swift-composable-architecture-mirror/Sources/ComposableArchitecture/Documentation.docc/Articles/GettingStarted.md

Getting started

Learn how to integrate the Composable Architecture into your project and write your first application.

Adding the Composable Architecture as a dependency

To use the Composable Architecture in a SwiftPM project, add it to the dependencies of your Package.swift and specify the ComposableArchitecture product in any targets that need access to the library:

let package = Package(
  dependencies: [
    .package(
      url: "https://github.com/pointfreeco/swift-composable-architecture",
      from: "1.0.0"
    ),
  ],
  targets: [
    .target(
      name: "<target-name>",
      dependencies: [
        .product(
          name: "ComposableArchitecture",
          package: "swift-composable-architecture"
        )
      ]
    )
  ]
)

Writing your first feature

To build a feature using the Composable Architecture you define some types and values that model your domain:

• State: A type that describes the data your feature needs to perform its logic and render its UI.
• Action: A type that represents all of the actions that can happen in your feature, such as user actions, notifications, event sources and more.
• Reducer: A function that describes how to evolve the current state of the app to the next state given an action. The reducer is also responsible for returning any effects that should be run, such as API requests, which can be done by returning an Effect value.
• Store: The runtime that actually drives your feature. You send all user actions to the store so that the store can run the reducer and effects, and you can observe state changes in the store so that you can update UI.

The benefits of doing this are that you will instantly unlock testability of your feature, and you will be able to break large, complex features into smaller domains that can be glued together.

As a basic example, consider a UI that shows a number along with "+" and "−" buttons that increment and decrement the number. To make things interesting, suppose there is also a button that when tapped makes an API request to fetch a random fact about that number and then displays the fact in an alert.

To implement this feature we create a new type that will house the domain and behavior of the feature, and you will annotate the type with the Reducer() macro:

@Reducer
struct Feature {
}

In here we need to define a type for the feature's state, which consists of an integer for the current count, as well as an optional string that represents the title of the alert we want to show (optional because nil represents not showing an alert):

@Reducer
struct Feature {
  struct State: Equatable {
    var count = 0
    var numberFactAlert: String?
  }
}

We also need to define a type for the feature's actions. There are the obvious actions, such as tapping the decrement button, increment button, or fact button. But there are also some slightly non-obvious ones, such as the action of the user dismissing the alert, and the action that occurs when we receive a response from the fact API request:

@Reducer
struct Feature {
  struct State: Equatable { /* ... */ }
  enum Action {
    case factAlertDismissed
    case decrementButtonTapped
    case incrementButtonTapped
    case numberFactButtonTapped
    case numberFactResponse(String)
  }
}

And then we implement the Reducer/body-swift.property property, which is responsible for handling the actual logic and behavior for the feature. In it we can use the Reduce reducer to describe how to change the current state to the next state, and what effects need to be executed. Some actions don't need to execute effects, and they can return Effect/none to represent that:

@Reducer
struct Feature {
  struct State: Equatable { /* ... */ }
  enum Action { /* ... */ }

  var body: some Reducer<State, Action> {
    Reduce { state, action in
      switch action {
      case .factAlertDismissed:
        state.numberFactAlert = nil
        return .none

      case .decrementButtonTapped:
        state.count -= 1
        return .none

      case .incrementButtonTapped:
        state.count += 1
        return .none

      case .numberFactButtonTapped:
        return .run { [count = state.count] send in
          let (data, _) = try await URLSession.shared.data(
            from: URL(string: "http://numbersapi.com/\(count)/trivia")!
          )
          await send(
            .numberFactResponse(String(decoding: data, as: UTF8.self))
          )
        }

      case let .numberFactResponse(fact):
        state.numberFactAlert = fact
        return .none
      }
    }
  }
}

And then finally we define the view that displays the feature. It holds onto a StoreOf<Feature> so that it can observe all changes to the state and re-render, and we can send all user actions to the store so that state changes. We must also introduce a struct wrapper around the fact alert to make it Identifiable, which the .alert view modifier requires:

struct FeatureView: View {
  let store: StoreOf<Feature>

  var body: some View {
    WithViewStore(self.store, observe: { $0 }) { viewStore in
      VStack {
        HStack {
          Button("−") { viewStore.send(.decrementButtonTapped) }
          Text("\(viewStore.count)")
          Button("+") { viewStore.send(.incrementButtonTapped) }
        }

        Button("Number fact") { viewStore.send(.numberFactButtonTapped) }
      }
      .alert(
        item: viewStore.binding(
          get: { $0.numberFactAlert.map(FactAlert.init(title:)) },
          send: .factAlertDismissed
        ),
        content: { Alert(title: Text($0.title)) }
      )
    }
  }
}

struct FactAlert: Identifiable {
  var title: String
  var id: String { self.title }
}

It is also straightforward to have a UIKit controller driven off of this store. You subscribe to the store in viewDidLoad in order to update the UI and show alerts. The code is a bit longer than the SwiftUI version:

class FeatureViewController: UIViewController {
  let viewStore: ViewStoreOf<Feature>
  var cancellables: Set<AnyCancellable> = []

  init(store: StoreOf<Feature>) {
    self.viewStore = ViewStore(store, observe: { $0 })
    super.init(nibName: nil, bundle: nil)
  }

  required init?(coder: NSCoder) {
    fatalError("init(coder:) has not been implemented")
  }

  override func viewDidLoad() {
    super.viewDidLoad()

    let countLabel = UILabel()
    let incrementButton = UIButton()
    let decrementButton = UIButton()
    let factButton = UIButton()

    // Omitted: Add subviews and set up constraints...

    self.viewStore.publisher
      .map { "\($0.count)" }
      .assign(to: \.text, on: countLabel)
      .store(in: &self.cancellables)

    self.viewStore.publisher.numberFactAlert
      .sink { [weak self] numberFactAlert in
        let alertController = UIAlertController(
          title: numberFactAlert, message: nil, preferredStyle: .alert
        )
        alertController.addAction(
          UIAlertAction(
            title: "OK",
            style: .default,
            handler: { _ in self?.viewStore.send(.factAlertDismissed) }
          )
        )
        self?.present(alertController, animated: true, completion: nil)
      }
      .store(in: &self.cancellables)
  }

  @objc private func incrementButtonTapped() {
    self.viewStore.send(.incrementButtonTapped)
  }
  @objc private func decrementButtonTapped() {
    self.viewStore.send(.decrementButtonTapped)
  }
  @objc private func factButtonTapped() {
    self.viewStore.send(.numberFactButtonTapped)
  }
}

Once we are ready to display this view, for example in the app's entry point, we can construct a store. This can be done by specifying the initial state to start the application in, as well as the reducer that will power the application:

@main
struct MyApp: App {
  var body: some Scene {
    FeatureView(
      store: Store(initialState: Feature.State()) {
        Feature()
      }
    )
  }
}

And that is enough to get something on the screen to play around with. It's definitely a few more steps than if you were to do this in a vanilla SwiftUI way, but there are a few benefits. It gives us a consistent manner to apply state mutations, instead of scattering logic in some observable objects and in various action closures of UI components. It also gives us a concise way of expressing side effects. And we can immediately test this logic, including the effects, without doing much additional work.

Testing your feature

To test use a TestStore, which can be created with the same information as the Store, but it does extra work to allow you to assert how your feature evolves as actions are sent:

@MainActor
func testFeature() async {
  let store = TestStore(initialState: Feature.State()) {
    Feature()
  }
}

Once the test store is created we can use it to make an assertion of an entire user flow of steps. Each step of the way we need to prove that state changed how we expect. For example, we can simulate the user flow of tapping on the increment and decrement buttons:

// Test that tapping on the increment/decrement buttons changes the count
await store.send(.incrementButtonTapped) {
  $0.count = 1
}
await store.send(.decrementButtonTapped) {
  $0.count = 0
}

Further, if a step causes an effect to be executed, which feeds data back into the store, we must assert on that. For example, if we simulate the user tapping on the fact button we expect to receive a fact response back with the fact, which then causes the alert to show:

await store.send(.numberFactButtonTapped)

await store.receive(\.numberFactResponse) {
  $0.numberFactAlert = "???"
}

However, how do we know what fact is going to be sent back to us?

Currently our reducer is using an effect that reaches out into the real world to hit an API server, and that means we have no way to control its behavior. We are at the whims of our internet connectivity and the availability of the API server in order to write this test.

It would be better for this dependency to be passed to the reducer so that we can use a live dependency when running the application on a device, but use a mocked dependency for tests. We can do this by adding a property to the Feature reducer:

@Reducer
struct Feature {
  let numberFact: (Int) async throws -> String
  // ...
}

Then we can use it in the reduce implementation:

case .numberFactButtonTapped:
  return .run { [count = state.count] send in
    let fact = try await self.numberFact(count)
    await send(.numberFactResponse(fact))
  }

And in the entry point of the application we can provide a version of the dependency that actually interacts with the real world API server:

@main
struct MyApp: App {
  var body: some Scene {
    FeatureView(
      store: Store(initialState: Feature.State()) {
        Feature(
          numberFact: { number in
            let (data, _) = try await URLSession.shared.data(
              from: .init(string: "http://numbersapi.com/\(number)")!
            )
            return String(decoding: data, as: UTF8.self)
          }
        )
      }
    )
  }
}

But in tests we can use a mock dependency that immediately returns a deterministic, predictable fact:

@MainActor
func testFeature() async {
  let store = TestStore(initialState: Feature.State()) {
    Feature(numberFact: { "\($0) is a good number Brent" })
  }
}

With that little bit of upfront work we can finish the test by simulating the user tapping on the fact button, receiving the response from the dependency to trigger the alert, and then dismissing the alert:

await store.send(.numberFactButtonTapped)

await store.receive(\.numberFactResponse) {
  $0.numberFactAlert = "0 is a good number Brent"
}

await store.send(.factAlertDismissed) {
  $0.numberFactAlert = nil
}

We can also improve the ergonomics of using the numberFact dependency in our application. Over time the application may evolve into many features, and some of those features may also want access to numberFact, and explicitly passing it through all layers can get annoying. There is a process you can follow to “register” dependencies with the library, making them instantly available to any layer in the application.

We can start by wrapping the number fact functionality in a new type:

struct NumberFactClient {
  var fetch: (Int) async throws -> String
}

And then registering that type with the dependency management system, which is quite similar to how SwiftUI's environment values works, except you specify the live implementation of the dependency to be used by default:

private enum NumberFactClientKey: DependencyKey {
  static let liveValue = NumberFactClient(
    fetch: { number in
      let (data, _) = try await URLSession.shared.data(
        from: .init(string: "http://numbersapi.com/\(number)")!
      )
      return String(decoding: data, as: UTF8.self)
    }
  )
}

extension DependencyValues {
  var numberFact: NumberFactClient {
    get { self[NumberFactClientKey.self] }
    set { self[NumberFactClientKey.self] = newValue }
  }
}

With that little bit of upfront work done you can instantly start making use of the dependency in any feature:

@Reducer
struct Feature {
  struct State { /* ... */ }
  enum Action { /* ... */ }
  @Dependency(\.numberFact) var numberFact
  // ...
}

This code works exactly as it did before, but you no longer have to explicitly pass the dependency when constructing the feature's reducer. When running the app in previews, the simulator or on a device, the live dependency will be provided to the reducer, and in tests the test dependency will be provided.

This means the entry point to the application no longer needs to construct dependencies:

@main
struct MyApp: App {
  var body: some Scene {
    FeatureView(
      store: Store(initialState: Feature.State()) {
        Feature()
      }
    )
  }
}

And the test store can be constructed without specifying any dependencies, but you can still override any dependency you need to for the purpose of the test:

let store = TestStore(initialState: Feature.State()) {
  Feature()
} withDependencies: {
  $0.numberFact.fetch = { "\($0) is a good number Brent" }
}

await store.send(.numberFactButtonTapped)
await store.receive(\.numberFactResponse) {
  $0.numberFactAlert = "0 is a good number Brent"
}

That is the basics of building and testing a feature in the Composable Architecture. There are a lot more things to be explored, such as doc:DependencyManagement, doc:Performance, doc:SwiftConcurrency and more about doc:Testing. Also, the Examples directory has a bunch of projects to explore to see more advanced usages.