What is @Observable (the Observation framework, iOS 17+)?
The gist
@Observable is a macro from the Observation framework (iOS 17, Swift 5.9) that makes a class observable without Combine. The macro rewrites every stored property into a computed one wrapped with access/withMutation on an ObservationRegistrar. While evaluating body, SwiftUI records exactly which properties were read and invalidates the view only when those change — fine-grained tracking, unlike ObservableObject, where any @Published change redrew every subscriber.
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