Redux-like state container in SwiftUI. Derived stores.

import SwiftUI

struct RootView: View {
    @EnvironmentObject var store: Store<AppState, AppAction>

    var body: some View {
        TabView {
            NavigationView {
                SummaryContainerView()
                    .navigationBarTitle("today")
                    .environmentObject(
                        store.derived(
                            deriveState: \.summary,
                            deriveAction: AppAction.summary
                        )
                )
            }.tabItem {
                Image(systemName: "heart.fill")
                    .imageScale(.large)
                Text("today")
            }

            NavigationView {
                TrendsContainerView()
                    .navigationBarTitle("trends")
                    .environmentObject(
                        store.derived(
                            deriveState: \.trends,
                            deriveAction: AppAction.trends
                        )
                )
            }.tabItem {
                Image(systemName: "chevron.up.circle.fill")
                    .imageScale(.large)
                Text("trends")
            }
        }
    }
}

func derived<DerivedState: Equatable, ExtractedAction>(
    deriveState: @escaping (State) -> DerivedState,
    embedAction: @escaping (ExtractedAction) -> Action
) -> Store<DerivedState, ExtractedAction> {
    let store = Store<DerivedState, ExtractedAction>(
        initialState: deriveState(state),
        reducer: Reducer { _, action, _ in
            self.send(embedAction(action))
            return Empty().eraseToAnyPublisher()
        },
        environment: ()
    )
    $state
        .map(deriveState)
        .removeDuplicates()
        .receive(on: DispatchQueue.main)
        .assign(to: &store.$state)
    return store
}