Structure Reducer

Printing is only done in debug (#if DEBUG) builds.

Printing is only done in debug (#if DEBUG) builds.

Printing is only done in debug (#if DEBUG) builds.

To use, build your app for Instruments (⌘I), create a blank instrument, and then use the "+" icon at top right to add the signpost instrument. Start recording your app (red button at top left) and then you should see timing information for every action sent to the store and every effect executed.

Effect instrumentation can be particularly useful for inspecting the lifecycle of long-living effects. For example, if you start an effect (e.g. a location manager) in onAppear and forget to tear down the effect in onDisappear, it will clearly show in Instruments that the effect never completed.

It is important to note that the order of combining reducers matter. Combining reducerA with reducerB is not necessarily the same as combining reducerB with reducerA.

This can become an issue when working with reducers that have overlapping domains. For example, if reducerA embeds the domain of reducerB and reacts to its actions or modifies its state, it can make a difference if reducerA chooses to modify reducerB's state before or after reducerB runs.

This is perhaps most easily seen when working with optional(file:line:) reducers, where the parent domain may listen to the child domain and nil out its state. If the parent reducer runs before the child reducer, then the child reducer will not be able to react to its own action.

Similar can be said for a forEach(state:action:environment:file:line:)-gvte reducer. If the parent domain modifies the child collection by moving, removing, or modifying an element before the forEach(state:action:environment:file:line:)-gvte reducer runs, the forEach(state:action:environment:file:line:)-gvte reducer may perform its action against the wrong element, an element that no longer exists, or an element in an unexpected state.

Running a parent reducer before a child reducer can be considered an application logic error, and can produce assertion failures. So you should almost always combine reducers in order from child to parent domain.

Here is an example of how you should combine an optional(file:line:) reducer with a parent domain:

let parentReducer = Reducer<ParentState, ParentAction, ParentEnvironment>.combine(
  // Combined before parent so that it can react to `.dismiss` while state is non-`nil`.
  childReducer.optional().pullback(
    state: \.child,
    action: /ParentAction.child,
    environment: { $0.child }
  ),
  // Combined after child so that it can `nil` out child state upon `.child(.dismiss)`.
  Reducer { state, action, environment in
    switch action
    case .child(.dismiss):
      state.child = nil
      return .none
    ...
    }
  },
)

It is important to note that the order of combining reducers matter. Combining reducerA with reducerB is not necessarily the same as combining reducerB with reducerA.

This can become an issue when working with reducers that have overlapping domains. For example, if reducerA embeds the domain of reducerB and reacts to its actions or modifies its state, it can make a difference if reducerA chooses to modify reducerB's state before or after reducerB runs.

This is perhaps most easily seen when working with optional(file:line:) reducers, where the parent domain may listen to the child domain and nil out its state. If the parent reducer runs before the child reducer, then the child reducer will not be able to react to its own action.

Similar can be said for a forEach(state:action:environment:file:line:)-gvte reducer. If the parent domain modifies the child collection by moving, removing, or modifying an element before the forEach(state:action:environment:file:line:)-gvte reducer runs, the forEach(state:action:environment:file:line:)-gvte reducer may perform its action against the wrong element, an element that no longer exists, or an element in an unexpected state.

Running a parent reducer before a child reducer can be considered an application logic error, and can produce assertion failures. So you should almost always combine reducers in order from child to parent domain.

Here is an example of how you should combine an optional(file:line:) reducer with a parent domain:

let parentReducer = Reducer<ParentState, ParentAction, ParentEnvironment>.combine(
  // Combined before parent so that it can react to `.dismiss` while state is non-`nil`.
  childReducer.optional().pullback(
    state: \.child,
    action: /ParentAction.child,
    environment: { $0.child }
  ),
  // Combined after child so that it can `nil` out child state upon `.child(.dismiss)`.
  Reducer { state, action, environment in
    switch action
    case .child(.dismiss):
      state.child = nil
      return .none
    ...
    }
  },
)

It is important to note that the order of combining reducers matter. Combining reducerA with reducerB is not necessarily the same as combining reducerB with reducerA.

This can become an issue when working with reducers that have overlapping domains. For example, if reducerA embeds the domain of reducerB and reacts to its actions or modifies its state, it can make a difference if reducerA chooses to modify reducerB's state before or after reducerB runs.

This is perhaps most easily seen when working with optional(file:line:) reducers, where the parent domain may listen to the child domain and nil out its state. If the parent reducer runs before the child reducer, then the child reducer will not be able to react to its own action.

Similar can be said for a forEach(state:action:environment:file:line:)-gvte reducer. If the parent domain modifies the child collection by moving, removing, or modifying an element before the forEach(state:action:environment:file:line:)-gvte reducer runs, the forEach(state:action:environment:file:line:)-gvte reducer may perform its action against the wrong element, an element that no longer exists, or an element in an unexpected state.

Running a parent reducer before a child reducer can be considered an application logic error, and can produce assertion failures. So you should almost always combine reducers in order from child to parent domain.

Here is an example of how you should combine an optional(file:line:) reducer with a parent domain:

let parentReducer: Reducer<ParentState, ParentAction, ParentEnvironment> =
  // Run before parent so that it can react to `.dismiss` while state is non-`nil`.
  childReducer
    .optional()
    .pullback(
      state: \.child,
      action: /ParentAction.child,
      environment: { $0.child }
    )
    // Combined after child so that it can `nil` out child state upon `.child(.dismiss)`.
    .combined(
      with: Reducer { state, action, environment in
        switch action
        case .child(.dismiss):
          state.child = nil
          return .none
        ...
        }
      }
    )

• A writable key path that can get/set a piece of local state from the global state.

• A case path that can extract/embed a local action into a global action.

• A function that can transform the global environment into a local environment.

This operation is important for breaking down large reducers into small ones. When used with the combine(_:)-1ern2 operator you can define many reducers that work on small pieces of domain, and then pull them back and combine them into one big reducer that works on a large domain.

 // Global domain that holds a local domain:
 struct AppState { var settings: SettingsState, /* rest of state */ }
 enum AppAction { case settings(SettingsAction), /* other actions */ }
 struct AppEnvironment { var settings: SettingsEnvironment, /* rest of dependencies */ }

 // A reducer that works on the local domain:
 let settingsReducer = Reducer<SettingsState, SettingsAction, SettingsEnvironment> { ... }

 // Pullback the settings reducer so that it works on all of the app domain:
 let appReducer: Reducer<AppState, AppAction, AppEnvironment> = .combine(
   settingsReducer.pullback(
     state: \.settings,
     action: /AppAction.settings,
     environment: { $0.settings }
   ),

   /* other reducers */
 )

It accomplishes this by providing 3 transformations to the method:

• A case path that can extract/embed a piece of local state from the global state, which is typically an enum.

• A case path that can extract/embed a local action into a global action.

• A function that can transform the global environment into a local environment.

This overload of pullback(state:action:environment:) differs from the other in that it takes a CasePath transformation for the state instead of a WritableKeyPath. This makes it perfect for working on enum state as opposed to struct state. In particular, you can use this operator to pullback a reducer that operates on a single case of some state enum to work on the entire state enum.

When used with the combine(_:)-994ak operator you can define many reducers that work each case of the state enum, and then pull them back and combine them into one big reducer that works on a large domain.

// Global domain that holds a local domain:
enum AppState { case loggedIn(LoggedInState), /* rest of state */ }
enum AppAction { case loggedIn(LoggedInAction), /* other actions */ }
struct AppEnvironment { var loggedIn: LoggedInEnvironment, /* rest of dependencies */ }

// A reducer that works on the local domain:
let loggedInReducer = Reducer<LoggedInState, LoggedInAction, LoggedInEnvironment> { ... }

// Pullback the logged-in reducer so that it works on all of the app domain:
let appReducer: Reducer<AppState, AppAction, AppEnvironment> = .combine(
  loggedInReducer.pullback(
    state: /AppState.loggedIn,
    action: /AppAction.loggedIn,
    environment: { $0.loggedIn }
  ),

  /* other reducers */
)

Take care when combining a child reducer for a particular case of enum state into its parent domain. A child reducer cannot process actions in its domain if it fails to extract its corresponding state. If a child action is sent to a reducer when its state is unavailable, it is generally considered a logic error, and a runtime warning will be logged. There are a few ways in which these errors can sneak into a code base:

• A parent reducer sets child state to a different case when processing a child action and runs before the child reducer:

  let parentReducer = Reducer<ParentState, ParentAction, ParentEnvironment>.combine(
    // When combining reducers, the parent reducer runs first
    Reducer { state, action, environment in
      switch action {
      case .child(.didDisappear):
        // And `nil`s out child state when processing a child action
        state.child = .anotherChild(AnotherChildState())
        return .none
      ...
      }
    },
    // Before the child reducer runs
    childReducer.pullback(state: /ParentState.child, ...)
  )
  
  let childReducer = Reducer<
    ChildState, ChildAction, ChildEnvironment
  > { state, action environment in
    case .didDisappear:
      // This action is never received here because child state cannot be extracted
    ...
  }
  

  To ensure that a child reducer can process any action that a parent may use to change its state, combine it before the parent:

  let parentReducer = Reducer<ParentState, ParentAction, ParentEnvironment>.combine(
    // The child runs first
    childReducer.pullback(state: /ParentState.child, ...),
    // The parent runs after
    Reducer { state, action, environment in
      ...
    }
  )
  

• A child effect feeds a child action back into the store when child state is unavailable:

  let childReducer = Reducer<
    ChildState, ChildAction, ChildEnvironment
  > { state, action environment in
    switch action {
    case .onAppear:
      // An effect may want to later feed a result back to the child domain in an action
      return environment.apiClient
        .request()
        .map(ChildAction.response)
  
    case let .response(response):
      // But the child cannot process this action if its state is unavailable
    ...
    }
  }
  

  It is perfectly reasonable to ignore the result of an effect when child state is nil, for example one-off effects that you don't want to cancel. However, many long-living effects should be explicitly canceled when tearing down a child domain:

  let childReducer = Reducer<
    ChildState, ChildAction, ChildEnvironment
  > { state, action environment in
    struct MotionId: Hashable {}
  
    switch action {
    case .onAppear:
      // Mark long-living effects that shouldn't outlive their domain cancellable
      return environment.motionClient
        .start()
        .map(ChildAction.motion)
        .cancellable(id: MotionId())
  
    case .onDisappear:
      // And explicitly cancel them when the domain is torn down
      return .cancel(id: MotionId())
    ...
    }
  }
  

• A view store sends a child action when child state is nil:

  WithViewStore(self.parentStore) { parentViewStore in
    // If child state is `nil`, it cannot process this action.
    Button("Child Action") { parentViewStore.send(.child(.action)) }
    ...
  }
  

  Use Store/scope(state:action:) with SwitchStore to ensure that views can only send child actions when the child domain is available.

  SwitchStore(self.parentStore) {
    CaseLet(state: /ParentState.child, action: ParentAction.child) { childStore in
      // This destination only appears when child state matches
      WithViewStore(childStore) { childViewStore in
        // So this action can only be sent when child state is available
        Button("Child Action") { childViewStore.send(.action) }
      }
    }
    ...
  }
  

Often used in tandem with pullback(state:action:environment:) to transform a reducer on a non-optional child domain into a reducer that can be combined with a reducer on a parent domain that contains some optional child domain:

// Global domain that holds an optional local domain:
struct AppState { var modal: ModalState? }
enum AppAction { case modal(ModalAction) }
struct AppEnvironment { var mainQueue: AnySchedulerOf<DispatchQueue> }

// A reducer that works on the non-optional local domain:
let modalReducer = Reducer<ModalState, ModalAction, ModalEnvironment { ... }

// Pullback the local modal reducer so that it works on all of the app domain:
let appReducer = Reducer<AppState, AppAction, AppEnvironment>.combine(
  modalReducer.optional().pullback(
    state: \.modal,
    action: /AppAction.modal,
    environment: { ModalEnvironment(mainQueue: $0.mainQueue) }
  ),
  Reducer { state, action, environment in
    ...
  }
)

Take care when combining optional reducers into parent domains. An optional reducer cannot process actions in its domain when its state is nil. If a child action is sent to an optional reducer when child state is nil, it is generally considered a logic error. There are a few ways in which these errors can sneak into a code base:

• A parent reducer sets child state to nil when processing a child action and runs before the child reducer:

  let parentReducer = Reducer<ParentState, ParentAction, ParentEnvironment>.combine(
    // When combining reducers, the parent reducer runs first
    Reducer { state, action, environment in
      switch action {
      case .child(.didDisappear):
        // And `nil`s out child state when processing a child action
        state.child = nil
        return .none
      ...
      }
    },
    // Before the child reducer runs
    childReducer.optional().pullback(...)
  )
  
  let childReducer = Reducer<
    ChildState, ChildAction, ChildEnvironment
  > { state, action environment in
    case .didDisappear:
      // This action is never received here because child state is `nil` in the parent
    ...
  }
  

  To ensure that a child reducer can process any action that a parent may use to nil out its state, combine it before the parent:

  let parentReducer = Reducer<ParentState, ParentAction, ParentEnvironment>.combine(
    // The child runs first
    childReducer.optional().pullback(...),
    // The parent runs after
    Reducer { state, action, environment in
      ...
    }
  )
  

• A child effect feeds a child action back into the store when child state is nil:

  let childReducer = Reducer<
    ChildState, ChildAction, ChildEnvironment
  > { state, action environment in
    switch action {
    case .onAppear:
      // An effect may want to feed its result back to the child domain in an action
      return environment.apiClient
        .request()
        .map(ChildAction.response)
  
    case let .response(response):
      // But the child cannot process this action if its state is `nil` in the parent
    ...
    }
  }
  

  It is perfectly reasonable to ignore the result of an effect when child state is nil, for example one-off effects that you don't want to cancel. However, many long-living effects should be explicitly canceled when tearing down a child domain:

  let childReducer = Reducer<
    ChildState, ChildAction, ChildEnvironment
  > { state, action environment in
    struct MotionId: Hashable {}
  
    switch action {
    case .onAppear:
      // Mark long-living effects that shouldn't outlive their domain cancellable
      return environment.motionClient
        .start()
        .map(ChildAction.motion)
        .cancellable(id: MotionId())
  
    case .onDisappear:
      // And explicitly cancel them when the domain is torn down
      return .cancel(id: MotionId())
    ...
    }
  }
  

• A view store sends a child action when child state is nil:

  WithViewStore(self.parentStore) { parentViewStore in
    // If child state is `nil`, it cannot process this action.
    Button("Child Action") { parentViewStore.send(.child(.action)) }
    ...
  }
  

  Use Store/scope(state:action:) with IfLetStore or Store/ifLet(then:else:) to ensure that views can only send child actions when the child domain is non-nil.

  IfLetStore(
    self.parentStore.scope(state: { $0.child }, action: { .child($0) }
  ) { childStore in
    // This destination only appears when child state is non-`nil`
    WithViewStore(childStore) { childViewStore in
      // So this action can only be sent when child state is non-`nil`
      Button("Child Action") { childViewStore.send(.action) }
    }
    ...
  }
  

// Global domain that holds a collection of local domains:
struct AppState { var todos: IdentifiedArrayOf<Todo> }
enum AppAction { case todo(id: Todo.ID, action: TodoAction) }
struct AppEnvironment { var mainQueue: AnySchedulerOf<DispatchQueue> }

// A reducer that works on a local domain:
let todoReducer = Reducer<Todo, TodoAction, TodoEnvironment> { ... }

// Pullback the local todo reducer so that it works on all of the app domain:
let appReducer = Reducer<AppState, AppAction, AppEnvironment>.combine(
  todoReducer.forEach(
    state: \.todos,
    action: /AppAction.todo(id:action:),
    environment: { _ in TodoEnvironment() }
  ),
  Reducer { state, action, environment in
    ...
  }
)

Take care when combining forEach(state:action:environment:file:line:)-gvte reducers into parent domains, as order matters. Always combine forEach(state:action:environment:file:line:)-gvte reducers before parent reducers that can modify the collection.

Take care when combining forEach(state:action:environment:file:line:)-21wow reducers into parent domains, as order matters. Always combine forEach(state:action:environment:file:line:)-21wow reducers before parent reducers that can modify the dictionary.