I'm following an exercise from a book and we're building out the view controllers programmatically and creating unit tests. The book also says this, and from previous experience with reading Apple reference docs, Apple references always say to not invoke viewDidLoad, viewWillAppear, viewDidDisappear, etc directly. I'm curious to know why this is and what happens if we do?
viewDidLoad, viewWillAppear, viewDidDisappear and so on are the runtime's way of letting you know that certain important things are happening:
viewDidLoad, the view controller has just obtained its view
viewWillAppear, the view controller's view is about to be placed into the interface
viewDidDisappear, the view controller's view is about to be removed from the interface
These are events that the runtime is responsible for, and it sends you messages to let you know they are happening so that you can respond, if you wish, at the appropriate moment.
It would make no sense for you to call them, because you do not know when these things are happening (except insofar as the runtime calls them)! If you called them at some arbitrary moment, you would be lying, e.g. saying that the view has just loaded when in fact it has not just loaded, and so you would break your own code or worse.