Observation Framework (Swift)

The Observation framework, built on Swift macros, replaces Combine-based data flow. Mark a class @Observable and any property read becomes trackable, in SwiftUI automatically and elsewhere through withObservationTracking.

Observing Outside SwiftUI

withObservationTracking takes two closures: the first reads the properties you care about, the second fires when any of those properties changes.

withObservationTracking {
    render(store.state)
} onChange: {
    print("State changed")
}

Two gotchas live here. The onChange closure fires only once, so re-register recursively for continuous observation. And it runs before the change applies, so defer the re-read into a new task.

func startObservation() {
    withObservationTracking {
        render(store.state)
    } onChange: {
        Task { startObservation() }
    }
}

Observing in SwiftUI

SwiftUI tracks any observable property the body reads. A plain let on the view is enough; no property wrapper.

struct ProductsView: View {
    let store: Store<AppState, AppAction>

    var body: some View {
        List(store.state.products, id: \.self) { Text($0) }
            .onAppear { store.send(.fetch) }
    }
}

The wrapper set collapses: @State replaces @StateObject for owning the object across the view lifecycle, and @Environment(Store.self) with .environment(store) replaces the EnvironmentObject pair.

Bindings with @Bindable

@Bindable derives bindings from an observable type's properties, for text fields and other two-way controls.

struct AuthView: View {
    @Bindable var viewModel: AuthViewModel

    var body: some View {
        TextField("username", text: $viewModel.username)
        SecureField("password", text: $viewModel.password)
    }
}

When the object arrives through the environment, declare @Bindable inline at the top of the body instead.

struct InlineAuthView: View {
    @Environment(AuthViewModel.self) var viewModel

    var body: some View {
        @Bindable var viewModel = viewModel
        TextField("username", text: $viewModel.username)
    }
}