.defaultHTTPHeaders
        
        return SessionManager(configuration: configuration)
    }()
}

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

XXBackgroundManger.shared.manager
    .download(self.urlDownloadStr) { (url, response) -> (destinationURL: URL, options: DownloadRequest.DownloadOptions) in
        //下载完成
    }
    .response { (downloadResponse) in
        //下载回调信息
    }
    .downloadProgress { (progress) in
        //下载进度
}

SessionManger流程分析

SessionManager.swift

 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)
    }

SessionManger初始化

• configuration
• delegate
• serverTrustPolicyManager

configuration

设置了一些基本的 SessionManager.defaultHTTPHeaders 请求头信息，上文提到的session的三种模式等

delegate

SessionDelegate 是集合所有的代理，所有事件统一处理

代理集合.png

SessionManager.swift
在我们初始化manager的时候，会在commonInit()方法中

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

因为前边保存了delegate,所以会回到系统的delegate

extension ViewController:URLSessionDownloadDelegate{
    func urlSession(_ session: URLSession, downloadTask: URLSessionDownloadTask, didFinishDownloadingTo location: URL) {}
    
    func urlSession(_ session: URLSession, downloadTask: URLSessionDownloadTask, didWriteData bytesWritten: Int64, totalBytesWritten: Int64, totalBytesExpectedToWrite: Int64) {}
    
    func urlSessionDidFinishEvents(forBackgroundURLSession session: URLSession) {
    //实现调用
    }    
}

注意urlSessionDidFinishEvents，因为SessionDelegate重写的方法，

#if !os(macOS)
    /// Tells the delegate that all messages enqueued for a session have been delivered.
    ///
    /// - parameter session: The session that no longer has any outstanding requests.
    open func urlSessionDidFinishEvents(forBackgroundURLSession session: URLSession) {
        sessionDidFinishEventsForBackgroundURLSession?(session)
    }
#endif

会回到
SessionDelegate.swift

delegate.sessionDidFinishEventsForBackgroundURLSession = { [weak self] session in
    guard let strongSelf = self else { return }
    DispatchQueue.main.async { strongSelf.backgroundCompletionHandler?() }
}

完成主线程的毁掉

serverTrustPolicyManager

/// Responsible for managing the mapping of ServerTrustPolicy objects to a given host
负责管理“ServerTrustPolicy”对象到给定主机的映射
待补充