Swift. Thread running even while being in other apps

I need to keep a thread running even when the user is inside another application. I tried the following thread that works only if I keep the application visible, once I press the home button, the thread is paused.

func prepareSync() {
    DispatchQueue.global(qos .background).async {
        self.sync()
        sleep(5)
        self.prepareSync()
    }
}