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The Composable Architecture

CI Slack

The Composable Architecture (TCA, for short) is a library for building applications in a consistent and understandable way, with composition, testing, and ergonomics in mind. It can be used in SwiftUI, UIKit, and more, and on any Apple platform (iOS, macOS, visionOS, tvOS, and watchOS).

What is the Composable Architecture?

This library provides a few core tools that can be used to build applications of varying purpose and complexity. It provides compelling stories that you can follow to solve many problems you encounter day-to-day when building applications, such as:

  • State management
    How to manage the state of your application using simple value types, and share state across many screens so that mutations in one screen can be immediately observed in another screen.

  • Composition
    How to break down large features into smaller components that can be extracted to their own, isolated modules and be easily glued back together to form the feature.

  • Side effects
    How to let certain parts of the application talk to the outside world in the most testable and understandable way possible.

  • Testing
    How to not only test a feature built in the architecture, but also write integration tests for features that have been composed of many parts, and write end-to-end tests to understand how side effects influence your application. This allows you to make strong guarantees that your business logic is running in the way you expect.

  • Ergonomics
    How to accomplish all of the above in a simple API with as few concepts and moving parts as possible.

Learn More

The Composable Architecture was designed over the course of many episodes on Point-Free, a video series exploring functional programming and the Swift language, hosted by Brandon Williams and Stephen Celis.

You can watch all of the episodes here, as well as a dedicated, multipart tour of the architecture from scratch.

video poster image

Examples

Screen shots of example applications

This repo comes with lots of examples to demonstrate how to solve common and complex problems with the Composable Architecture. Check out this directory to see them all, including:

Looking for something more substantial? Check out the source code for isowords, an iOS word search game built in SwiftUI and the Composable Architecture.

Basic Usage

Note

For a step-by-step interactive tutorial, be sure to check out Meet the Composable Architecture.

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 displays it in the view.

To implement this feature we create a new type that will house the domain and behavior of the feature, and it will be annotated with the @Reducer macro:

import ComposableArchitecture

@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 fact being presented:

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

Note

We've applied the @ObservableState macro to State in order to take advantage of the observation tools in the library.

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 that occurs when we receive a response from the fact API request:

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

And then we implement the body property, which is responsible for composing 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 .none to represent that:

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

  var body: some Reducer<State, Action> {
    Reduce { state, action in
      switch action {
      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.numberFact = 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:

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

  var body: some View {
    Form {
      Section {
        Text("\(store.count)")
        Button("Decrement") { store.send(.decrementButtonTapped) }
        Button("Increment") { store.send(.incrementButtonTapped) }
      }

      Section {
        Button("Number fact") { store.send(.numberFactButtonTapped) }
      }
      
      if let fact = store.numberFact {
        Text(fact)
      }
    }
  }
}

It is also straightforward to have a UIKit controller driven off of this store. You can observe state changes in the store in viewDidLoad, and then populate the UI components with data from the store. The code is a bit longer than the SwiftUI version, so we have collapsed it here:

Click to expand! 
class FeatureViewController: UIViewController {
  let store: StoreOf<Feature>

  init(store: StoreOf<Feature>) {
    self.store = store
    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 decrementButton = UIButton()
    let incrementButton = UIButton()
    let factLabel = UILabel()
    
    // Omitted: Add subviews and set up constraints...
    
    observe { [weak self] in
      guard let self 
      else { return }
      
      countLabel.text = "\(self.store.text)"
      factLabel.text = self.store.numberFact
    }
  }

  @objc private func incrementButtonTapped() {
    self.store.send(.incrementButtonTapped)
  }
  @objc private func decrementButtonTapped() {
    self.store.send(.decrementButtonTapped)
  }
  @objc private func factButtonTapped() {
    self.store.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:

import ComposableArchitecture

@main
struct MyApp: App {
  var body: some Scene {
    WindowGroup {
      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

Note

For more in-depth information on testing, see the dedicated testing article.

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:

@Test
func basics() 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 numberFact state to be populated:

await store.send(.numberFactButtonTapped)

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

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 {
    WindowGroup {
      FeatureView(
        store: Store(initialState: Feature.State()) {
          Feature(
            numberFact: { number in
              let (data, _) = try await URLSession.shared.data(
                from: URL(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:

@Test
func basics() 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, and then receiving the response from the dependency to present the fact:

await store.send(.numberFactButtonTapped)

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

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.

Note

For more in-depth information on dependency management, see the dedicated dependencies article.

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 by conforming the client to the DependencyKey protocol, which requires you to specify the live value to use when running the application in simulators or devices:

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

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

With that little bit of upfront work done you can instantly start making use of the dependency in any feature by using the @Dependency property wrapper:

 @Reducer
 struct Feature {
-  let numberFact: (Int) async throws -> String
+  @Dependency(\.numberFact) var numberFact
   
   …

-  try await self.numberFact(count)
+  try await self.numberFact.fetch(count)
 }

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 {
    WindowGroup {
      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" }
}

// ...

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 composition, modularity, adaptability, and complex effects. The Examples directory has a bunch of projects to explore to see more advanced usages.

Documentation

The documentation for releases and main are available here:

Other versions

There are a number of articles in the documentation that you may find helpful as you become more comfortable with the library:

Community

If you want to discuss the Composable Architecture or have a question about how to use it to solve a particular problem, there are a number of places you can discuss with fellow Point-Free enthusiasts:

Installation

You can add ComposableArchitecture to an Xcode project by adding it as a package dependency.

  1. From the File menu, select Add Package Dependencies...
  2. Enter "https://github.com/pointfreeco/swift-composable-architecture" into the package repository URL text field
  3. Depending on how your project is structured:
    • If you have a single application target that needs access to the library, then add ComposableArchitecture directly to your application.
    • If you want to use this library from multiple Xcode targets, or mix Xcode targets and SPM targets, you must create a shared framework that depends on ComposableArchitecture and then depend on that framework in all of your targets. For an example of this, check out the Tic-Tac-Toe demo application, which splits lots of features into modules and consumes the static library in this fashion using the tic-tac-toe Swift package.

Companion libraries

The Composable Architecture is built with extensibility in mind, and there are a number of community-supported libraries available to enhance your applications:

If you'd like to contribute a library, please open a PR with a link to it!

Translations

The following translations of this README have been contributed by members of the community:

If you'd like to contribute a translation, please open a PR with a link to a Gist!

FAQ

We have a dedicated article for all of the most frequently asked questions and comments people have concerning the library.

Credits and thanks

The following people gave feedback on the library at its early stages and helped make the library what it is today:

Paul Colton, Kaan Dedeoglu, Matt Diephouse, Josef Doležal, Eimantas, Matthew Johnson, George Kaimakas, Nikita Leonov, Christopher Liscio, Jeffrey Macko, Alejandro Martinez, Shai Mishali, Willis Plummer, Simon-Pierre Roy, Justin Price, Sven A. Schmidt, Kyle Sherman, Petr Šíma, Jasdev Singh, Maxim Smirnov, Ryan Stone, Daniel Hollis Tavares, and all of the Point-Free subscribers 😁.

Special thanks to Chris Liscio who helped us work through many strange SwiftUI quirks and helped refine the final API.

And thanks to Shai Mishali and the CombineCommunity project, from which we took their implementation of Publishers.Create, which we use in Effect to help bridge delegate and callback-based APIs, making it much easier to interface with 3rd party frameworks.

Other libraries

The Composable Architecture was built on a foundation of ideas started by other libraries, in particular Elm and Redux.

There are also many architecture libraries in the Swift and iOS community. Each one of these has their own set of priorities and trade-offs that differ from the Composable Architecture.

License

This library is released under the MIT license. See LICENSE for details.