Alamofire-后台下载问题

问题一 -999

被释放了，会报-999，所以可以确定URLSession.init是一个单例

问题二 掉帧

测试之前先把模拟器上的App删掉，然后Xcode关掉，重新打开Xcode编译一下代码，再运行
Alamofire-系统URLSession中后台下载
1、开通后台下载权限，就会在下载完成的时候自己去执行func urlSessionDidFinishEvents(forBackgroundURLSession session: URLSession) {}方法
2、AppDelegate中执行self.backgroundSessionCompletionHandler = completionHandler

    var backgroundSessionCompletionHandler: (() -> Void)?

    func application(_ application: UIApplication, handleEventsForBackgroundURLSession identifier: String, completionHandler: @escaping () -> Void) {
        self.backgroundSessionCompletionHandler = completionHandler
    }

3、urlSessionDidFinishEvents中执行DispatchQueue.main.async { guard let appDelegate = UIApplication.shared.delegate as? AppDelegate, let backgroundHandle = appDelegate.backgroundSessionCompletionHandler else { return } backgroundHandle() }

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

问题三 identifier

    func application(_ application: UIApplication, handleEventsForBackgroundURLSession identifier: String, completionHandler: @escaping () -> Void) {        
        let configuration = URLSessionConfiguration.background(withIdentifier: identifier)
        let session = URLSession(configuration: configuration)
    }