A blog exploring functional programming and Swift.

Announcing the Reducer Protocol

Monday Oct 10, 2022

Just two months ago we announced the biggest update to the Composable Architecture since its first release. Today we are announcing an even bigger update by introducing the ReducerProtocol and a brand new dependency management system to everyone who upgrades to version 0.41.0.

To celebrate, we are releasing a free episode to show off some of the changes, and demonstrate how it massively simplifies many of our case studies and demo applications, as well as isowords, our open-source word game built entirely in the Composable Architecture and SwiftUI.

And if you don’t have time to watch the entire episode, continue reading. 😀

The ReducerProtocol

The biggest change to the library is the introduction of the ReducerProtocol, which means one defines a type to conform to the protocol rather than constructing an instance of Reducer directly. This immediately gives you the benefit of having a natural place to nest your feature’s domain:

struct Feature: ReducerProtocol {
  struct State {
    // ...
  }
  enum Action {
    // ...
  }

  func reduce(into state: inout State, action: Action) -> Effect<Action, Never> {
    // ...
  }
}

…and in the process of doing this can improve the stability of the Swift compiler to properly type check your program and provide inline warnings.

The protocol also gives us the opportunity to reimagine what composition of reducers looks like. By leveraging result builders and Swift 5.7’s new support for constrained opaque types, we can provide a way to compose multiple reducers that looks similar to SwiftUI syntax. For example, the app-level reducer that handles the functionality for 3 tabs in a tab view can look like this:

struct App: ReducerProtocol {
  struct State {
    var activity: Activity.State
    var profile: Profile.State
    var search: Search.State
  }
  enum Action {
    case activity(Activity.Action)
    case profile(Profile.Action)
    case search(Search.Action)
  }

  var body: some ReducerProtocol<State, Action> {
    Scope(state: \.activity, action: /Action.activity) {
      Activity()
    }
    Scope(state: \.profile, action: /Action.profile) {
      Profile()
    }
    Scope(state: \.search, action: /Action.search) {
      Search()
    }
  }
}

There are also new operators that allow you to more safely compose reducers that work on optional state (ifLet), array state (forEach) and even enum state (ifCaseLet).

Dependencies made easy

The biggest new feature that the reducer protocol unlocked for the library is a brand new dependency management system. Now that reducers are types, they become the natural place to hold onto dependencies. There’s no need to define a separate “environment” type that holds all of the dependencies the feature needs to do its job, which means no need to maintain the boilerplate of an initializer if you decide to modularize later.

Even better, by taking some inspiration from SwiftUI, you can provide dependencies to your reducers via a property wrapper that pulls from a global store of dependencies:

struct Feature: ReducerProtocol {
  @Dependency(\.apiClient) var apiClient
  @Dependency(\.mainQueue) var mainQueue
  @Dependency(\.uuid) var uuid

  // ...
}

There’s no need to explicitly pass dependencies through every layer of the application. You can add a new dependency to a leaf node of your features with a single line of code and without updating any other feature. And it’s possible to override dependencies for a specific reducer and its effects.

The library also bakes in some extra safety around dependency usage. For example, “live” dependencies are not allowed to be used in tests, and if they are it will cause a test failure. This encourages you to be explicit with mocking dependencies so that you do not unknowingly interact with the real world in tests unless you explicitly state you want to.

Further, when registering a dependency with the library you can optionally provide extra implementations in addition to the live value. You can provide a “test” value that will be used when you test your feature in a TestStore. And you can provide a “preview” value that will be used when your feature is run in an Xcode preview.

Read our dedicated article to learn more about how to best leverage the dependency system in the library, and how to best design your own dependencies.

Performance improvements

Once you have moved your features to the new ReducerProtocol you may see some performance improvements, especially for deeply nested features. The Swift compiler can do a much better job optimizing and inlining methods than it can with escaping closures, causing stack traces to become slimmer and stack memory usage to drop.

We demonstrated this in this week’s episode where we showed that drilling many layers deep into a recursive UI led to a stack trace of 164 frames, but only 29 of those frames were actually from the application. To contrast, the same application running on the previous version of the library produced a stack trace of 191 stack frames, and 113 of those frames were due to the application. This means the ReducerProtocol version of the application has about a fourth of the number of stack frames for that use case.

For more information on maximizing performance when using the library, be sure to check out our dedicated performance article.

Updated documentation

This release has also brought a massive improvement to the documentation of the library. Many types and methods now have fuller descriptions and explanations, some articles have been updated, and a few new articles have been added.

There is now a dedicated article on dependency management. It covers the “why” and “how” of dependency management, and shows how to make use of the controllable dependencies the library comes with, as well as how to register your own. It also gives a detailed explanation of how the new dependency management system works with testing and Xcode previews, as well as some tips on how to best design your own dependency clients.

There is also a dedicated article on migration strategies to the new ReducerProtocol style of building features. It outlines many scenarious you are likely to encounter while migrating your existing application, and if something is not covered we highly recommend you open a GitHub discussion about it.

Get started today

We have barely scratched the surface of the far reaching consequences of this update. Be sure to watch this week’s free episode and update to version 0.41.0 today!


Subscribe to Point-Free

👋 Hey there! If you got this far, then you must have enjoyed this post. You may want to also check out Point-Free, a video series on functional programming and Swift.