protocol MovieLoader {
func load() async throws -> [Movie]
}
struct MovieList: View {
@State var movies = [Movie]()
let loader: MovieLoader
var body: some View {
List(movies, rowContent: MovieRow.init)
.task { await load() }
}
func load() async {
movies = (try? await loader.load()) ?? []
}
}
func test_load_loadsMovies() async {
let expected = [anyMovie()]
let stubbed = StubMovieLoader(stubs: expected)
let sut = MovieList(loader: stubbed)
await sut.load()
XCTAssertEqual(sut.movies, expected)
}
If you try to run this test as-is, it will fail.
Thehe reason is that @State in a SwiftUI view only connects to an in-memory store if the view has been mounted into a real view hierarchy. The most common workaround is to move the logic into an @Observable.
@Observable class MovieListViewModel {
var movies = [Movie]()
let loader: MovieLoader
init(loader: MovieLoader = RemoteMovieLoader()) { ... }
func load() async {
movies = (try? await loader.load()) ?? []
}
}
struct MovieList: View {
@State var vm = MovieListViewModel()
var body: some View {
List(vm.movies, content: MovieRow.init)
.task { await vm.load() }
}
}
func test_load_loadsMovies() async {
let expected = [anyMovie()]
let stubbed = StubMovieLoader(stubs: expected)
let sut = MovieListViewModel(loader: stubbed)
await sut.load()
XCTAssertEqual(sut.movies, expected)
}
This pattern, widely used and considered a de facto standard, comes with some drawbacks:
- We abandon value types purely for the sake of testability.
- We introduce reference types with all their baggage (lifecycle management and potential leaks) into a framework designed to be as value type-oriented as possible.
- We keep mixing UI state logic (how and when it updates) with the storage itself, despite introducing an extra object to abstract it — even though stateless MVVM implementations exist even in UIKit.
The solution, curiously, has been in SwiftUI from the start.
In this article I'd like to present an alternative, building on the work of Lazar Otasevic.
Binding
@State is not testable outside a view hierarchy, but @Binding offers a way to expose state for testing — something that seems to have gone largely unnoticed by the community.
Conceptually, a @Binding can be understood as a simple pair of closures (get and set):
struct Binding<Value> {
let get: () -> Value
let set: (Value) -> Void
}
Which makes a view using @Binding fully testable:
struct MovieList: View {
@Binding var movies: [Movie]
let loader: MovieLoader
var body: some View {
List(movies, rowContent: MovieRow.init)
.task { await load() }
}
func load() async {
movies = (try? await loader.load()) ?? []
}
}
func test_load_loadsMovies() async {
var storage = [Movie]()
let binding = Binding(get: { storage }, set: { storage = $0 })
let expected = [anyMovie()]
let stubLoader = StubMovieLoader(stubs: expected)
let sut = MovieList(movies: binding, loader: stubLoader)
await sut.load()
XCTAssertEqual(storage, expected)
}
The view no longer owns its state, becoming a purely functional component that delegates persistence to its ancestor.
SwiftUI is already a state engine;
@Bindingis simply the wire that lets us connect that engine to our unit tests.
We can build some helper utilities. Analogous to Apple's static Binding.constant(), we can have a Binding.variable():
extension Binding {
static func variable(_ initialValue: Value) -> Self {
var copy = initialValue
return Binding(get: { copy }, set: { copy = $0 })
}
}
func test_load_loadsMovies() async {
let expected = [anyMovie()]
let stubLoader = StubMovieLoader(stubs: expected)
let (sut, movies) = makeSUT(loader: stubLoader)
await sut.load()
XCTAssertEqual(movies(), expected)
}
func makeSUT(movies: [Movie] = [], loader: MovieLoader) -> (MovieList, () -> [Movie]) {
let movies = Binding.variable(movies)
let sut = MovieList(movies: movies, loader: loader)
return (sut, { movies.wrappedValue })
}
Binding.variable doesn't create a special container like @State ; it simply captures a local variable via closures. When the view writes:
movies = newValue
...it's actually executing the set closure, which mutates copy.
And when you read in the test:
movies.wrappedValue
...you're executing the get closure, which returns that same copy. The view and the test share the same storage, since both use the captured variable.
The view's state logic is fully testable. Whoever builds it needs to provide it with its state:
import Movies
import MoviesUI
// Composition Root
struct MovieListComposer: View {
@State var movies = [Movie]()
let loader: MovieLoader
var body: some View {
MovieList(movies: $movies, loader: loader)
}
}
We can also create a generic wrapper for views using this pattern:
struct Host<Initial, Content: View>: View {
typealias Binding = SwiftUI.Binding<Initial>
@State var state: Initial
let content: (Binding) -> Content
init(
_ state: Initial,
@ViewBuilder content: @escaping (Binding) -> Content
) {
self.state = state
self.content = content
}
var body: some View {
content($state)
}
}
struct SomeApp: App {
var body: some Scene {
WindowGroup {
Host([Movie]()) {
MovieList(movies: $0)
}
}
}
}
Conclusions and Considerations
This pattern enables a high level of testability while preserving the simplicity of SwiftUI's declarative system, without intermediate layers.
- You don't need a @observable ViewModel to test state logic in SwiftUI if you don't need idenitity for your specific use case (most cases).
@Statehas storage that is inaccessible outside the SwiftUI runtime.@Bindinghas no storage of its own and only represents access to external storage via closures.- By using
@Binding, we keep the view as a lightweight, testable struct, without forcing the creation of a class just to satisfy the test runner. - You can still use
@Observablefor state, since@Bindingis the communication interface, not the storage. This decouples the view from how the data is stored (whether in@State, an@Observable, or a property wrapper from CoreData/SwiftData/etc.). - Encapsulating logic in a dedicated struct for reuse across views and a more decoupled architecture is entirely possible. For more details, I recommend the the "True Logic: Stateless and Pure" section of this article from Lazar Otasevic. I've also published an example project that can serve as a reference: OnlyGoodMovies on how to architecture a project around this design pattern.