上文中大家已經(jīng)了解了NSURLSession的使用和機(jī)制，本文著重講解Alamofire的后臺(tái)下載機(jī)制和開發(fā)中可能會(huì)遇到的坑

根據(jù)Alamofire的鏈?zhǔn)巾憫?yīng)，我們暫且敲出如下代碼：

SessionManager.default.download(self.urlDownloadStr) { (url, response) -> (destinationURL: URL, options: DownloadRequest.DownloadOptions) in
            let documentUrl = FileManager.default.urls(for: .documentDirectory, in: .userDomainMask).first
            let fileUrl = documentUrl?.appendingPathComponent(response.suggestedFilename!)
            return (fileUrl!,[.removePreviousFile,.createIntermediateDirectories])
            }.downloadProgress { (progress) in
                print("下載進(jìn)度:\(progress)")
            }.response { (downloadResponse) in
                print("下載回掉:\(downloadResponse)")
            }

當(dāng)我們進(jìn)入后臺(tái)

下載出現(xiàn)中斷，直到我們回到前臺(tái)，才能繼續(xù)下載

來到GitHub，在Alamofire的Issues中，來搜索我們想要的答案

so,我們要自定義manager...

let configuration = URLSessionConfiguration.background(withIdentifier: "com.episode.backgroundDownload")
let manager = SessionManager(configuration: configuration)

緊接著

manager.download(self.urlDownloadStr) { (url, response) -> (destinationURL: URL, options: DownloadRequest.DownloadOptions) in
            let documentUrl = FileManager.default.urls(for: .documentDirectory, in: .userDomainMask).first
            let fileUrl = documentUrl?.appendingPathComponent(response.suggestedFilename!)
            return (fileUrl!,[.removePreviousFile,.createIntermediateDirectories])
            }.downloadProgress { (progress) in
                print("下載進(jìn)度:\(progress)")
            }.response { (downloadResponse) in
                print("下載回掉:\(downloadResponse)")
            }

run起來，你想到了啥，-999哈哈。。。

需要保持manager在整個(gè)控制器的生命周期，so

var manager = SessionManager()

run,額，進(jìn)入后臺(tái)中斷的問題還是有問題，so,AppDelegate下載權(quán)限

struct EpisodeBackgroundManger {
    static let shared = EpisodeBackgroundManger()
    let manager: SessionManager = {
        let configuration = URLSessionConfiguration.background(withIdentifier: "com.episode.Alamofire.testing")
        configuration.httpAdditionalHeaders = SessionManager.defaultHTTPHeaders
        configuration.timeoutIntervalForRequest = 10
        configuration.timeoutIntervalForResource = 10
        return SessionManager(configuration: configuration)
    }()
}

EpisodeBackgroundManger.shared.manager
            .download(self.urlDownloadStr) { (url, response) -> (destinationURL: URL, options: DownloadRequest.DownloadOptions) in
            let documentUrl = FileManager.default.urls(for: .documentDirectory, in: .userDomainMask).first
            let fileUrl     = documentUrl?.appendingPathComponent(response.suggestedFilename!)
            return (fileUrl!,[.removePreviousFile,.createIntermediateDirectories])
            }
            .response { (downloadResponse) in
                print("下載回調(diào): \(downloadResponse)")
            }
            .downloadProgress { (progress) in
                print("下載進(jìn)度 : \(progress)")
            }

來到AppDelegate.swift

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

        print("hello - \(identifier)")
       EpisodeBackgroundManger.shared.manager.backgroundCompletionHandler = completionHandler
        self.backgroundSessionCompletionHandler = completionHandler
    }

控制器還需要處理回掉么，NO，牛逼的Alamofire已經(jīng)處理了，將依賴下沉

SessionDelegate 類中實(shí)現(xiàn)了NSURLSessionDelegate協(xié)議的urlSessionDidFinishEvents這個(gè)后臺(tái)下載任務(wù)完成后回調(diào)的代理方法

open func urlSessionDidFinishEvents(forBackgroundURLSession session: URLSession) {
        sessionDidFinishEventsForBackgroundURLSession?(session)
    }

并在SessionManager的初始化中進(jìn)行了實(shí)現(xiàn)

總結(jié)

Alamofire是對(duì)URLSession進(jìn)行封裝，所以這兩種方式進(jìn)行后臺(tái)下載，原理是一樣的。只是 Alamofire 使用更加簡(jiǎn)潔方便，依賴下沉，網(wǎng)絡(luò)層下沉。