问题
I see many applications that display onboarding view controllers before allowing access to the main content. I have noticed that a lot of the onboarding tutorials initially display the onboarding content and then dismiss that view controller to allow access to the content.
I know that Xcode gives warnings when you try to present a view controller on a view that is not visible, such as in viewDidLoad and viewWillAppear:. So instantiating the content's view controller inside of a navigation controller, and then trying to present the onboarding view controller seemingly only works without errors if it's presented in viewDidAppear:. However, in the applications that I am referencing, the onboarding is the initially visible view before it gets dismissed to allow access to the application's content.
So what type of hierarchy allows this type of transition between view controllers when navigation controllers are presenting the onboarding without seeing the application's content prior to viewDidAppear: being called?
来源：https://stackoverflow.com/questions/44231639/what-type-of-hierarchy-is-responsible-for-this-style-of-onboarding