restorationIdentifier

This property indicates whether the view controller and its contents should be preserved and is used to identify the view controller during the restoration process. The value of this property is nil by default, which indicates that the view controller should not be saved. Assigning a string object to the property lets the system know that the view controller should be saved. In addition, the contents of the string are your way to identify the purpose of the view controller.

During subsequent launches, UIKit asks your app for help in recreating the view controllers that were installed the last time your app ran. When it asks for a specific view controller, UIKit provides your app with this restoration identifier and the restoration identifiers of any parent view controllers in the view controller hierarchy. Your app must use this information to create or locate the appropriate view controller object.