AppDelegate is a class which defines methods that are called by the singleton UIApplication object in response to important events in the lifetime of an app.
Normally used to perform tasks on application startup (setup app environment, analitycs (ex.: Mixpanel/GoogleAnalytics/Crashlitics), DB stack etc.) and shutdown (ex.: save DB context), handling URL open requests and similar application-wide tasks.
All States of Application through AppDelegate methods
For Getting updated or to do something before app goes live to user you can use below method.