while (true)
{
processInput()
updateState()
paint()
}
That will kill battery unless you write code to not update until input or app state changes, in which case you're building up to your own retained-mode system.As for building up to your own retained mode system: not really, at least not in my view. Whether you update at a constant rate, or update only in response to user input (or whatever), you're still doing the key things that differentiate immediate mode-type GUIs from retained mode-type GUIs: firstly, you're regenerating the entire UI from scratch each time; and secondly, you're handling the input events while doing the regeneration.
This is what makes immediate mode-type GUIs so easy to use in many cases. You never have to make sure each widget is linked up with whatever it relates to, and you never have to ensure the widget list is kept in sync with the list of things, and so on. You're always building the widget list from the (authoritative) list of things, and processing the related input events at the same time.
That's the hard part that callbacks try to make easier.