Alamofire (2)--后台下载

2019-10-09  本文已影响0人  iOS打工犭袁

这一篇主要讲解后台下载，后台下载对于应用程序来说，是一个非常重要也比较好用的功能。虽然用好后台下载的确能够大大提升用户体验，但是又很多时候我们也会遇到很多坑点以及疑惑点。其中会通过 URLSession 和 Alamofire 两种形式分别展开讨论，对比学习才能更能体会 Alamofire 的设计思维。Alamofire持续更新中，希望大家希望！

一、URLSession处理后台下载

URLSession在后台处理方面还是比较简单的。

// 1：初始化一个background的模式的configuration
let configuration = URLSessionConfiguration.background(withIdentifier: self.createID())
// 2：通过configuration初始化网络下载会话
let session = URLSession.init(configuration: configuration, delegate: self, delegateQueue: OperationQueue.main)
// 3：session创建downloadTask任务-resume启动
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 - 开始沙盒迁移
        print("下载完成 - \(location)")
        let locationPath = location.path
        //拷贝到用户目录（文件名以时间戳命名）
        let documnets = NSHomeDirectory() + "/Documents/" + self.lgCurrentDataTurnString() + ".mp4"
        print("移动地址:\(documnets)")
        //创建文件管理器
        let fileManager = FileManager.default
        try! fileManager.moveItem(atPath: locationPath, toPath: documnets)
    }

    func urlSession(_ session: URLSession, downloadTask: URLSessionDownloadTask, didWriteData bytesWritten: Int64, totalBytesWritten: Int64, totalBytesExpectedToWrite: Int64) {
        print(" bytesWritten \(bytesWritten)\n totalBytesWritten \(totalBytesWritten)\n totalBytesExpectedToWrite \(totalBytesExpectedToWrite)")
        print("下载进度: \(Double(totalBytesWritten)/Double(totalBytesExpectedToWrite))\n")
    }
}

这里实现了下载功能，但是对于我们需要的后台下载还差一段

Applications using an NSURLSession with a background configuration may be launched or resumed in the background in order to handle the completion of tasks in that session, or to handle authentication. This method will be called with the identifier of the session needing attention. Once a session has been created from a configuration object with that identifier, the session's delegate will begin receiving callbacks. If such a session has already been created (if the app is being resumed, for instance), then the delegate will start receiving callbacks without any action by the application. You should call the completionHandler as soon as you're finished handling the callbacks.

苹果爸爸总是能在合适时间给你优秀的建议，阅读文档的能力决定你是否能够在这个时代站稳自己的脚尖

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
    }
}

在urlSessionDidFinishEvents的代理实现调用

func urlSessionDidFinishEvents(forBackgroundURLSession session: URLSession) {
    print("后台任务下载回来")
    DispatchQueue.main.async {
        guard let appDelegate = UIApplication.shared.delegate as? AppDelegate, let backgroundHandle = appDelegate.backgroundSessionCompletionHandler else { return }
        backgroundHandle()
    }
}

那么如果不实现这个代理里面的回调函数的执行，那么会发生什么呢

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

二、Alamofire后台下载

Alamofire框架还是比较有感觉的，这个节奏也是函数式回调，还支持链式请求和响应！事务逻辑非常清晰，还有代码可读性也是非常简洁

LGBackgroundManger.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("下载回调信息: \(downloadResponse)")
    }
    .downloadProgress { (progress) in
        print("下载进度 : \(progress)")
}


struct LGBackgroundManger {    
    static let shared = LGBackgroundManger()

    let manager: SessionManager = {
        let configuration = URLSessionConfiguration.background(withIdentifier: "com.lgcooci.AlamofireTest.demo")
        configuration.httpAdditionalHeaders = SessionManager.defaultHTTPHeaders
        configuration.timeoutIntervalForRequest = 10
        configuration.timeoutIntervalForResource = 10
        configuration.sharedContainerIdentifier = "group.com.lgcooci.AlamofireTest"
        return SessionManager(configuration: configuration)
    }()
}

可能很多同学都在质疑为什么要做成单利，URLSession的时候不是挺好的？

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

三、SessionManger流程分析

一篇优秀的博客，毕竟还要跟大家交代这样清晰的代码的背后流程

1、SessionManger初始化
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)
}
2、代理完成回调

SessionDelegate 是一个非常重要的类，集合所有的代理

这里我们根据需求来到 urlSessionDidFinishEvents 的代理

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

在我们的 SessionManger 里面的初始化的时候，有一个方法commonInit

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

3、流程总结

无论你是使用 URLSession 的方式，还是 Alamofire 进行后台下载，但是原理还是一样的，只是 Alamofire 使用更加达到依赖下沉，网络层下沉，使用更简洁，这也是很多时候我们需要第三方框架的原因。这一篇你估计已经感受到了 Alamofire 的舒服，那么如果你喜欢的话，麻烦点心，关注一下。我会持续更新一个 Alamofire 的系列专题，谢谢！

原文作者：集才华帅气于一身的—Cooci哥

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