A Reactive Data-Binding and Dependency Injection Library for SwiftUI x Concurrency

by iOS Example

SwiftUI Atom Properties

A Reactive Data-Binding and Dependency Injection Libraryfor SwiftUI x Concurrency

? API Reference

Introduction

Reactive Data Binding Effective Data Caching Compile SafeDependency Injection
Piece of app data that can be accessed from anywhere propagates changes reactively. Recompute atom data and views only when truly need, otherwise it caches data until no longer used. Successful compilation guarantees that dependency injection is ready.

SwiftUI Atom Properties offers practical capabilities to manage the complexity of modern apps. It effectively integrates the solution for both data-binding and dependency injection while allowing us to rapidly building an application.

Motivation

SwiftUI offers a simple and understandable data-binding solution with built-in property wrappers, but is a little uneasiness for building middle to large scale production apps. As a typical example, view data can only be shared by pushing it up to a common ancestor. Software development is not all set in advance; it evolves over time to meet business and customer needs. Therefore, you may need to radically redesign it so that local data used only in one part of the view-tree can be shared elsewhere, as the app grows. EnvironmentObject was hoped to be a solution to the problem, but it ended up with let us to create a huge state-holder object – Big Ball of Mud being provided from the root of an app, so it could not be an ideal. Ultimately, pure SwiftUI needs state-drilling from the root to descendants in anyway, which not only makes code-splitting difficult, but also causes gradual performance degradation due to the huge view-tree computation as the app grow up.

This library solves these problems by defining application data as distributed pieces called atom, allowing data to be shared throughout the app as the source of truth. That said, atom itself doesn’t have internal state, but rather retrieves the associated state from the context in which they are used, and ensures that the app is testable. Furthermore, it manages a directed graph of atoms and propagates data changes transitively from upstream to downstream, such that it updates only the views truly need update while preventing expensive data recomputation, resulting in effortlessly high performance and efficient memory use.

This approach guarantees the following principles:

  • Reactively reflects data changes into views.
  • Boilerplate-free interface where shared data has the same simple interface as SwiftUI built-ins.
  • Compatible with any other libraries (e.g. TCA) of your choice if needed.
  • Accelerates code-splitting by distributed & incremental state definition.
  • Ensures testable code over time with capabilities of dependency injection.
  • Provides simplified interfaces for asynchronous process.
  • Swift Concurrency based thread-safety.

Quick Overview

To get a feel for this library, let’s first look at the state management for a tiny counter app.

The CounterAtom in the example below represents the shared data of a mutable count value.

struct CounterAtom: StateAtom, Hashable {
    func defaultValue(context: Context) -> Int {
        0
    }
}

Bind the atom to the view using @WatchState property wrapper so that it can obtain the value and write new values.

struct CountStepper: View {
    @WatchState(CounterAtom())
    var count

    var body: some View {
        Stepper(value: $count) {}
    }
}

@Watch property wrapper obtains the atom value read-only. Now that the app can share the state among multiple views without passing it down through initializer.

struct CounterView: View {
    @Watch(CounterAtom())
    var count

    var body: some View {
        VStack {
            Text("Count: \(count)")
            CountStepper()
        }
    }
}

If you like the principles, see the sample apps and the basic tutorial to learn more about this library.

Examples
  • Counter Demonstrates the minimum app using this library.
  • Todo A simple todo app that has user interactions, showing how multiple atoms interact with each other.
  • The Movie DB Demonstrates practical usage which close to a real-world app, using TMDB API for asynchronous networking.
  • Map A simple but effective app that demonstrates how to wrap a framework in this library.
  • Voice Memo Demonstrates how to manage complex state with multiple dependencies using MVVM pattern on an atom. Created with imitate TCA’s example.
  • Time Travel A simple demo that demonstrates how to do time travel debugging with this library.

Each example has test target to show how to test your atoms with dependency injection as well. Open Examples/App.xcodeproj and play around with it!

Getting Started

Requirements

Minimum Version
Swift 5.6
Xcode 13.3
iOS 14.0
macOS 11.0
tvOS 14.0
watchOS 7.0

Installation

The module name of the package is Atoms. Choose one of the instructions below to install and add the following import statement to your source code.

import Atoms

Xcode Package Dependency

From Xcode menu: File > Swift Packages > Add Package Dependency

https://github.com/ra1028/swiftui-atom-properties

Swift Package Manager

In your Package.swift file, first add the following to the package dependencies:

.package(url: "https://github.com/ra1028/swiftui-atom-properties"),

And then, include “Atoms” as a dependency for your target:

.target(name: "<target>", dependencies: [
    .product(name: "Atoms", package: "swiftui-atom-properties"),
]),

Documentation

Basic Tutorial

In this tutorial, we will create a simple todo app as an example. This app will support:

  • Create todo items
  • Edit todo items
  • Filter todo items

Every view that uses atom must have an AtomRoot somewhere in the ancestor. In SwiftUI lifecycle apps, it’s recommended to put it right under WindowGroup.

@main
struct TodoApp: App {
    var body: some Scene {
        WindowGroup {
            AtomRoot {
                TodoList()
            }
        }
    }
}

First, define a todo structure and an enum to filter todo list, and declare state with StateAtom that represents a mutable value.

struct Todo {
    var id: UUID
    var text: String
    var isCompleted: Bool
}

enum Filter: CaseIterable, Hashable {
    case all, completed, uncompleted
}

struct TodosAtom: StateAtom, Hashable {
    func defaultValue(context: Context) -> [Todo] {
        []
    }
}

struct FilterAtom: StateAtom, Hashable {
    func defaultValue(context: Context) -> Filter {
        .all
    }
}

The FilteredTodosAtom below represents the derived data that combines the above two atoms. You can think of derived data as the output of passing values to a pure function that derives a new value from the depending values.

When dependent data changes, the derived data reactively updates, and the output value is cached until it truly needs to be updated, so don’t need to worry about low performance due to the filter function being called each time the view recomputes.

struct FilteredTodosAtom: ValueAtom, Hashable {
    func value(context: Context) -> [Todo] {
        let filter = context.watch(FilterAtom())
        let todos = context.watch(TodosAtom())

        switch filter {
        case .all:         return todos
        case .completed:   return todos.filter(\.isCompleted)
        case .uncompleted: return todos.filter { !$0.isCompleted }
        }
    }
}

To create a new todo item, you need to access to a writable value that update the value of TodosAtom we defined previously. We can use @WatchState property wrapper to obtain a read-write access to it.

struct TodoCreator: View {
    @WatchState(TodosAtom())
    var todos

    @State
    var text = ""

    var body: some View {
        HStack {
            TextField("Enter your todo", text: $text)
            Button("Add") {
                todos.append(Todo(id: UUID(), text: text, isCompleted: false))
                text = ""
            }
        }
    }
}

Similarly, build a view to switch the value of FilterAtom. Get a Binding to the value exposed by @WatchState using $ prefix.

struct TodoFilters: View {
    @WatchState(FilterAtom())
    var current

    var body: some View {
        Picker("Filter", selection: $current) {
            ForEach(Filter.allCases, id: \.self) { filter in
                switch filter {
                case .all:         Text("All")
                case .completed:   Text("Completed")
                case .uncompleted: Text("Uncompleted")
                }
            }
        }
        .pickerStyle(.segmented)
    }
}

Next, create a view to display and edit individual todo items.

struct TodoItem: View {
    @WatchState(TodosAtom())
    var allTodos

    @State
    var text: String

    @State
    var isCompleted: Bool

    let todo: Todo

    init(todo: Todo) {
        self.todo = todo
        self._text = State(initialValue: todo.text)
        self._isCompleted = State(initialValue: todo.isCompleted)
    }

    var index: Int {
        allTodos.firstIndex { $0.id == todo.id }!
    }

    var body: some View {
        Toggle(isOn: $isCompleted) {
            TextField("Todo", text: $text) {
                allTodos[index].text = text
            }
        }
        .onChange(of: isCompleted) { isCompleted in
            allTodos[index].isCompleted = isCompleted
        }
    }
}

Use @Watch to obtain the value of FilteredTodosAtom read-only. Updates to any of the dependent atoms are propagated to this view, and it re-render the todo list. Finally, assemble the views we’ve created so far and complete.

struct TodoList: View {
    @Watch(FilteredTodosAtom())
    var filteredTodos

    var body: some View {
        List {
            TodoCreator()
            TodoFilters()

            ForEach(filteredTodos, id: \.id) { todo in
                TodoItem(todo: todo)
            }
        }
    }
}

That is the basics for building apps using SwiftUI Atom Properties, but even asynchronous processes and more complex state management can be settled according to the same steps. See Guides section for more detail. Also, the Examples directory has several projects to explore concrete usage.

Guides

This section introduces the available APIs and their uses. To look into the APIs in more detail, visit the API referrence.

AtomRoot

Provides the internal store which provides atoms to view-tree through environment values. It must be the root of any views to manage atoms used throughout the application.

@main
struct ExampleApp: App {
    var body: some Scene {
        WindowGroup {
            AtomRoot {
                ExampleView()
            }
        }
    }
}

Atoms

An atom represents a piece of state and is the source of truth for your app. It can also represent a derived data by combining and transforming one or more other atoms. Each atom does not actually have a global data inside, and retrieve values from the internal store provided by the AtomRoot. That’s why they can be accessed from anywhere, but never lose testability.

An atom and its value are associated using a unique key which is automatically defined if the atom conforms to Hashable, but you can also define it explicitly without Hashable.

struct UserNameAtom: StateAtom {
   let userID: Int

   var key: Int {
       userID
   }

   func defaultValue(context: Context) -> String {
       "Robert"
   }
}

In order to provide the best interface and effective data-binding for the type of the resulting values, there are several variants of atoms as following.

ValueAtom

? Click to expand example code 

struct LocaleAtom: ValueAtom, Hashable {
    func value(context: Context) -> Locale {
        .current
    }
}

struct LocaleView: View {
    @Watch(LocaleAtom())
    var locale

    var body: some View {
        Text(locale.identifier)
    }
}
Description
Summary Provides a read-only value.
Output T
Use Case Computed property, Derived data, Dependency injection

StateAtom

? Click to expand example code 

struct CounterAtom: StateAtom, Hashable {
    func defaultValue(context: Context) -> Int {
        0
    }

    // Does nothing by default.
    func willSet(newValue: Int, oldValue: Int, context: Context) {
        print("Will change")
    }

    // Does nothing by default.
    func didSet(newValue: Int, oldValue: Int, context: Context) {
        print("Did change")
    }
}

struct CounterView: View {
    @WatchState(CounterAtom())
    var count

    var body: some View {
        Stepper("Count: \(count)", value: $count)
    }
}
Description
Summary Provides a read-write data.
Output T
Use Case Mutable data, Derived data

TaskAtom

? Click to expand example code 

struct FetchUserAtom: TaskAtom, Hashable {
    func value(context: Context) async -> User? {
        await fetchUser()
    }
}

struct UserView: View {
    @Watch(FetchUserAtom())
    var userTask

    var body: some View {
        Suspense(userTask) { user in
            Text(user?.name ?? "Unknown")
        }
    }
}
Description
Summary Initiates a non-throwing Task from the given async function.
Output Task<T, Never>
Use Case Non-throwing asynchronous operation e.g. Expensive calculation

ThrowingTaskAtom

? Click to expand example code 

struct FetchMoviesAtom: ThrowingTaskAtom, Hashable {
    func value(context: Context) async throws -> [Movie] {
        try await fetchMovies()
    }
}

struct MoviesView: View {
    @Watch(FetchMoviesAtom())
    var moviesTask

    var body: some View {
        List {
            Suspense(moviesTask) { movies in
                ForEach(movies, id: \.id) { movie in
                    Text(movie.title)
                }
            } catch: { error in
                Text(error.localizedDescription)
            }
        }
    }
}
Description
Summary Initiates a throwing Task from the given async throws function.
Output Task<T, Error>
Use Case Throwing asynchronous operation e.g. API call

AsyncSequenceAtom

? Click to expand example code 

struct NotificationAtom: AsyncSequenceAtom, Hashable {
    let name: Notification.Name

    func sequence(context: Context) -> NotificationCenter.Notifications {
        NotificationCenter.default.notifications(named: name)
    }
}

struct NotificationView: View {
    @Watch(NotificationAtom(name: UIApplication.didBecomeActiveNotification))
    var notificationPhase

    var body: some View {
        switch notificationPhase {
        case .suspending, .failure:
            Text("Unknown")

        case .success:
            Text("Active")
        }
    }
}
Description
Summary Provides a AsyncPhase value that represents asynchronous, sequential elements of the given AsyncSequence.
Output AsyncPhase<T, Error>
Use Case Handle multiple asynchronous values e.g. web-sockets

PublisherAtom

? Click to expand example code 

struct TimerAtom: PublisherAtom, Hashable {
    func publisher(context: Context) -> AnyPublisher<Date, Never> {
        Timer.publish(every: 1, on: .main, in: .default)
            .autoconnect()
            .eraseToAnyPublisher()
    }
}

struct TimerView: View {
    @Watch(TimerAtom())
    var timerPhase

    var body: some View {
        if let date = timerPhase.value {
            Text(date.formatted(date: .numeric, time: .shortened))
        }
    }
}
Description
Summary Provides a AsyncPhase value that represents sequence of values of the given Publisher.
Output AsyncPhase<T, E: Error>
Use Case Handle single or multiple asynchronous value(s) e.g. API call

ObservableObjectAtom

? Click to expand example code 

class Contact: ObservableObject {
    @Published var name = ""
    @Published var age = 20

    func haveBirthday() {
        age += 1
    }
}

struct ContactAtom: ObservableObjectAtom, Hashable {
    func object(context: Context) -> Contact {
        Contact()
    }
}

struct ContactView: View {
    @WatchStateObject(ContactAtom())
    var contact

    var body: some View {
        VStack {
            TextField("Enter your name", text: $contact.name)
            Text("Age: \(contact.age)")
            Button("Celebrate your birthday!") {
                contact.haveBirthday()
            }
        }
    }
}
Description
Summary Instantiates an observable object.
Output T: ObservableObject
Use Case Mutable complex state object

Modifiers

Modifiers can be applied to an atom to produce a different versions of the original atom to make it more coding friendly or to reduce view re-computation for performance optimization.

select(_:)

? Click to expand example code 

struct CountAtom: StateAtom, Hashable {
    func defaultValue(context: Context) -> Int {
        12345
    }
}

struct CountDisplayView: View {
    @Watch(CountAtom().select(\.description))
    var description  // : String

    var body: some View {
        Text(description)
    }
}
Description
Summary Selects a partial property with the specified key path from the original atom. The selected property doesn’t notify updates if the new value is equivalent to the old value.
Output T: Equatable
Compatible All atoms types. The selected property must be Equatable compliant.
Use Case Performance optimization, Property scope restriction

phase

? Click to expand example code 

struct FetchWeatherAtom: ThrowingTaskAtom, Hashable {
    func value(context: Context) async throws -> Weather {
        try await fetchWeather()
    }
}

struct WeatherReportView: View {
    @Watch(FetchWeatherAtom().phase)
    var weatherPhase  // : AsyncPhase<Weather, Error>

    var body: some View {
        switch weatherPhase {
        case .suspending:
            Text("Loading.")

        case .success(let weather):
            Text("It's \(weather.description) now!")

        case .failure:
            Text("Failed to get weather data.")
        }
    }
}
Description
Summary Converts the Task that the original atom provides into AsyncPhase.
Output AsyncPhase<T, E: Error>
Compatible TaskAtom, ThrowingTaskAtom
Use Case Consume asynchronous result as AsyncPhase

Property Wrappers

The following property wrappers are used to bind atoms to view and recompute the view with data changes. By retrieving the atom through these property wrappers, the internal system marks the atom as in-use and the values are cached until that view is dismantled.

@Watch

? Click to expand example code 

struct UserNameAtom: StateAtom, Hashable {
    func defaultValue(context: Context) -> String {
        "John"
    }
}

struct UserNameDisplayView: View {
    @Watch(UserNameAtom())
    var name

    var body: some View {
        Text("User name: \(name)")
    }
}
Description
Summary This property wrapper is similar to @State or @Environment, but is always read-only. It recomputes the view with value changes.
Compatible All atom types

@WatchState

? Click to expand example code 

struct UserNameAtom: StateAtom, Hashable {
    func defaultValue(context: Context) -> String {
        "Jim"
    }
}

struct UserNameInputView: View {
    @WatchState(UserNameAtom())
    var name

    var body: some View {
        VStack {
            TextField("User name", text: $name)
            Button("Clear") {
                name = ""
            }
        }
    }
}
Description
Summary This property wrapper is read-write as the same interface as @State. It recomputes the view with data changes. You can get a Binding to the value using $ prefix.
Compatible StateAtom

@WatchStateObject

? Click to expand example code 

class Counter: ObservableObject {
    @Published var count = 0

    func plus(_ value: Int) {
        count += value
    }
}

struct CounterAtom: ObservableObjectAtom, Hashable {
    func object(context: Context) -> Counter {
        Counter()
    }
}

struct CounterView: View {
    @WatchStateObject(CounterObjectAtom())
    var counter

    var body: some View {
        VStack {
            Text("Count: \(counter.count)")
            Stepper(value: $counter.count) {}
            Button("+100") {
                counter.plus(100)
            }
        }
    }
}
Description
Summary This property wrapper has the same interface as @StateObject and @ObservedObject. It recomputes the view when the observable object updates. You can get a Binding to one of the observable object’s properties using $ prefix.
Compatible ObservableObjectAtom

@ViewContext

? Click to expand example code 

struct FetchBookAtom: ThrowingTaskAtom, Hashable {
    let id: Int

    func value(context: Context) async throws -> Book {
        try await fetchBook(id: id)
    }
}

struct BookView: View {
    @ViewContext
    var context

    let id: Int

    var body: some View {
        let task = context.watch(FetchBookAtom(id: id))

        Suspense(task) { book in
            Text(book.content)
        } suspending: {
            ProgressView()
        }
    }
}

Unlike the property wrappers described the above, this property wrapper is not intended to bind single atom. It provides an AtomViewContext to the view, allowing for more functional control of atoms. For instance, the following controls can only be done through the context.

  • refresh(_:) operator that to reset an asynchronous atom value and wait for its completion.
await context.refresh(FetchMoviesAtom())
  • reset(_:) operator that to clear the current atom value.
context.reset(CounterAtom())

The context also provides a flexible solution for passing dynamic parameters to atom’s initializer. See Contexts section for more detail.

Contexts

Contexts are context structure for using and interacting with the data of other atoms from a view or an another atom. The basic API common to all contexts is as follows:

API Use
watch(_:) Obtains an atom value and starts watching its update.
read(_:) Obtains an atom value but does not watch its update.
set(_:for:) Sets a new value to the atom.
[:_] subscript Read-write access for applying mutating methods.
state(_:) Gets a binding to the atom state.
refresh(_:) Reset an atom and await until asynchronous operation is complete.
reset(_:) Reset an atom to the default value or a first output.

There are the following types context as different contextual environments, and they have some specific APIs for each.

AtomViewContext

? Click to expand example code 

struct SearchQueryAtom: StateAtom, Hashable {
    func defaultValue(context: Context) -> String {
        ""
    }
}

struct FetchBooksAtom: ThrowingTaskAtom, Hashable {
    func value(context: Context) async throws -> [Book] {
        let query = context.watch(SearchQueryAtom())
        return try await fetchBooks(query: query)
    }
}

struct BooksView: View {
    @ViewContext
    var context: AtomViewContext

    var body: some View {
        // watch
        let booksTask = context.watch(FetchBooksAtom())     // Task<[Book], Error>
        // state
        let searchQuery = context.state(SearchQueryAtom())  // Binding<String>

        List {
            Suspense(booksTask) { books in
                ForEach(books, id: \.isbn) { book in
                    Text("\(book.title): \(book.isbn)")
                }
            }
        }
        .searchable(text: searchQuery)
        .refreshable { [context] in  // NB: Unfortunately, SwiftUI has a memory leak when capturing `self` implicitly inside a `refreshable` modifier.
            // refresh
            await context.refresh(FetchBooksAtom())
        }
        .toolbar {
            ToolbarItem(placement: .bottomBar) {
                HStack {
                    Button("Reset") {
                        // reset
                        context.reset(SearchQueryAtom())
                    }
                    Button("All") {
                        // set
                        context.set("All", for: SearchQueryAtom())
                    }
                    Button("Space") {
                        // subscript
                        context[SearchQueryAtom()].append(" ")
                    }
                    Button("Print") {
                        // read
                        let query = context.read(SearchQueryAtom())
                        print(query)
                    }
                }
            }
        }
    }
}

Context available through the @ViewContext property wrapper when using atoms from a view. There is no specific API for this context.

AtomRelationContext

? Click to expand example code 

class LocationManagerDelegate: NSObject, CLLocationManagerDelegate { ... }

struct LocationManagerAtom: ValueAtom, Hashable {
    func value(context: Context) -> LocationManagerProtocol {
        let manager = CLLocationManager()
        let delegate = LocationManagerDelegate()

        manager.delegate = delegate
        context.addTermination(manager.stopUpdatingLocation)
        context.keepUntilTermination(delegate)

        return manager
    }
}

struct CoordinateAtom: ValueAtom, Hashable {
    func value(context: Context) -> CLLocationCoordinate2D? {
        let manager = context.watch(LocationManagerAtom())
        return manager.location?.coordinate
    }
}

Context passed as a parameter to the primary function of each atom type.

API Use
addTermination(_:) Calls the passed closure when the atom is updated or is no longer used.
keepUntilTermination(_:) Retains the given object instance until the atom is updated or is no longer used.

AtomTestContext

? Click to expand example code 

protocol APIClientProtocol {
    func fetchMusics() async throws -> [Music]
}

struct APIClient: APIClientProtocol { ... }
struct MockAPIClient: APIClientProtocol { ... }

struct APIClientAtom: ValueAtom, Hashable {
    func value(context: Context) -> APIClientProtocol {
        APIClient()
    }
}

struct FetchMusicsAtom: ThrowingTaskAtom, Hashable {
    func value(context: Context) async throws -> [Music] {
        let api = context.watch(APIClientAtom())
        return try await api.fetchMusics()
    }
}

@MainActor
class FetchMusicsTests: XCTestCase {
    func testFetchMusicsAtom() async throws {
        let context = AtomTestContext()

        context.override(APIClientAtom()) { _ in
            MockAPIClient()
        }

        let musics = try await context.watch(FetchMusicsAtom()).value

        XCTAssertTrue(musics.isEmpty)
    }
}

Context that can simulate any scenarios in which atoms are used from a view or another atom and provides a comprehensive means of testing.

API Use
unwatch(_:) Simulates a scenario in which the atom is no longer watched.
override(_:with:) Overwrites the output of a specific atom or all atoms of the given type with the fixed value.
observe(_:) Observes changes in any atom values and its lifecycles.
onUpdate Sets a closure that notifies there has been an update to one of the atoms.

KeepAlive

KeepAlive allows the atom to preserve its data even if it’s no longer watched to from anywhere. In the example case below, once master data is obtained from the server, it can be cached in memory until the app process terminates.

struct FetchMasterDataAtom: ThrowingTaskAtom, KeepAlive, Hashable {
    func value(context: Context) async throws -> MasterData {
        try await fetchMasterData()
    }
}

Suspense

Suspense awaits the resulting value of the given Task and displays the content depending on its phase. Optionally, you can pass suspending content to be displayed until the task completes, and pass catch content to be displayed if the task fails.

struct NewsView: View {
    @Watch(LatestNewsAtom())
    var newsTask: Task<News, Error>

    var body: some View {
        Suspense(newsTask) { news in
            Text(news.content)
        } suspending: {
            ProgressView()
        } catch: { error in
            Text(error.localizedDescription)
        }
    }
}

Testing

This library naturally integrates dependency injection and data-binding to provide a comprehensive means of testing. It allows you to test per small atom such that you can keep writing simple test cases per smallest unit of state without compose all states into a huge object and supposing complex integration test scenarios. In order to fully test your app, this library guarantees the following principles:

  • Hermetic environment that no data is shared between test cases.
  • Dependencies are replaceable with any of mock/stub/fake/spy per test case.
  • Test cases can reproduce any possible scenarios at the view-layer.

In the test case, you first create an AtomTestContext instance that behaves similarly to other context types. The context allows for flexible reproduction of expected scenarios for testing using the control functions described in the Contexts section. In addition, it’s able to replace the atom value with test-friendly dependencies with override function. It helps you to write a reproducible & stable testing. Since atom needs to be used from the main actor to guarantee thread-safety, XCTestCase class that to test atoms should have @MainActor attribute.

Click to expand the classes to be tested 

struct Book: Equatable {
    var title: String
    var isbn: String
}

protocol APIClientProtocol {
    func fetchBook(isbn: String) async throws -> Book
}

struct APIClient: APIClientProtocol {
    func fetchBook(isbn: String) async throws -> Book {
        ... // Networking logic.
    }
}

class MockAPIClient: APIClientProtocol {
    var response: Book?

    func fetchBook(isbn: String) async throws -> Book {
        guard let response = response else {
            throw URLError(.unknown)
        }
        return response
    }
}

struct APIClientAtom: ValueAtom, Hashable {
    func value(context: Context) -> APIClientProtocol {
        APIClient()
    }
}

struct FetchBookAtom: ThrowingTaskAtom, Hashable {
    let isbn: String

    func value(context: Context) async throws -> Book {
        let api = context.watch(APIClientAtom())
        return try await api.fetchBook(isbn: isbn)
    }
}


@MainActor
class FetchBookTests: XCTestCase {
    func testFetch() async throws {
        let context = AtomTestContext()
        let api = MockAPIClient()

        // Override the atom value with the mock instance.
        context.override(APIClientAtom()) { _ in
            api
        }

        let expected = Book(title: "A book", isbn: "ISBN000–0–0000–0000–0")

        // Inject the expected response to the mock.
        api.response = expected

        let book = try await context.watch(FetchBookAtom(isbn: "ISBN000–0–0000–0000–0")).value

        XCTAssertEqual(book, expected)
    }
}

Preview

Even in SwiftUI previews, the view must have an AtomRoot somewhere in the ancestor. However, since This library offers the new solution for dependency injection, you don’t need to do painful DI each time you create previews anymore. You can to override the atoms that you really want to inject substitutions.

struct NewsList_Preview: PreviewProvider {
    static var previews: some View {
        AtomRoot {
            NewsList()
        }
        .override(APIClientAtom()) { _ in
            StubAPIClient()
        }
    }
}

Observability

? Click to expand example code 

struct Logger: AtomObserver {
    func atomAssigned<Node: Atom>(atom: Node) {
        print("\(atom) started to be used somewhere.")
    }

    func atomUnassigned<Node: Atom>(atom: Node) {
        print("\(atom) is no longer used.")
    }

    func atomChanged<Node: Atom>(snapshot: Snapshot<Node>) {
        print("The value of `\(snapshot.atom)` is changed to `\(snapshot.value)`.")
    }
}

@main
struct ExampleApp: App {
    var body: some Scene {
        WindowGroup {
            AtomRoot {
                VStack {
                    NavigationLink("Home") {
                        Home()
                    }

                    NavigationLink("Setting") {
                        AtomRelay {
                            Setting()
                        }
                        .observe(Logger())  // Observes setting related atoms only.
                    }
                }
            }
            .observe(Logger())  // Observes all atoms used in the app.
        }
    }
}

You can monitor the updates and lifecycle of atoms used in your app by registering an AtomObserver compliant instance through the observe(_:) function in AtomRoot or AtomRelay. Registering an observer in AtomRoot observes all atoms used in the app, but in contrast, using AtomRelay can observe partial atoms that used in the descendant views. In addition, this observability can be applied to do time travel debugging and is demonstrated in one of the examples.

Advanced Usage

Obtain an atom value without watching to it

? Click to expand example code 

struct TextAtom: StateAtom, Hashable {
    func value(context: Context) -> String {
        ""
    }
}

struct TextCopyView: View {
    @ViewContext
    var context

    var body: some View {
        Button("Copy") {
            UIPasteboard.general.string = context.read(TextAtom())
        }
    }
}

The read(_:) function is a way to get the data of an atom without having watch to and receiving future updates of it. It’s commonly used inside functions triggered by call-to-actions.

Dynamically initiate an atom with external parameters

? Click to expand example code 

struct FetchUserAtom: ThrowingTaskAtom {
    let id: Int

    // This atom can also conforms to `Hashable` in this case,
    // but this example specifies the key explicitly.
    var key: Int {
        id
    }

    func value(context: Context) async throws -> Value {
        try await fetchUser(id: id)
    }
}

struct UserView: View {
    let id: Int

    @ViewContext
    var context

    var body: some View {
        let task = context.watch(FetchUserAtom(id: id))

        Suspense(task) { user in
            VStack {
                Text("Name: \(user.name)")
                Text("Age: \(user.age)")
            }
        }
    }
}

Each atom must have a unique key to be uniquely associated with its value. As described in the Atoms section, it is automatically synthesized by conforming to Hashable, but with explicitly specifying a key allowing you to pass arbitrary external parameters to the atom. It is commonly used, for example, to retrieve user information associated with a dynamically specified ID from a server.

Pass a context to your object to interact with other atoms

? Click to expand example code 

@MainActor
class MessageLoader: ObservableObject {
    let context: AtomContext

    @Published
    var phase = AsyncPhase<[Message], Error>.suspending

    init(context: AtomContext) {
        self.context = context
    }

    func load() async {
        do {
            let api = context.read(APIClientAtom())
            let messages = try await api.fetchMessages(offset: 0)
            phase = .success(messages)
        }
        catch {
            phase = .failure(error)
        }
    }

    func loadNext() async {
        guard let messages = phase.value else {
            return
        }

        do {
            let api = context.read(APIClientAtom())
            let next = try await api.fetchMessages(offset: messages.count)
            phase = .success(messages + next)
        }
        catch {
            phase = .failure(error)
        }
    }
}

struct MessageLoaderAtom: ObservableObjectAtom, Hashable {
    func object(context: Context) -> MessageLoader {
        MessageLoader(context: context)
    }
}

You can pass a context to your object and interact with other atoms at any asynchronous timing. However, in that case, when the watch is called, it end up with the object instance itself will be re-created with fresh data. Therefore, you can explicitly prevent the use of the watch by passing it as AtomContext type.

Dealing with Known SwiftUI Bugs

In iOS14, modal presentation causes assertionFailure when dismissing it

? Click to expand workaround 

struct RootView: View {
    @State
    var isPresented = false

    @ViewContext
    var context

    var body: some View {
        VStack {
            Text("Example View")
        }
        .sheet(isPresented: $isPresented) {
            AtomRelay(context) {
                MailView()
            }
        }
    }
}

Unfortunately, SwiftUI has a bug in iOS14 where the EnvironmentValue is removed from a screen presented with .sheet just before dismissing it. Since this library is designed based on EnvironmentValue, this bug end up triggering the friendly assertionFailure that is added so that developers can easily aware of forgotten AtomRoot implementation. As a workaround, AtomRelay has the ability to explicitly inherit the internal store through AtomViewContext from the parent view.

Some SwiftUI modifiers cause memory leak

? Click to expand workaround 

@ViewContext
var context

...

.refreshable { [context] in
    await context.refresh(FetchDataAtom())
}

@State
var isShowingSearchScreen = false

...

.onSubmit { [$isShowingSearchScreen] in
    $isShowingSearchScreen.wrappedValue = true
}

Some modifiers in SwiftUI seem to cause an internal memory leak if it captures self implicitly or explicitly. To avoid that bug, make sure that self is not captured when using those modifiers. Below are the list of modifiers I found that cause memory leaks:

Contributing

Any type of contribution is welcome! e.g.

  • Give it star ⭐ & fork this repository.
  • Report bugs with reproducible steps.
  • Propose new features.
  • Add more documentations.
  • Provide repos of sample apps using this library.
  • Become a maintainer after making multiple contributions.
  • Become a sponsor.

Acknowledgements

License

MIT © Ryo Aoyama

GitHub

View Github