您可以在用户打开应用程序时使用后台线程。但如果应用程序将被终止，则无法运行函数。请在此处查找应用程序生命周期并重新设计您的体系结构：https://developer.apple.com/library/ios/documentation/iPhone/Conceptual/iPhoneOSProgrammingGuide/TheAppLifeCycle/TheAppLifeCycle.html

如果用户终止应用程序，它将不再运行，因此您的代码将不再运行。您的代码/应用程序不可能处于这种状态。
我说的“kill”，并不是指“background”，一个应用程序的background是不同的，查看苹果关于不同应用程序状态的文档（见m.阿尔宾的答案）以及各种strategies for handling those app states。

func applicationWillTerminate(_ application: UIApplication) {
    // Called when the user discards a scene session.
    // If any sessions were discarded while the application was not running, this will be called shortly after application:didFinishLaunchingWithOptions.
    // Use this method to release any resources that were specific to the discarded scenes, as they will not return.
    print("Application Will Terminate")
}