Alamofire實現後臺下載

咔咔咔，敲完一個Alamofire的下載實現：

func downLoadFile() {
    SessionManager.default.download(urlString) { (url, response) -> (destinationURL: URL, options: DownloadRequest.DownloadOptions) in
        let docUrl = FileManager.default.urls(for: FileManager.SearchPathDirectory.documentDirectory, in: FileManager.SearchPathDomainMask.userDomainMask).first
        let fileUrl = docUrl?.appendingPathComponent(response.suggestedFilename!)
        return (fileUrl!, [.removePreviousFile, .createIntermediateDirectories])
        }.downloadProgress { (progress) in
            print("\(progress)")
        }.response { (respond) in
            print("\(respond)")
    }
}
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切到後臺時，下載不繼續執行，切回後，下載繼續執行，後臺下載的目的沒有達到啊。。。 一一般規操做，目的沒有達到啊，爲何？確定哪裏忽略了，default有木有很刺眼？看下唄

public static let `default`: SessionManager = {
    let configuration = URLSessionConfiguration.default
    configuration.httpAdditionalHeaders = SessionManager.defaultHTTPHeaders
    
    return SessionManager(configuration: configuration)
}()
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• 是SessionManager的一個單例
• URLSessionConfiguration是default模式
• 後臺下載須要的模式是background

再看下SessionManager的init方法

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)
}
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• 第一個參數configuration的默認值仍是URLSessionConfiguration的defualt模式

這個時候咱們就須要從新配置爲background模式了：

func downLoadBackground() {
    let configuration = URLSessionConfiguration.background(withIdentifier: "com.zimi")
    let backgroundManager = SessionManager(configuration: configuration)
    backgroundManager.download(urlString) { (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!, [.createIntermediateDirectories, .removePreviousFile])
    }
        .response { (response) in
            print("\(response)")
        }.downloadProgress { (progress) in
            print("\(progress.fractionCompleted)")
    }
}
// 控制檯打印：<1> load failed with error Error Domain=NSURLErrorDomain Code=-999 "cancelled"
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竟然報錯了。。。

原來是SessionManager在downLoadBackground方法中是局部變量，進入後臺下載時被釋放了 改爲這樣

let backgroundManager: SessionManager = {
    let configuration = URLSessionConfiguration.background(withIdentifier: "com.zimi")
    let sessionManager = SessionManager(configuration: configuration)
    
    return sessionManager
}()
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URLSession的官方文檔關於後臺下載有四步，在Swift - 網絡 URLSession中有介紹，固然不能忘了這個重要的步驟了：

func application(_ application: UIApplication, handleEventsForBackgroundURLSession identifier: String, completionHandler: @escaping () -> Void) {
    BackgroundDLViewModel().backgroundManager.backgroundCompletionHandler = completionHandler
}
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否則在控制檯會打印一個警告，切回的時候也會出下卡頓

Warning: Application delegate received call to -application:handleEventsForBackgroundURLSession:completionHandler: 
but the completion handler was never called.
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那麼重點來了，在用URLSession來處理後臺下載的時候，須要經過urlSessionDidFinishEvents(forBackgroundURLSession session: URLSession)代理方法來執行completionHandler的回調，那麼，既然SessionManager的屬性backgroundCompletionHandler幫咱們保存了completionHandler這個閉包，它是怎麼幫咱們來調用的呢？ 在前面貼出的init方法中有commonInit這個方法的調用，那麼咱們來看下：

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?() }
    }
}
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• sessionDidFinishEventsForBackgroundURLSession代理的閉包聲明裏，作了backgroundCompletionHandler閉包回到主線程異步的回調
• Alamofire中有一個專職delegate的類SessionDelegate，對URLSession的代理方法urlSessionDidFinishEvents(forBackgroundURLSession session: URLSession)進行了實現

open func urlSessionDidFinishEvents(forBackgroundURLSession session: URLSession) {
    sessionDidFinishEventsForBackgroundURLSession?(session)
}
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• SessionDelegate重寫了NSObject的responds方法，經過sessionDidFinishEventsForBackgroundURLSession閉包是否爲空來判斷是否執行urlSessionDidFinishEvents(forBackgroundURLSession session: URLSession)方法

open override func responds(to selector: Selector) -> Bool {
    #if !os(macOS)
        if selector == #selector(URLSessionDelegate.urlSessionDidFinishEvents(forBackgroundURLSession:)) {
            return sessionDidFinishEventsForBackgroundURLSession != nil
        }
    #endif
   //省略了一些代碼
}
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是否是很6啊？不用咱們再寫代理，也不用再寫代理方法的實現了，Alamofire幫咱們省了這一步了。

像Alamofire這些優秀的框架能幫咱們省不少的代碼，但咱們也不能忘了原生API的基礎哦