App background to foreground

with this code my app works good,but when i get back in foreground all function are disable....who can I get work in foreground?

func applicationDidEnterBackground(_ application: UIApplication) {
  var finished = false
  var bgTask: UIBackgroundTaskIdentifier = 0;
  bgTask = application.beginBackgroundTask(withName:"MyBackgroundTask", expirationHandler: {() -> Void in
  // Time is up.
  if bgTask != UIBackgroundTaskInvalid {
  // Do something to stop our background task or the app will be killed
  finished = true
  }
  })
  // Perform your background task here
  print("The task has started")
  while !finished {
  print("Not finished")
  // when done, set finished to true
  // If that doesn't happen in time, the expiration handler will do it for us
  }
  // Indicate that it is complete
  application.endBackgroundTask(bgTask)
  bgTask = UIBackgroundTaskInvalid
}