SessionManager
SessionManager 作为管理类，只做配置相关的设置，定义上传、下载等功能的入口函数，具体功能的实现，数据的处理则下沉到其他相关类处理，使用非常简洁，方便，容易理解
初始化
public static let `default`: SessionManager = {
let configuration = URLSessionConfiguration.default
configuration.httpAdditionalHeaders = SessionManager.defaultHTTPHeaders
return SessionManager(configuration: configuration)
}()
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)
}
- 自定义
URLSessionConfiguration的httpAdditionalHeaders信息，有助于后台阻止非法请求 -
URLSession的代理移交给SessionDelegate，达到任务分离的目的
后台下载
后台下载需要我们定义一个用于后台下载的sessionManager，这里我们包装成一个单例
struct BackgroundManager {
static let shared = BackgroundManager()
let manager: SessionManager = {
let config = URLSessionConfiguration.background(withIdentifier: "com.alamofire.background")
config.httpAdditionalHeaders = SessionManager.defaultHTTPHeaders
config.sharedContainerIdentifier = "group.com.alamofire"
return SessionManager(configuration: config)
}()
}
下载代码：
BackgroundManager.shared.manager.download(url) { (url, res) -> (destinationURL: URL, options: DownloadRequest.DownloadOptions) in
let documentUrl = FileManager.default.urls(for: .documentationDirectory, in: .userDomainMask).first
let fileUrl = documentUrl?.appendingPathComponent(res.suggestedFilename!)
return (fileUrl!, [.removePreviousFile, .createIntermediateDirectories])
}
.downloadProgress { (progress) in
print("------\(progress)")
}
.response { (response) in
print("完成")
}
从下面的SessionManager.init方法可以看到，sessionDelegate实现了闭包sessionDidFinishEventsForBackgroundURLSession
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?() }
}
}
//sessionDelegate 后台下载完成代理方法
open func urlSessionDidFinishEvents(forBackgroundURLSession session: URLSession) {
sessionDidFinishEventsForBackgroundURLSession?(session)
}
delegate.sessionDidFinishEventsForBackgroundURLSession会在session后台下载完成后执行，故而我们只要strongSelf.backgroundCompletionHandler保存AppDelegate授权后台执行的闭包completionHandler就可以了,即
func application(_ application: UIApplication, handleEventsForBackgroundURLSession identifier: String, completionHandler: @escaping () -> Void) {
BackgroundManager.shared.manager.backgroundCompletionHandler = completionHandler
}
如此，当后台下载完成后，最终会回调执行系统的闭包completionHandler
如果最后不执行系统的completionHandler，会有以下影响：
- 造成界面刷新卡顿
- 打印台会输出警告
Warning: Application delegate received call to -
application:handleEventsForBackgroundURLSession:completionHandler:
but the completion handler was never called.
执行结果：
当下载过程中，进入后台，则会停止进度打印
进入后台
当过一会回到前台时，直接输出完成，说明在后台是在下载的
回到前台
