Swiftpack.co - Package - pointfreeco/swift-composable-architecture

The Composable Architecture


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, 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: part 1, part 2, part 3 and part 4.


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:

Basic Usage

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.
  • Environment: A type that holds any dependencies the feature needs, such as API clients, analytics clients, etc.
  • 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 is 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.

The state of this feature would consist 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):

struct AppState: Equatable {
  var count = 0
  var numberFactAlert: String?

Next we have the actions in the feature. 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:

enum AppAction: Equatable {
  case factAlertDismissed
  case decrementButtonTapped
  case incrementButtonTapped
  case numberFactButtonTapped
  case numberFactResponse(Result<String, ApiError>)

struct ApiError: Error, Equatable {}

Next we model the environment of dependencies this feature needs to do its job. In particular, to fetch a number fact we need to construct an Effect value that encapsulates the network request. So that dependency is a function from Int to Effect<String, ApiError>, where String represents the response from the request. Further, the effect will typically do its work on a background thread (as is the case with URLSession), and so we need a way to receive the effect's values on the main queue. We do this via a main queue scheduler, which is a dependency that is important to control so that we can write tests. We must use an AnyScheduler so that we can use a live DispatchQueue in production and a test scheduler in tests.

struct AppEnvironment {
  var mainQueue: AnySchedulerOf<DispatchQueue>
  var numberFact: (Int) -> Effect<String, ApiError>

Next, we implement a reducer that implements the logic for this domain. It describes how to change the current state to the next state, and describes what effects need to be executed. Some actions don't need to execute effects, and they can return .none to represent that:

let appReducer = Reducer<AppState, AppAction, AppEnvironment> { state, action, environment 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 environment.numberFact(state.count)
      .receive(on: environment.mainQueue)

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

  case .numberFactResponse(.failure):
    state.numberFactAlert = "Could not load a number fact :("
    return .none

And then finally we define the view that displays the feature. It holds onto a Store<AppState, AppAction> 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 AppView: View {
  let store: Store<AppState, AppAction>

  var body: some View {
    WithViewStore(self.store) { viewStore in
      VStack {
        HStack {
          Button("−") { viewStore.send(.decrementButtonTapped) }
          Button("+") { viewStore.send(.incrementButtonTapped) }

        Button("Number fact") { viewStore.send(.numberFactButtonTapped) }
        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's important to note that we were able to implement this entire feature without having a real, live effect at hand. This is important because it means features can be built in isolation without building their dependencies, which can help compile times.

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, so we have collapsed it here:

class AppViewController: UIViewController {
  let viewStore: ViewStore<AppState, AppAction>
  var cancellables: Set<AnyCancellable> = []

  init(store: Store<AppState, AppAction>) {
    self.viewStore = ViewStore(store)
    super.init(nibName: nil, bundle: nil)

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

  override func viewDidLoad() {

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

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

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

      .sink { [weak self] numberFactAlert in
        let alertController = UIAlertController(
          title: numberFactAlert, message: nil, preferredStyle: .alert
            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() {
  @objc private func decrementButtonTapped() {
  @objc private func factButtonTapped() {

Once we are ready to display this view, for example in the scene delegate, we can construct a store. This is the moment where we need to supply the dependencies, and for now we can just use an effect that immediately returns a mocked string:

let appView = AppView(
  store: Store(
    initialState: AppState(),
    reducer: appReducer,
    environment: AppEnvironment(
      mainQueue: DispatchQueue.main.eraseToAnyScheduler(),
      numberFact: { number in Effect(value: "\(number) is a good number Brent") }

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.


To test, you first create a TestStore with the same information that you would to create a regular Store, except this time we can supply test-friendly dependencies. In particular, we use a test scheduler instead of the live DispatchQueue.main scheduler because that allows us to control when work is executed, and we don't have to artificially wait for queues to catch up.

let scheduler = DispatchQueue.testScheduler

let store = TestStore(
  initialState: AppState(),
  reducer: appReducer,
  environment: AppEnvironment(
    mainQueue: scheduler.eraseToAnyScheduler(),
    numberFact: { number in Effect(value: "\(number) is a good number Brent") }

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. Further, if a step causes an effect to be executed, which feeds data back into the store, we must assert that those actions were received properly.

The test below has the user increment and decrement the count, then they ask for a number fact, and the response of that effect triggers an alert to be shown, and then dismissing the alert causes the alert to go away.

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

  // Test that tapping the fact button causes us to receive a response from the effect. Note
  // that we have to advance the scheduler because we used `.receive(on:)` in the reducer.
  .do { scheduler.advance() },
  .receive(.numberFactResponse(.success("0 is a good number Brent"))) {
    $0.numberFactAlert = "0 is a good number Brent"

  // And finally dismiss the alert
  .send(.factAlertDismissed) {
    $0.numberFactAlert = nil

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.


The Composable Architecture comes with a number of tools to aid in debugging.

  • reducer.debug() enhances a reducer with debug-printing that describes every action the reducer receives and every mutation it makes to state.

    received action:
        index: 0
        todos: [
    -       isComplete: false,
    +       isComplete: true,
            description: "Milk",
            id: 5834811A-83B4-4E5E-BCD3-8A38F6BDCA90
            isComplete: false,
            description: "Eggs",
            id: AB3C7921-8262-4412-AA93-9DC5575C1107
            isComplete: true,
            description: "Hand Soap",
            id: 06E94D88-D726-42EF-BA8B-7B4478179D19
  • reducer.signpost() instruments a reducer with signposts so that you can gain insight into how long actions take to execute, and when effects are running.

Supplementary libraries

One of the most important principles of the Composable Architecture is that side effects are never performed directly, but instead are wrapped in the Effect type, returned from reducers, and then the Store later performs the effect. This is crucial for simplifying how data flows through an application, and for gaining testability on the full end-to-end cycle of user action to effect execution.

However, this also means that many libraries and SDKs you interact with on a daily basis need to be retrofitted to be a little more friendly to the Composable Architecture style. That's why we'd like to ease the pain of using some of Apple's most popular frameworks by providing wrapper libraries that expose their functionality in a way that plays nicely with our library. So far we support:

  • ComposableCoreLocation: A wrapper around CLLocationManager that makes it easy to use from a reducer, and easy to write tests for how your logic interacts with CLLocationManager's functionality.
  • ComposableCoreMotion: A wrapper around CMMotionManager that makes it easy to use from a reducer, and easy to write tests for how your logic interacts with CMMotionManager's functionality.
  • More to come soon. Keep an eye out 😉

If you are interested in contributing a wrapper library for a framework that we have not yet covered, feel free to open an issue expressing your interest so that we can discuss a path forward.


  • How does the Composable Architecture compare to Elm, Redux, and others?

    In some ways TCA is a little more opinionated than the other libraries. For example, Redux is not prescriptive with how one executes side effects, but TCA requires all side effects to be modeled in the Effect type and returned from the reducer.

    In other ways TCA is a little more lax than the other libraries. For example, Elm controls what kinds of effects can be created via the Cmd type, but TCA allows an escape hatch to any kind of effect since Effect conforms to the Combine Publisher protocol.

    And then there are certain things that TCA prioritizes highly that are not points of focus for Redux, Elm, or most other libraries. For example, composition is very important aspect of TCA, which is the process of breaking down large features into smaller units that can be glued together. This is accomplished with the pullback and combine operators on reducers, and it aids in handling complex features as well as modularization for a better-isolated code base and improved compile times.

  • Why isn't Store thread-safe? Why isn't send queued? Why isn't send run on the main thread?

    When an action is sent to the Store, a reducer is run on the current state, and this process cannot be done from multiple threads. A possible work around is to use a queue in sends implementation, but this introduces a few new complications:

    1. If done simply with DispatchQueue.main.async you will incur a thread hop even when you are already on the main thread. This can lead to unexpected behavior in UIKit and SwiftUI, where sometimes you are required to do work synchronously, such as in animation blocks.

    2. It is possible to create a scheduler that performs its work immediately when on the main thread and otherwise uses DispatchQueue.main.async (e.g. see ReactiveSwift's UIScheduler). This introduces a lot more complexity, and should probably not be adopted without having a very good reason.

    At the end of the day, we require Store to be used in much the same way that you interact with Apple's APIs. Just as URLSession delivers its results on a background thread, thus making you responsible for dispatching back to the main thread, the Composable Architecture makes you responsible for making sure to send actions on the main thread. If you are using an effect that may deliver its output on a non-main thread, you must explicitly perform .receive(on:) in order to force it back on the main thread.

    This approach makes the fewest number of assumptions about how effects are created and transformed, and prevents unnecessary thread hops and re-dispatching. It also provides some testing benefits. If your effects are not responsible for their own scheduling, then in tests all of the effects would run synchronously and immediately. You would not be able to test how multiple in-flight effects interleave with each other and affect the state of your application. However, by leaving scheduling out of the Store we get to test these aspects of our effects if we so desire, or we can ignore if we prefer. We have that flexibility.

    However, if you are still not a fan of our choice, then never fear! The Composable Architecture is flexible enough to allow you to introduce this functionality yourself if you so desire. It is possible to create a higher-order reducer that can force all effects to deliver their output on the main thread, regardless of where the effect does its work:

    extension Reducer {
      func receive<S: Scheduler>(on scheduler: S) -> Self {
        Self { state, action, environment in
          self(&state, action, environment)
            .receive(on: scheduler)

    You would probably still want something like a UIScheduler so that you don't needlessly perform thread hops.


The Composable Architecture depends on the Combine framework, so it requires minimum deployment targets of iOS 13, macOS 10.15, Mac Catalyst 13, tvOS 13, and watchOS 6. If your application must support older OSes, there are forks for ReactiveSwift and RxSwift that you can adopt!


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

  1. From the File menu, select Swift Packages › Add Package Dependency…
  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 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 TicTacToeCommon framework.


The latest documentation for the Composable Architecture APIs is available here.


If you want to discuss the Composable Architecture or have a question about how to use it to solve a particular problem, ask around on its Swift forum.

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.


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


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0.9.0 -

0.8.0 -

  • Improved: better error messaging around testing uncompleted effects: it will now print the action the effect was kicked off from (thanks @mackoj).
  • Bug fix: fixed identified array implementations of shuffle, reverse (thanks @jeffersonsetiawan).
  • Bug fix: test stores should now perform effects in the same order as a live store.
  • Infrastructure: better document TCA acronym.
  • Infrastructure: document Apple bug with WithViewStore and ScrollViewReader.
  • Infrastructure: fix recursive higher-order reducer demo to map its effects recursively.
  • Infrastructure: Xcode 12 CI.

0.7.0 -

  • Changed: LocationManager mock initializers now take the correct, optional signature for the location dependency (thanks @mackoj ).
  • Changed: Reducer.optional is now a method so that its assertions can include file/line context (thanks @alexito4). The optional property has been deprecated in favor of the method.
  • Changed: previously-deprecated interfaces have been obsoleted.
  • Bug fixed: Store.ifLet no longer evaluates the else branch too often (thanks @mluisbrown).
  • Bug fixed: more than one alert can now be presented in a row, as can more than one action sheet.
  • Infrastructure: documentation fixes (thanks @jasdev, @artnest, @mmatoszko).
  • Infrastructure: Store.ifLet can take advantage of a scope overload (thanks @fonkadelic).
  • Infrastructure: fixed Voice Memos bug around deleting a memo that is playing.
  • Infrastructure: new demo for tvOS focus.
  • Infrastructure: added key frame effect to animation demo (thanks @boudavid).
  • Infrastructure: fixed Tic-Tac-Toe demo bug around error display (thanks @heiberg).

0.6.0 -

  • Added: SwiftUI alert and action sheet helpers via AlertState and ActionSheetState.
  • Changed: ViewStore no longer uses @Published to coordinate its state with ObservableObject.

0.5.0 -

  • Added: ComposableCoreMotion, a micro library for integrating the Composable Architecture with Core Motion.
  • Improved: Store.send now prints the action sent in its assertion failure for better debugging (thanks @ollitapa).
  • Changed: Reducer.debug now uses Publishers.MergeMany instead of Publishers.Concatenate to avoid a Combine bug that results in a memory leak.
  • Infrastructure: document rxswift-composable-architecture (thanks @dannyhertz), move ViewStore.swift into the root directory (thanks @dannyhertz), reorganize VoiceMemos demo for readability.

0.4.0 -

  • Added: ComposableCoreLocation types now conform to Codable (thanks @alskipp).
  • Added: Reducer debug methods now take an actionFormat parameter.
  • Changed: depend on CombineSchedulers instead of vendoring AnyScheduler and TestScheduler. The library contains several bug fixes over the code that was included in 0.3.0.
  • Fixed: use subscribe method over receive(subscriber:), per Apple documentation (thanks @asiliuk).
  • Improved: ComposableCoreLocation delegate ergonomics (thanks @klundberg).
  • Optimized: signposts should be a little more performant now.
  • Optimized: store.send no longer recursively processes effects and instead processes them in a while loop, avoiding stack issues (thanks @klop).
  • Optimized: IfLetStore view content types been simplified.
  • Infrastructure: link to ReactiveSwift fork (thanks @mluisbrown).
  • Infrastructure: encapsulate Publishers.Create and AnyPublisher.init(_:) (thanks @kaandedeoglu).
  • Infrastructure: clean up tests around cancellation (thanks @klop).
  • Infrastructure: added a new web socket case study.
  • Infrastructure: fixed voice memo assertion failure.

0.3.0 -

  • Added: reducers can now be instrumented with signposts using the signposts higher-order reducer.
  • Changed: debugged reducers will now omit printing state when no state changes.
  • Bug fixed: several bugs around automatically synthesized mutating methods on IdentifiedArray, including replaceSubrange and sort (thanks @peterkovacs).
  • Bug fixed: chaining multiple cancellable calls onto a single effect with the same identifier no longer crashes.
  • Infrastructure: better support for iPad in the demo apps.
  • Infrastructure: cleaned up location manager app annotation behavior (thanks @kaandedeoglu).

0.2.0 -

  • Added: ComposableCoreLocation, a micro library for interfacing the Composable Architecture with Core Location.

0.1.5 -

  • Added: a TestStore.assert method that takes an array of steps (thanks @pteasima).
  • Changed: misusing identified array's id-based subscript (attempting to update an element not in the array, or attempting to nil out an element) will now crash with a helpful error message in development.
  • Changed: WithViewStore.debug logic is now a no-op in release mode.
  • Changed: Reducer.debug and WithViewStore.debug can now omit the prefix argument name to more closely align with Publisher.print.
  • Bug fixed: test schedulers now properly order concurrent work items.
  • Bug fixed: debug output will no longer overflow on infinitely recursive references.
  • Infrastructure: README fixes (thanks @OfftheCode), documentation fixes (thanks @johankool), new location manager demo, animation case study fixed for dark mode.

0.1.4 -

  • Added: IdentifiedArray.remove(id:) method for removing elements by identifier.
  • Added: Reducer.combined(with:) method for combining reducers using dot-chaining (thanks @alexito4).
  • Added: Reducer.run method for invoking the reducer in a more ergonomic fashion than callAsFunction in Swift 5.1.
  • Changed: optimized test support code to dynamically load XCTest just once per test process (rather than once per assertion failure).
  • Changed: IdentifiedArray's id-based subscript no longer removes elements from the array when passed nil. Use remove(id:) instead.
  • Changed: CasePath is no longer vendored. It is instead included as an SPM dependency.
  • Changed: Effect.async has been renamed to Effect.run.
  • Changed: Effect.sync has been renamed to Effect.catching.
  • Bug fixed: TestScheduler now always performs/schedules simultaneous work by earliest-scheduled.
  • Infrastructure: added binding-based animation demo to case studies.
  • Infrastructure: add Swift forum to README.
  • Infrastructure: fixed typo in IfLetStore documentation (thanks @jasdev).

0.1.3 -

Breaking changes:

  • ComposableArchitecture is no longer a dynamic library. To upgrade:
    • You will need to reintegrate the library into your application. You can reintegrate by removing and re-adding the library to your application or shared framework target.
    • If you include ComposableArchitecture in more than one target in your project, you must move it to a single, dynamic framework, instead. Then this framework is what you can share among your app, framework, and test targets. For an example, check out the Tic-Tac-Toe demo application, which has been modularized and integrates ComposableArchitecture in this fashion.
  • ComposableArchitectureTestSupport has been removed and TestStore has moved into ComposableArchitecture. To upgrade:
    • Change import ComposableArchitectureTestSupport to import ComposableArchitecture in your tests.

Other changes:

  • Added: stateless and actionless transformations to Store to more easily derive stores that should only send actions or only read state.
  • Changed: IfLetStore no longer wraps its content in a Group.
  • Bug fixed: TestStore's assert no longer crashes when working with empty state structs (thanks @jlainog).
  • Infrastructure: decluttered CI output using xcodebuild -quiet (thanks @tarjamorgado).
  • Infrastructure: demos now build in Xcode 11.3.1.

0.1.2 -

  • Bug fixed: Xcode previews should no longer always crash during compilation.
  • Infrastructure: simpler Tic-Tac-Toe winning condition logic (thanks @thomasmarsh).

0.1.1 -

  • Added: TestStore now has scope methods that mirrors its Store counterpart for testing view store transformations.
  • Removed: TestStore no longer has an initializer that takes state and action transformations. Use scope instead.
  • Changed: identified array's id-based subscript now takes an explicit id parameter name. This avoids ambiguity when subscripting into an array of Int-identified elements.
  • Bug fixed: annotating reducers with debug() no longer breaks them in release builds (thanks @tulushev).
  • Bug fixed: nil-ing out an element of an identified array no longer puts it in an invalid state.
  • Bug fixed: removing more than one element of an identified array via index set should no longer remove any incorrect elements or crash.
  • Improved: messaging for out-of-bounds forEach errors (thanks @ryantstone for bringing up the issue).
  • Improved: documentation (thanks @alohaglenn, @ReadmeCritic :wink:, @natemann).

0.1.0 -

A library for building applications in a consistent and understandable way, with composition, testing, and ergonomics in mind.