NSNotificationCenter - UIApplicationWillEnterForegroundNotification需要太长时间

Question

我的代码：

overide func viewDidLoad() {
   NSNotificationCenter.defaultCenter().addObserver(self, selector: "activeAgain", name: UIApplicationWillEnterForegroundNotification, object:nil)
}

func activeAgain() {
   println("ActiveAgain") //Comes immediately
   callSeparateFunction() //Takes 5s
}

为什么println立即出现，单独的函数调用最多需要5s？当应用程序回到前台时，我必须调用一个新函数。谢谢！