ComposableViewModel.swift

@MainActor protocol ComposableViewModel: DynamicProperty {
    associatedtype Body: View
    @ViewBuilder @MainActor func build(content: NonVisibleView) -> Self.Body
}

struct NonVisibleView: View {
    var body: some View {
        EmptyView()
    }
}

extension View {
    func compose(_ viewModel: some ComposableViewModel) -> some View {
        return self.background {
            viewModel.build(content: NonVisibleView())
        }
    }
}

struct FooViewModel: DynamicProperty, ComposableViewModel {
    @Environment(\.colorScheme) var colorScheme

    @Observable final class UIState {
        var value: Int
        init(value: Int) {
            self.value = value
        }
    }
    @State var state: UIState

    init(value: Int) {
        self.state = State(value: value)
    }

    func foo() {
        state.value += 1
        print("colorScheme", colorScheme)
    }

    func build(content: NonVisibleView) -> some View {
        content
            .onAppear {
                print("onAppear")
            }
    }
}

struct ContentView: View {
    var viewModel: FooViewModel
    init(value: Int) {
        self.viewModel = FooViewModel(value: value)
    }

    var body: some View {
        VStack {
            Text("value: \(viewModel.state.value)")
            Button("Tap!!") {
                viewModel.foo()
            }
        }
        .compose(viewModel)
    }
}