The Composable Architecture (TCA), which just updated to version 1.13, is a well-designed Swift library providing a cohesive framework that simplifies common challenges in app development such as state management, composing features, managing side effects, and facilitating testing.
The framework of the Composable Architecture is built around several principal elements: state, actions, reducers, and store. These concepts, particularly reducers and store, might ring a bell for developers accustomed to using Redux with React. However, these are relatively fresh ideas for native iOS development. The developers of the library, Brandon Williams and Stephen Celis, designed it in such a way to allow large, intricate features to be broken down into manageable domains that are easily integrated.
The Composable Architecture is compatible with both UIKit and Swift, and it draws significant inspiration from SwiftUI which enhances its compatibility, particularly for developers who are well-versed in SwiftUI and its idioms. The designers claim this connection makes the API of the framework particularly streamlined.
A notable offering of TCA is the @ObservableState macro that functions similarly to the iOS 16 @Observable, enabling minimal state observations akin to SwiftUI’s functionality. Moreover, the library borrows ideas related to composability from SwiftUI, allowing each feature of an application to possess a body property, similar to SwiftUI’s structure. These features are complemented by the use of the @Dependency property wrapper for specifying necessary dependencies.
The following code snippet illustrates how to implement a basic up-down counter functionality:
The example demonstrates how the app’s state and the user’s possible actions are defined using specific types, and how the body method updates the state corresponding to each action.
An important feature in TCA is state sharing, achieved through the @Shared macro. This is akin to SwiftUI’s @Binding that provides a publisher property for tracking changes to a reference across different parts of the application. The @Shared property wrapper also facilitates data storage in memory, user defaults, generic files, or through custom methods.
TCA primarily utilizes standard UI elements of SwiftUI, such as sheet(item:), popover(item:), and NavigationStack. However, it also features its own navigation mechanism that suits state-driven applications better. The framework endorses two key navigation styles: tree-based and stack-based navigation.
Adopting a library as the foundation for the architecture of an app can be quite a challenging decision to make, the library authors admit, but there are cases where it could be the best approach instead of trying to reimplement everything from scratch:
If a library’s core tenets align with your priorities for building your app, then adopting a library can be a sensible choice. It would be better to coalesce on a well-defined set of tools with a consistent history of maintenance and a strong community than to glue together many “tips and tricks” found in blog posts scattered around the Internet.
The Swift Composable Architecture can be cloned from GitHub or installed from the Swift Package Index. It has over 12,000 stars and 1,400 forks on GitHub with over 200 contributors, which makes it the most popular Swift architectural library among those listed on the Swift Package Index.
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