Architecture: MVVM, observation and dependency injection

Separate view from logic with view models, use the Observation framework instead of ObservableObject, inject dependencies through the environment, and design for tests.

A view model with @Observable

protocol CounterStoring {
    func load() -> Int
    func save(_ value: Int)
}

@Observable
final class CounterViewModel {
    var count = 0
    var history: [Int] = []
    var lastError: String?

    private let store: CounterStoring
    init(store: CounterStoring) { self.store = store }

    func increment() {
        count += 1
        history.append(count)
        store.save(count)
    }

    func restore() {
        count = store.load()
    }
}

struct CounterView: View {
    @State private var model: CounterViewModel

    init(store: CounterStoring) {
        _model = State(initialValue: CounterViewModel(store: store))
    }

    var body: some View {
        VStack(spacing: 16) {
            Text("Count: \(model.count)").font(.title)
            Button("Increment") { model.increment() }
        }
        .task { model.restore() }
    }
}

• @Observable replaces ObservableObject and @Published: SwiftUI reads the properties a view touches and only re-renders that view.
• Create the model with @State in the view that owns it — @State keeps the instance alive across re-renders.
• Inject the model into children with .environment(model) and read it with @Environment(CounterViewModel.self).
• Keep view models free of UIKit types so they can run in a unit test with no simulator.

Where the layers stop

The rule is not "always three layers". It is that a type should have one reason to change. A settings screen that only writes a boolean needs no view model at all.

Injecting for tests

final class InMemoryStore: CounterStoring {
    private var value = 0
    func load() -> Int { value }
    func save(_ newValue: Int) { value = newValue }
}

@Test
func incrementStoresTheNewValue() {
    let store = InMemoryStore()
    let model = CounterViewModel(store: store)

    model.increment()
    model.increment()

    #expect(model.count == 2)
    #expect(store.load() == 2)
}

FAQ

Related

Networking with URLSession and async/await Testing, debugging and Instruments

Last refreshed 2026-09-18.