pfw-composable-architecture

The Composable Architecture

IMPORTANT

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Goal

Build Swift application features using the Composable Architecture (TCA) library.

Quick start

Add the swift-composable-architecture package dependency from 1.0.0 or newer.
Add the ComposableArchitecture product to your target's dependencies.
import ComposableArchitecture as needed

API interface

Full API interface for when solution can't be found elsewhere: references/interface/ComposableArchitecture.swiftinterface

How to build a basic feature

Use this template and customize as needed:

import ComposableArchitecture

@Reducer struct Counter {
  @ObservableState struct State {
    var count = 0
  }
  enum Action {
    case decrementButtonTapped
    case incrementButtonTapped
  }
  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
      }
    }
  }
}

DO name action cases literally after what the user does (e.g. decrementButtonTapped, not decrement) or data the effect returns (e.g. apiResponse, timerTick, etc.)
DO NOT append Reducer suffix to the @Reducer's type name (e.g. Counter, not CounterReducer)
DO NOT conform Action to Equatable
DO NOT use legacy reduce(into:) API (use body and Reduce, instead)
DO group and alphabetize properties, enum cases, dependencies, feature-local state, etc., unless the user has another style preference.

How to interact with a feature from SwiftUI

Use this template and customize as needed:

import ComposableArchitecture
import SwiftUI

struct CounterView: View {
  let store: StoreOf<Counter>

  var body: some View {
    HStack {
      Button {
        store.send(.decrementButtonTapped)
      } label: {
        Image(systemName: "minus")
      }
      Text("\(store.count)")
      Button {
        store.send(.incrementButtonTapped)
      } label: {
        Image(systemName: "plus")
      }
    }
  }
}

#Preview {
  CounterView(
    store: Store(initialState: Counter.State()) {
      Counter()
    }
  )
}

DO NOT use legacy ViewStore or WithViewStore APIs

How to group multiple features together and modify them

Use CombineReducers to collect features together before applying reducer modifiers like ifLet and forEach:

var body: some Reducer<State, Action> {
  CombineReducers {
    Scope(state: \.child1, action: \.child1) {
      Child1()
    }
    Scope(state: \.child2, action: \.child2) {
      Child2()
    }
  }
  .ifLet(\.$child3, action: \.child3) {
    Child3()
  }
}

DO NOT use CombineReducers to wrap features if no modifier (like ifLet, forEach) is applied to CombineReducers.

How to perform asynchronous work

Return a run effect from a Reduce closure:

@Reducer struct LoginFeature {
  @ObservableState struct State { ... }
  enum Action {
    case loginButtonTapped
    case response(Result<LoginResponse, any Error>)
  }
  @Dependency(APIClient.self) var apiClient
  var body: some Reducer<State, Action>
    Reduce { state, action in
      switch action {
      case .loginButtonTapped:
        return .run { send in
          await send(
            .response(
              Result {
                try await apiClient.login()
              }
            )
          )
        }
      ...
      }
    }
  }
}

DO NOT use deprecated TaskResult type. Use Result, instead.

How to explicitly cancel asynchronous work

Step 1: Introduce a cancellation identifier

@Reducer struct Screen {
  @ObservableState struct State { ... }
  enum Action { ... }
  enum CancelID { case apiRequest }
  var body: some Reducer<State, Action> { ... }
}

Step 2: Identify the cancellable effect

var body: some Reducer<State, Action> {
  Reduce { state, action in
    switch action {
    ...
    case .fetchButtonTapped:
      return .run { store in
        ...
      }
      .cancellable(id: CancelID.apiRequest)
    }
  }
}

Step 3: Use the identifier to cancel the effect

var body: some Reducer<State, Action> {
  Reduce { state, action in
    switch action {
    ...
    case .cancelButtonTapped:
      return .cancel(id: CancelID.apiRequest)
    }
  }
}

How to control a feature's dependency

Use the @Dependency property wrapper to access a controllable dependency from a feature:

@Reducer struct WhatTimeIsIt {
  @ObservableState struct State {
    var message = ""
  }
  enum Action {
    case questionButtonTapped
  }
  @Dependency(\.date.now) var now
  var body: some Reducer<State, Action> {
    Reduce { state, action in
      switch action {
      case .questionButtonTapped:
        state.message = "It's \(now)"
        return .none
      }
    }
  }
}

DO declare the @Dependency directly inside the @Reducer

Use the dependency reducer modifier to override a controllable dependency for a feature:

Scope(state: \.onboarding, action: \.onboarding) {
  Onboarding()
    .dependency(\.apiClient, MockAPIClient())
}

If a @Dependency is declared, always use it instead of the uncontrolled alternative. For example, if @Dependency(\.uuid) var uuid is declared, use uuid() instead of UUID(), and if @Dependency(\.date.now) var now is declared, use now instead of Date().

DO prefer controlled dependencies over uncontrolled dependencies (e.g. @Dependency(\.uuid) var uuid and uuid() instead of UUID()).

How to modify store state directly and via bindings

Step 1: Add bindable domain to the feature

Conform Action to BindableAction, introduce a binding case, and add BindingReducer() to the feature body:

@Reducer struct Counter {
  @ObservableState struct State {
    var count = 0
  }
  enum Action: BindableAction {
    case binding(BindingAction<State>)
  }
  var body: some Reducer<State, Action> {
    BindingReducer()
  }
}

DO NOT apply legacy @BindableState property wrapper to State properties

Step 2: Derive bindings from a bindable store

@Bindable var store: StoreOf<Counter>
...
Stepper("\(store.count)", value: $store.count)

How to derive a binding for a state/action pair

For a given reducer:

@Reducer struct Counter {
  @ObservableState struct State {
    var count = 0
  }
  enum Action {
    case stepperChanged(Int)
  }
  var body: some Reducer<State, Action> {
    Reduce { state, action in
      switch action {
      case .stepperChanged(let int):
        state.count = int
        return .none
      }
    }
  }
}

Derive a binding from a bindable store:

@Bindable var store: StoreOf<Counter>
...
Stepper(
  "\(store.count)",
  value: $store.count.sending(\.stepperChanged)
)

How to compose a child feature into a parent

Step 1: Add child domain to parent

@Reducer struct Parent {
  @ObservableState struct State {
    ...
    var child: Child.State
  }
  enum Action {
    ...
    case child(Child.Action)
  }
  var body: some Reducer<State, Action> {
    ...
    Scope(state: \.child, action: \.child) {
      Child()
    }
  }
}

Step 2: Scope parent store into child store to hand off to child feature

struct ParentView: View {
  let store: StoreOf<Parent>

  var body: some View {
    ...
    ChildView(store: store.scope(state: \.child, action: \.child))
  }
}

How to present a child feature from a parent

Step 1: Add optional child domain to parent

@Reducer struct Parent {
  @ObservableState struct State {
    ...
    @Presents var child: Child.State?
  }
  enum Action {
    ...
    case child(PresentationAction<Child.Action>)
  }
  var body: some Reducer<State, Action> {
    Reduce { state, action in
      ...
    }
    .ifLet(\$.child, action: \.child) {
      Child()
    }
  }
}

DO NOT use legacy @PresentationState property wrapper

Step 2: Scope parent store into child store to hand off to child feature

struct ParentView: View {
  @Bindable var store: StoreOf<Parent>

  var body: some View {
    ...
    .sheet(item: $store.scope(state: \.child, action: \.child)) { childStore in
      ChildView(store: childStore)
    }
  }
}

DO NOT use legacy sheet(store:) or similar APIs

How to manage a collection of features in a parent

Step 1: Add collection of child domain to parent

@Reducer struct Parent {
  @ObservableState struct State {
    ...
    var children: IdentifiedArrayOf<Child.State> = []
  }
  enum Action {
    ...
    case children(IdentifiedActionOf<Child>)
  }
  var body: some Reducer<State, Action> {
    Reduce { state, action in
      ...
    }
    .forEach(\.children, action: \.children) {
      Child()
    }
  }
}

Step 2: Scope parent store into child stores to hand off to child features

Display a list of child features:

let store: StoreOf<Parent>
...
ForEach(
  store.scope(state: \.children, action: \.children),
  id: \.state.id
) { childStore in
  ChildView(store: childStore)
}

How to provide stack navigation to a feature

Step 1: Add "path" feature that models all navigable destinations

@Reducer enum Path {
  case counter(Counter)
  case scoreboard(Scoreboard)
}

DO NOT nest Path feature inside parent feature (prefix the feature with the parent name, instead, e.g. ParentPath for the feature Parent)

Step 2: Add path domain to parent

@Reducer struct Parent {
  @ObservableState struct State {
    ...
    var path: StackState<Path.State> = []
  }
  enum Action {
    ...
    case path(StackActionOf<Path>)
  }
  var body: some Reducer<State, Action> {
    Reduce { state, action in
      ...
    }
    .forEach(\.path, action: \.path) {
      Path.body
    }
  }
}

Step 3: Scope parent store into child stores to hand off to child features

@Bindable var store: StoreOf<Parent>
...
NavigationStack(path: $store.scope(state: \.path, action: \.path)) {
  RootView()
} destination: { pathStore in
  switch pathStore.case {
  case .counter(let counterStore):
    CounterView(store: counterStore)
  case .scoreboard(let scoreboardStore):
    ScoreboardView(store: scoreboardStore)
  }
}

How to dismiss a feature

A child feature can dismiss/remove itself from its parent:

@Reducer struct Child {
  @ObservableState struct State {
    ...
  }
  enum Action {
    ...
  }
  @Dependency(\.dismiss) var dismiss
  var body: some Reducer<State, Action> {
    Reduce { state, action in
      switch action {
      ...
      case .closeButtonTapped:
        return .run { _ in
          await dismiss()
        }
      }
    }
  }
}

How to debug a feature

Use the _printChanges modifier to add logging to a feature.

var body: some Reducer<State, Action> {
  CombineReducers {
    ...
  }
  ._printChanges()
}

About this skill

IMPORTANT

Goal

Quick start

API interface

How to build a basic feature

How to interact with a feature from SwiftUI

How to group multiple features together and modify them

How to perform asynchronous work

How to explicitly cancel asynchronous work

Step 1: Introduce a cancellation identifier

Step 2: Identify the cancellable effect

Step 3: Use the identifier to cancel the effect

How to control a feature's dependency

How to modify store state directly and via bindings

Step 1: Add bindable domain to the feature

Step 2: Derive bindings from a bindable store

How to derive a binding for a state/action pair

How to compose a child feature into a parent

Step 1: Add child domain to parent

Step 2: Scope parent store into child store to hand off to child feature

How to present a child feature from a parent

Step 1: Add optional child domain to parent

Step 2: Scope parent store into child store to hand off to child feature

How to manage a collection of features in a parent

Step 1: Add collection of child domain to parent

Step 2: Scope parent store into child stores to hand off to child features

How to provide stack navigation to a feature

Step 1: Add "path" feature that models all navigable destinations

Step 2: Add path domain to parent

Step 3: Scope parent store into child stores to hand off to child features

How to dismiss a feature

How to debug a feature