A blog exploring functional programming and Swift.

The Composable Architecture ❤️ SwiftUI Bindings

Monday Sep 6, 2021

Correction

Due to a problem involving nested bindable state, we have since had to make things slightly less concise by trading dynamic member lookup for a more explicit method: e.g. viewStore.$field is now viewStore.binding(\.$field). For more information on the change see this release and this pull request.

Early this year, we did a series of episodes on “concise forms.” We showed how SwiftUI comes with some amazing tools for handling state through the use of two-way bindings that can be derived from property wrappers like @State and @ObservedObject. For simple forms it almost feels like magic.

We then compared this to the Composable Architecture, which adopts a “unidirectional” data flow, wherein the only way to mutate state is by sending actions to a runtime store, which holds all of the app’s business logic and is responsible for mutating the state inside. From its very first release the Composable Architecture shipped with tools that integrate deeply with SwiftUI applications, including ways of deriving two-way bindings for various SwiftUI controls, by describing a field in state and an action that can mutate it.

This unfortunately means adding an action per mutable field, and logic for handling that action in the reducer, which is boilerplate that really begins to add up.

For example, a settings screen may model its state with the following struct:

struct SettingsState {
  var digest = Digest.daily
  var displayName = ""
  var enableNotifications = false
  var protectMyPosts = false
  var sendEmailNotifications = false
  var sendMobileNotifications = false
}

Each of these fields should be editable, and in the Composable Architecture this means that each field requires a corresponding action that can be sent to the store. Typically this comes in the form of an enum with a case per field:

enum SettingsAction {
  case digestChanged(Digest)
  case displayNameChanged(String)
  case enableNotificationsChanged(Bool)
  case protectMyPostsChanged(Bool)
  case sendEmailNotificationsChanged(Bool)
  case sendMobileNotificationsChanged(Bool)
}

And we’re not even done yet. In the reducer we must now handle each action, which simply replaces the state at each field with a new value:

let settingsReducer = Reducer<
  SettingsState, SettingsAction, SettingsEnvironment
> { state, action, environment in
  switch action {
  case let digestChanged(digest):
    state.digest = digest
    return .none

  case let displayNameChanged(displayName):
    state.displayName = displayName
    return .none

  case let enableNotificationsChanged(isOn):
    state.enableNotifications = isOn
    return .none

  case let protectMyPostsChanged(isOn):
    state.protectMyPosts = isOn
    return .none

  case let sendEmailNotificationsChanged(isOn):
    state.sendEmailNotifications = isOn
    return .none

  case let sendMobileNotificationsChanged(isOn):
    state.sendMobileNotifications = isOn
    return .none
  }
}

And then, in the view, we can derive two-way bindings from the view store to pass them along, which is pretty verbose to do:

Form {
  TextField(
    "Display name",
    text: viewStore.binding(
      get: \.displayName,
      send: SettingsAction.displayNameChanged
    )
  )
  Toggle(
    "Protect my posts",
    isOn: viewStore.binding(
      get: \.protectMyPosts,
      send: SettingsAction.protectMyPostsChanged
    )
  )
  Toggle(
    "Send notifications",
    isOn: viewStore.binding(
      get: \.sendNotifications,
      send: SettingsAction.sendNotificationsChanged
    )
  )

  if viewStore.sendNotifications {
    Toggle(
      "Mobile",
      isOn: viewStore.binding(
        get: \.sendMobileNotifications,
        send: SettingsAction.sendMobileNotificationsChanged
      )
    )
    Toggle(
      "Email",
      isOn: viewStore.binding(
        get: \.sendEmailNotifications,
        send: SettingsAction.sendEmailNotificationsChanged
      )
    )
    Picker(
      "Top posts digest",
      selection: viewStore.binding(
        get: \.digest,
        send: SettingsAction.digestChanged
      )
    ) {
      ForEach(Digest.allCases, id: \.self) { digest in
        Text(digest.rawValue)
      }
    }
  }
}
.alert(
  item: viewStore.binding(
    get: \.alert,
    send: SettingsAction.dismissAlert
  )
) { alert in
  Alert(title: Text(alert.title))
}

This is a ton of boilerplate for something that should be simple. Luckily, we were able to employ some advanced Swift techniques, like type erasure and key paths, to dramatically eliminate this boilerplate by introducing BindingAction. It allowed us to collapse all of these field-mutating actions into a single case that holds a FormAction generic over the reducer’s root SettingsState:

enum SettingsAction {
  case binding(BindingAction<SettingsState>)
}

Then we can collapse all of the field mutations in the reducer by instead tacking on the binding method, which performs all these field mutations for us:

let settingsReducer = Reducer<
  SettingsState, SettingsAction, SettingsEnvironment
> { state, action, environment in
  switch action {
  case .binding:
    return .none
  }
}
.binding(action: /SettingsAction.binding)

And finally, we used a view store helper that simplified the work of deriving a binding by specifying the a path and binding action:

TextField(
  "Display name",
  text: viewStore.binding(keyPath: \.displayName, send: SettingsAction.binding)
)
...

This was overall a huge improvement! We were able to eliminate a ton of boilerplate in action enums and reducers.

However, this was not without a cost. Adding a BindingAction case to your action enum immediately gives your view unfettered access to mutate its state, which goes completely against the grain of the Composable Architecture.

For instance, if we introduce some state that should not be mutated outside the reducer:

struct SettingsState {
  ...
  var isLoading = false
}

Nothing protects it in the view:

.onAppear {
  viewStore.send(.binding(.set(\.isLoading, true)))
}

And so, in our most recent two episodes, we faced this problem head-on, and along the way made things even more concise, by utilizing even more advanced Swift features, including property wrappers and dynamic member lookup. If you’re interested in how, we hope you’ll check them out today:

By introducing the @BindableState property wrapper, it is now possible to safely annotate which fields in state should be bindable:

struct SettingsState: Equatable {
  @BindableState var alert: AlertState? = nil
  @BindableState var digest = Digest.daily
  @BindableState var displayName = ""
  var isLoading = false
  @BindableState var protectMyPosts = false
  @BindableState var sendNotifications = false
  @BindableState var sendMobileNotifications = false
  @BindableState var sendEmailNotifications = false
}

Notably, isLoading is not annotated, and so it can not be mutated directly in the view.

Our action enum can stay mostly the same, except we have also introduced a BindableAction protocol, which can allow us to eliminate even more boilerplate.

enum SettingsAction: BindableAction {
  case binding(BindingAction<SettingsState>)
}

The protocol has a single requirement, which is a static binding method that can bundle up a BindingAction into itself, and because enum cases can conform to such requirements, SettingsAction already conforms!

With this protocol defined, we can simplify the reducer because we no longer have to specify the binding case explicitly:

let settingsReducer = Reducer<
  SettingsState, SettingsAction, SettingsEnvironment
> { state, action, environment in
  switch action {
  case .binding:
    return .none
  }
}
.binding()

But the real gains are seen in the view. Because the action can be inferred, we can adopt a syntax that matches the conciseness of vanilla SwiftUI!

Form {
  TextField("Display name", text: viewStore.$displayName)
  Toggle("Protect my posts", isOn: viewStore.$protectMyPosts)
  Toggle("Send notifications", isOn: viewStore.$sendNotifications)

  if viewStore.sendNotifications {
    Toggle("Mobile", isOn: viewStore.$sendMobileNotifications)
    Toggle("Email", isOn: viewStore.$sendEmailNotifications)
    Picker("Top posts digest", selection: viewStore.$digest) {
      ForEach(Digest.allCases, id: \.self) { digest in
        Text(digest.rawValue)
      }
    }
  }
}
.alert(item: viewStore.$alert) { alert in
  Alert(title: Text(alert.title))
}

That’s over 3x shorter than what we had before!

Say “hi” to safety, and “bye” to even more boilerplate today!

We’ve just released version 0.26.0 of the Composable Architecture, and so you can start simplifying your existing code today. Let us know what you think on Twitter, or start a discussion on GitHub.


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