Observation comes to the Composable Architecture

After 2 months in beta, we are finally releasing the Composable Architecture with Swift 5.9’s observation tools tightly integrated. This simplifies nearly every facet of the library, and allows us to drastically reduce the APIs in the library and leverage SwiftUI’s APIs more fully.

And best of all, this release is 100% backwards compatible with the last release of the library, which means you can start incrementally using these tools today. Oh, and we also backported all of the observation tools so that they work on older Apple platforms going all the way back to iOS 13!

Join us for a quick overview of the changes, and also be sure to check out the migration guide and update your project to 1.7 today to get all of the benefits.

👋 Goodbye view stores!

By far the most substantial improvement to the library is that view stores, and a flurry of related concepts, are no longer needed. View stores were necessary in the beginning because one needed to be able to hold a lot of state in a feature, yet observe only a small part of the state. Now that is all handled automatically for us thanks to Swift’s new observation tools.

To update your features, simply annotate the State type of your reducers with the @ObservableState macro (it works with both structs and enums):

 @Reducer
 struct Feature {
+  @ObservableState
   struct State { /* ... */ }
   …
 }

And then in the view you can drop any usage of WithViewStore and simply access state directly from the store:

 var body: some View {
-  WithViewStore(store, observe: ViewState.init) { store in
   Form {
-    Text(viewStore.count.description)
+    Text(store.count.description)
-    Button("+") { viewStore.send(.incrementButtonTapped) }
+    Button("+") { store.send(.incrementButtonTapped) }
   }
-  }
 }

All state access is immediately tracked so that the view will only observe changes to that state, and nothing else.

However, as the code is written above, this will only work on devices running iOS 17. If you are deploying to iOS 16 or lower, then there is one small additional step you must take. We recently backported Swift’s observation tools so that they work on iOS 16 and earlier. But, in order for observation to be tracked properly, you must wrap your view in WithPerceptionTracking:

 var body: some View {
+  WithPerceptionTracking {
     Form {
       Text(store.count.description)
       Button("+") { store.send(.incrementButtonTapped) }
     }
+  }
 }

That’s all it takes to automatically get observation tracking, and you can run the app on iOS versions going all the way back to 13!

👋 Goodbye IfLetStore, ForEachStore, SwitchStore, NavigationStackStore and navigation view modifiers!

Historically, the Composable Architecture needed to maintain a whole zoo of tools, views, and view modifiers in order to make SwiftUI work with the library, and for state to be observed in the most minimal way possible. But now that observation happens automatically, and is based on what state is accessed in the view, we can completely get rid of those tools.

For example, to derive a Store to an optional child domain, one would previously use the IfLetStore helper view. But now one can simply use a vanilla Swift if let statement:

-IfLetStore(store: store.scope(state: \.child, action: \.child)) { childStore in
+if let childStore = store.scope(state: \.child, action: \.child)) {
   ChildView(store: childStore)
 } else: {
   Text("Nothing to show")
 }

Similarly, the library needed to provide a ForEachStore view helper in order to efficiently observe collections of features, but now one can use a regular ForEach view and hand it a store scoped to a collection of features:

-ForEachStore(store.scope(state: \.rows, action: \.rows)) { childStore in
+ForEach(store.scope(state: \.rows, action: \.rows)) { childStore in
   ChildView(store: childStore)
 }

The library also had to provide tools for efficiently switching over enums of features called SwitchStore and CaseLet. These concepts now completely go away and one can use a regular switch statement:

-SwitchStore(store) {
-  switch $0 {
+  switch store.state {
   case .activity:
-    CaseLet(/Feature.State.activity, action: Feature.Action.activity) { store in
-      ActivityView(store: store)
-    }
+    store.scope(state: \.activity, action: \.activity).map(ActivityView.init)
   case .settings:
-    CaseLet(/Feature.State.settings, action: Feature.Action.settings) { store in
-      SettingsView(store: store)
-    }
+    store.scope(state: \.settings, action: \.settings).map(SettingsView.init)
   }
-}

Even navigation stacks needed their own helper, called NavigationStackStore. This too goes away and one can now use the custom initializer the library provides for specifying the path of features that drives navigation, as well as a trailing closure to describe the views to present for each feature:

-NavigationStackStore(store.scope(state: \.path, action: \.path)) {
+NavigationStack(path: $store.scope(state: \.path, action: \.path)) {
   RootView()
-} destination: {
-  switch $0 {
+} destination: { store in
+  switch store.state {
   case .activity:
-    CaseLet(/Feature.State.activity, action: Feature.Action.activity) { store in
-      ActivityView(store: store)
-    }
+    store.scope(state: \.activity, action: \.activity).map(ActivityView.init)
   case .settings:
-    CaseLet(/Feature.State.settings, action: Feature.Action.settings) { store in
-      SettingsView(store: store)
-    }
+    store.scope(state: \.settings, action: \.settings).map(SettingsView.init)
   }
 }

And the library also needed to supply a whole plethora of navigation view modifiers that mimic the vanilla SwiftUI APIs, but tuned specifically for the Composable Architecture. This included sheet(store:), popover(store:), fullScreenCover(store:), and more. But now all of those go away, and instead you can use the vanlla SwiftUI modifiers by deriving a binding from the store for the feature you want to present:

-.sheet(store: store.scope(state: \.$child, action: \.child)) { store in
+.sheet(item: $store.scope(state: \.child, action: \.child)) { store in
   ChildView(store: store)
 }

This is only scratching the surface of what Swift’s observation tools have allowed us to simplify. Be sure to follow the migration guide to update your application to use all the newest tools.

And once we decide to release our next major version (2.0), we will be able to delete thousands of lines of code and documentation, making the library lighter weight and improving compile times for complex features.

UIKit

While the initial version of Swift’s observation tools were clearly made with SwiftUI (and only SwiftUI) in mind, we still wanted to provide some tools for people using UIKit. Even if you were to build a brand new application today hoping to use only the newest of SwiftUI’s tools, you will inevitably need to escape out of the SwiftUI world in order to accomplish something that SwiftUI cannot do. For our isowords game we had to escape out of the SwiftUI world in order to fully interface with SceneKit.

So, the library comes with a tool called observe(_:) that lets you set up an observation loop for updating your UI when state changes. It is most appropriate to call once in the entry point of a view, such as viewDidLoad of a UIViewController:

override func viewDidLoad() {
  super.viewDidLoad()

  observe { [weak self] in
    guard let self else { return }

    countLabel.isHidden = store.isObservingCount
    if !countLabel.isHidden {
      countLabel.text = "\(store.count)"
    }
    factLabel.text = store.fact
  }
}

In the code above we are able to update the controller’s UI components with state from the store, and further the mere act of accessing state makes observe automatically observe changes to that state. So, if count changes the observe trailing closure will execute again, allowing us to update the label’s text.

Get started today

Follow the migration guide to upgrade your projects to version 1.7 of the Composable Architecture to start making use of its new observation tools today. And we have some big plans for the library in the coming months, so stay tuned!