当应用程序在SWIFT中进入前台时，如何呈现Lock_ViewController？

Please help me. I have an app that has a passcode lock feature, this gets activated everytime the user opens the app. If the user goes to home ( app goes to background) and then go back to the app ( foreground) I want to display the lock screen again ( or use other viewcontroller ). How am I going to do this?Please help me. I have an app that has a passco