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Composable Forms: Say "Bye" to Boilerplate!

Monday Feb 1, 2021

Based on the code we wrote in our latest episode on Concise Forms, today we are releasing first-party support for concisely handling form data in the Composable Architecture. This allows you to minimize the boilerplate caused by needing to have a unique action for every UI control. Instead, all UI bindings can be consolidated into a single form action.

Composable Forms

The latest version of the Composable Architecture comes with some tools that allow you to dramatically eliminate the boilerplate that is typically incurred when working with multiple mutable fields on state.

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

This is a lot of boilerplate for something that should be simple. Luckily, we can dramatically eliminate this boilerplate using FormAction. First, we can 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 form(FormAction<SettingsState>)

And then, we can simplify the settings reducer by tackling on the form method, which handle these field mutations for us:

let settingsReducer = Reducer<
  SettingsState, SettingsAction, SettingsEnvironment
> {
  switch action {
  case .form:
    return .none
.form(action: /SettingsAction.form)

That’s it 🤯.

Form actions are constructed and sent to the store by providing a writable key path from root state to the field being mutated. There is even a view store helper that simplifies this work. You can derive a binding by specifying the key path and form action case:

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

Should you need to layer additional functionality over your form, your reducer can pattern match the form action for a given key path:

case .form(\.displayName):
  // Validate display name

case .form(\.enableNotifications):
  // Return an authorization request effect

Form actions can event be tested in much the same way regular actions are tested. Rather than send a specific action describing how a binding changed, such as displayNameChanged("Blob"), you will send a .form action that describes which key path is being set to what value, such as .form(.set(\.displayName, "Blob")):

let store = TestStore(
  initialState: SettingsState(),
  reducer: settingsReducer,
  environment: SettingsEnvironment(...)

  .send(.form(.set(\.displayName, "Blob"))) {
    $0.displayName = "Blob"
  .send(.form(.set(\.protectMyPosts, true))) {
    $0.protectMyPosts = true

Say “bye” to boilerplate today!

We’ve just released version 0.12.0 of the Composable Architecture, and so you can start using this new feature immediately. Let us know what you think!

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