Model-View-Presenter, Form Validation, and a Unidirectional State Machine

As an exercise, I wrote a simple iOS scene transition from a login form (root view controller) to a success scene. I was experimenting with the Model-View-Presenter approach where view and presenter know each other, but communicate through protocols, so you program to an interface, not the implementation. I ended up using a simple state machine to represent the view's state, and employed form validation that is represented in the types. I like how the result reads, so here's a rundown of the exercise:

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