What is a formal definition of a first responder in iOS?

Question

I understand that a first responder object is the receives a callback signal according to input activity, etc and that it will bubble it up the chain until a responder willing t

Answer 1

It's a property of the window. The window has at all times no more than one first responder. Any UIResponder can claim first responder status by being sent becomeFirstResponder, and that is why you can cause a UITextField to get "focus" by sending that message to it.