MVVM
The point
MVVM puts a mediator between View and Model — the ViewModel. It is an object with no UIKit/SwiftUI dependency that holds the screen state in a display-ready form (strings, isLoading flags, filtered lists) and exposes commands (load(), delete(id:)). The View observes the state and re-renders itself; the ViewModel knows nothing about the View — so several Views can show one ViewModel, and it can be tested without UI.
Data flow in iOS MVVM is mostly unidirectional: events go from the View to the ViewModel, state goes back. Two-way binding is used sparingly ($viewModel.query for a text field).
Observation mechanisms by era: KVO/RxSwift/ReactiveCocoa → Combine @Published + ObservableObject (iOS 13) → Observation @Observable (iOS 17) — SwiftUI tracks only the properties actually read in body and does not re-render more than needed.
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