Alamofire(2)-SessionManager

前一篇文章大致说了下Swift版本的原生请求，以及使用Alamofire请求数据是如何的方便，这篇文章开始，慢慢的深入接入Alamofire的源码，仔细看看一个在GitHub上如此高星的框架源码多么的优秀！代码里使用的Alamofire版本为4.8.2的稳定release版本,本篇主要介绍的内容如下：

• SessionManager初始化
• SessionManager下载
• SessionManager后台下载
• SessionManager流程分析

直接进入request的源码开干

public func request(
    _ url: URLConvertible,
    method: HTTPMethod = .get,
    parameters: Parameters? = nil,
    encoding: ParameterEncoding = URLEncoding.default,
    headers: HTTPHeaders? = nil)
    -> DataRequest
{
    return SessionManager.default.request(
        url,
        method: method,
        parameters: parameters,
        encoding: encoding,
        headers: headers
    )
}

可以看到method参数是一个HTTPMethod的枚举,默认值必须是用的最多的get方法，自行进入一看源码，
清楚！String类型关联的枚举,里面是各种HTTP的请求方法。
SessionManager.default看着就像是一个单利的用法,验证一下就继续深入源码:

/// A default instance of `SessionManager`, used by top-level Alamofire request methods, and suitable for use
    /// directly for any ad hoc requests.
    public static let `default`: SessionManager = {
        let configuration = URLSessionConfiguration.default
        configuration.httpAdditionalHeaders = SessionManager.defaultHTTPHeaders

        return SessionManager(configuration: configuration)
    }()

果不其然，的确是个单利。这个跟AFNetworking里面的manager不一样，AFNetworking里的manager是一个工厂模式，并非一个单利。

SessionManager的初始化方法只传了一个configuration参数,深入看一下SessionManager的初始化方法：

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)
    }

可见初始化方法里的参数全有默认值,也就是说可以直接SessionManager()初始化出来一个实例的。那么在单利里面传的一个configuration时候把httpAdditionalHeaders赋了值,直接看源码：

/// Creates default values for the "Accept-Encoding", "Accept-Language" and "User-Agent" headers.
    public static let defaultHTTPHeaders: HTTPHeaders = {
        // Accept-Encoding HTTP Header; see https://tools.ietf.org/html/rfc7230#section-4.2.3
        let acceptEncoding: String = "gzip;q=1.0, compress;q=0.5"

        // Accept-Language HTTP Header; see https://tools.ietf.org/html/rfc7231#section-5.3.5
        let acceptLanguage = Locale.preferredLanguages.prefix(6).enumerated().map { index, languageCode in
            let quality = 1.0 - (Double(index) * 0.1)
            return "\(languageCode);q=\(quality)"
        }.joined(separator: ", ")

        // User-Agent Header; see https://tools.ietf.org/html/rfc7231#section-5.5.3
        // Example: `iOS Example/1.0 (org.alamofire.iOS-Example; build:1; iOS 10.0.0) Alamofire/4.0.0`
        let userAgent: String = {
            if let info = Bundle.main.infoDictionary {
                let executable = info[kCFBundleExecutableKey as String] as? String ?? "Unknown"
                let bundle = info[kCFBundleIdentifierKey as String] as? String ?? "Unknown"
                let appVersion = info["CFBundleShortVersionString"] as? String ?? "Unknown"
                let appBuild = info[kCFBundleVersionKey as String] as? String ?? "Unknown"

                let osNameVersion: String = {
                    let version = ProcessInfo.processInfo.operatingSystemVersion
                    let versionString = "\(version.majorVersion).\(version.minorVersion).\(version.patchVersion)"

                    let osName: String = {
                        #if os(iOS)
                            return "iOS"
                        #elseif os(watchOS)
                            return "watchOS"
                        #elseif os(tvOS)
                            return "tvOS"
                        #elseif os(macOS)
                            return "OS X"
                        #elseif os(Linux)
                            return "Linux"
                        #else
                            return "Unknown"
                        #endif
                    }()

                    return "\(osName) \(versionString)"
                }()

                let alamofireVersion: String = {
                    guard
                        let afInfo = Bundle(for: SessionManager.self).infoDictionary,
                        let build = afInfo["CFBundleShortVersionString"]
                    else { return "Unknown" }

                    return "Alamofire/\(build)"
                }()

                return "\(executable)/\(appVersion) (\(bundle); build:\(appBuild); \(osNameVersion)) \(alamofireVersion)"
            }

            return "Alamofire"
        }()

        return [
            "Accept-Encoding": acceptEncoding,
            "Accept-Language": acceptLanguage,
            "User-Agent": userAgent
        ]
    }()

设置了一些请求头信息,看下来觉得userAgent是个非常有意义的值，里面包含了项目、设备、Alamofire版本等信息，可以方便后端查找有异常的数据，能更好的定位异常。直接用Charles抓个包，将原生请求和Alamofire的请求对比一下：

原生.png Alamofire.png
在SessionManager的初始化方法里看到有一个delegate参数,这个东西就是用来实现代理移交的，SessionManager作为一个管理者，不可能直接管太多的细节，所以会把一些工作移交给对应的人来出来。同时看到有一个commonInit方法，直接干进去看源码：

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?() }
        }
    }

delegate.sessionManager = self这是什么骚操作,是不是感觉产生了循环引用，会导致内存泄漏，哈哈，这点大可放心，这么牛逼的一个开源库，这种低级错误不存在的，这个赋值主要是想标记一下，有了问题反馈给谁，可以这么理解：

管理者.png
SessionManager就是一个管理者，delegate有多种，如果说不同的delegate之间直接交流，会很乱，所以需要有一个管理者或者说中间者来统一调度管理。
commonInit方法里最后的一个方法看名字就很熟悉，是不是很像上一篇文章的后台下载，没错这个就是用来处理后台下载的。
下面来一个实实在在的用Alamofire下载文件的示例：

SessionManager.default.download(videoUrlStr) { (url, response) -> (destinationURL: URL, options: DownloadRequest.DownloadOptions) in
        let fileUrl = FileManager.default.urls(for: .documentDirectory, in: .userDomainMask).first?.appendingPathComponent(response.suggestedFilename ?? "video0818.mp4")
        return (fileUrl!,[.createIntermediateDirectories,.removePreviousFile])
     }
     .downloadProgress { (progress) in
          print("进度：\(progress)")
     }
     .response { (downloadResponse) in
          print("回调信息：\(downloadResponse)")
     }

漂亮的链式语法,看着就是舒服！return (fileUrl!,[.createIntermediateDirectories,.removePreviousFile])返回的是个目标地址和下载的一些可选项的元组，比较简单，我这里传的是移除目标路径里已经有的文件和创建需要的目录，也就是说下载完后将下载完的文件从tmp移动到返回的目标路径的时候，如果目标路径里是存在文件的会直接被移除掉。那么问题来了，文件比较大，下载到一半的时候用户进入后台了，这样写会继续下载吗 ？如果你还记得SessionManager的defalut的单利对象里面的let configuration = URLSessionConfiguration.default，那你就肯定知道不支持后台下载的。。。那就自己摸索一下先，一顿操作猛如虎之后代码如下：

let configuration=URLSessionConfiguration.background(withIdentifier: "com.BoxJing.backgroundDownload")
        let manager=SessionManager(configuration: configuration)
        manager.download(videoUrlStr) { (url, response) -> (destinationURL: URL, options: DownloadRequest.DownloadOptions) in
            let fileUrl = FileManager.default.urls(for: .documentDirectory, in: .userDomainMask).first?.appendingPathComponent(response.suggestedFilename ?? "video0818.mp4")
            return (fileUrl!,[.createIntermediateDirectories,.removePreviousFile])
            }
            .downloadProgress { (progress) in
                print("进度：\(progress)")
            }
            .response { (downloadResponse) in
                print("回调信息：\(downloadResponse)")
        }

会发现下面的结果(-999):

019-08-18 16:15:12.750608+0800 AlamofireDemo[3064:98404] Task <BC8DA9F9-9779-4181-99FA-3BF8C2EEBE32>.<1> load failed with error Error Domain=NSURLErrorDomain Code=-999 "cancelled" UserInfo={NSErrorFailingURLStringKey=http://cache.fotoplace.cc/181114/13404389/3832bf181a42f3b372156659dac6b731.mp4, NSErrorFailingURLKey=http://cache.fotoplace.cc/181114/13404389/3832bf181a42f3b372156659dac6b731.mp4, _NSURLErrorRelatedURLSessionTaskErrorKey=(
    "BackgroundDownloadTask <BC8DA9F9-9779-4181-99FA-3BF8C2EEBE32>.<1>",
    "LocalDownloadTask <BC8DA9F9-9779-4181-99FA-3BF8C2EEBE32>.<1>"
), _NSURLErrorFailingURLSessionTaskErrorKey=BackgroundDownloadTask <BC8DA9F9-9779-4181-99FA-3BF8C2EEBE32>.<1>, NSLocalizedDescription=cancelled} [-999]
回调信息：DefaultDownloadResponse(request: Optional(http://cache.fotoplace.cc/181114/13404389/3832bf181a42f3b372156659dac6b731.mp4), response: nil, temporaryURL: nil, destinationURL: nil, resumeData: nil, error: Optional(Error Domain=NSURLErrorDomain Code=-999 "cancelled" UserInfo={NSErrorFailingURLStringKey=http://cache.fotoplace.cc/181114/13404389/3832bf181a42f3b372156659dac6b731.mp4, NSErrorFailingURLKey=http://cache.fotoplace.cc/181114/13404389/3832bf181a42f3b372156659dac6b731.mp4, _NSURLErrorRelatedURLSessionTaskErrorKey=(
    "BackgroundDownloadTask <BC8DA9F9-9779-4181-99FA-3BF8C2EEBE32>.<1>",
    "LocalDownloadTask <BC8DA9F9-9779-4181-99FA-3BF8C2EEBE32>.<1>"
), _NSURLErrorFailingURLSessionTaskErrorKey=BackgroundDownloadTask <BC8DA9F9-9779-4181-99FA-3BF8C2EEBE32>.<1>, NSLocalizedDescription=cancelled}), timeline: Timeline: { "Request Start Time": 587808912.744, "Initial Response Time": 587808912.754, "Request Completed Time": 587808912.754, "Serialization Completed Time": 587808912.756, "Latency": 0.010 secs, "Request Duration": 0.010 secs, "Serialization Duration": 0.002 secs, "Total Duration": 0.012 secs }, _metrics: nil)

这就尴尬了！明明就是设置一个background模式，就下载不了了？？？如何解决？(想自己去探究的可以直接去Alamofire的issue里翻一翻会有结果的，每个开源项目的issue是最好的解决自己问题的地方)。
不多扯，自行去issue里继续找去吧，原来需要将manager持有一下保存住。来个属性保存一SessionManager对象，var manager = SessionManager()代码完善一下：

let configuration=URLSessionConfiguration.background(withIdentifier: "com.BoxJing.backgroundDownload")
manager = SessionManager(configuration: configuration)
manager.download(videoUrlStr) { (url, response) -> (destinationURL: URL, options: DownloadRequest.DownloadOptions) in
       let fileUrl = FileManager.default.urls(for: .documentDirectory, in: .userDomainMask).first?.appendingPathComponent(response.suggestedFilename ?? "video0818.mp4")
        return (fileUrl!,[.createIntermediateDirectories,.removePreviousFile])
     }
     .downloadProgress { (progress) in
         print("进度：\(progress)")
     }
     .response { (downloadResponse) in
         print("回调信息：\(downloadResponse)")
}

的确是可以后台下载了,这样还没完！为什么呢？记不记得前一篇文章里提到一个苹果文档里说的，如果有后台下载的任务，任务完成后一定要通知系统！要想通知系统，那肯定要有session的完成后的代理方法urlSessionDidFinishEvents,直接在工程里搜一下，相信Alamofire里面肯定会有对应的实现。

urlSessionDidFinishEvents.png

#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

在SessionDelegate类中追踪到了sessionDidFinishEventsForBackgroundURLSession,熟悉不熟悉，就是出现在前面commonInit方法里面的！再来看一下:

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

很明显会执行manager的一个backgroundCompletionHandler闭包，那就直接干出来一个试试会执行不会

manager.backgroundCompletionHandler = {
            print("下载完成了！！！")
        }

在后台下载完成后的确执行了我们写的回调闭包！那我们理论上就是应该在backgroundCompletionHandler闭包里告诉系统，任务完成了。一顿操作猛如虎：

AppDelegate中：
var backgroundSessionCompletionHandler:(() -> Void)?
//
    func application(_ application: UIApplication, handleEventsForBackgroundURLSession identifier: String, completionHandler: @escaping () -> Void) {
        print("Delagete 任务完成")
        self.backgroundSessionCompletionHandler=completionHandler
    }

backgroundCompletionHandler闭包中：
manager.backgroundCompletionHandler = {
            print("下载完成了！！！")
            DispatchQueue.main.async {
                guard let appDele = UIApplication.shared.delegate as? AppDelegate, let backgroundHandle=appDele.backgroundSessionCompletionHandler else {return}
                backgroundHandle()
            }
        }

后台下载完成通知系统.png
完美！但是，看着真的是很不爽！有木有啊，manager.backgroundCompletionHandler方法里面的代码跟屎一样的烦人！如何铲掉这坨屎嘞？过程就不再赘述了，可以自行去issue里找一找，这里直接给出思想：单利。直接干代码：

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

//下载
BoxBackgroundManger.shared.manager.download(videoUrlStr) { (url, response) -> (destinationURL: URL, options: DownloadRequest.DownloadOptions) in
            let fileUrl = FileManager.default.urls(for: .documentDirectory, in: .userDomainMask).first?.appendingPathComponent(response.suggestedFilename ?? "video0818.mp4")
            return (fileUrl!,[.createIntermediateDirectories,.removePreviousFile])
            }
            .downloadProgress { (progress) in
                print("进度：\(progress)")
            }
            .response { (downloadResponse) in
                print("回调信息：\(downloadResponse)")
        }

AppDelegate中：
func application(_ application: UIApplication, handleEventsForBackgroundURLSession identifier: String, completionHandler: @escaping () -> Void) {
        print("Delagete 任务完成")
//        self.backgroundSessionCompletionHandler=completionHandler
        BoxBackgroundManger.shared.manager.backgroundCompletionHandler = completionHandler
    }

直接看结果：

单利管理后台下载.png

非常的完美！！！有木有！到此为止，Alamofire的后台下载完美结束！