Swift Testing Scoping (SwiftUI)

Swift 6.1 adds test scoping to Swift Testing: a custom trait that runs code around a test or a whole suite. Pair it with a @TaskLocal dependency container and one annotation swaps the app's environment for mocks.

Setup without scoping repeats itself

Swift Testing uses init and deinit on a class suite for setup and teardown. Every suite that needs the same ModelContainer or seed data repeats that initializer.

class ModelTests {
  let container: ModelContainer

  init() throws {
    let config = ModelConfiguration(isStoredInMemoryOnly: true)
    container = try ModelContainer(for: User.self, configurations: config)
  }
}

A TaskLocal environment to swap

Hold dependencies in one struct and expose the live instance as a @TaskLocal with the production value as the default. Task locals can be overridden for the duration of one closure, which is exactly the shape a test scope needs.

struct Environment {
  let search: (String) async throws -> [String]
}

extension Environment {
  @TaskLocal static var current = Environment.production
}

The scoping trait

Conform to TestTrait, SuiteTrait, and TestScoping. The single requirement, provideScope, receives the test's performing function; you must call it or the test never runs. Wrap the call in the task-local override.

struct MockEnvironmentTrait: TestTrait, SuiteTrait, TestScoping {
  func provideScope(
    for test: Test,
    testCase: Test.Case?,
    performing function: @Sendable () async throws -> Void
  ) async throws {
    try await Environment.$current.withValue(Environment.mock) {
      try await function()
    }
  }
}

extension Trait where Self == MockEnvironmentTrait {
  static var mockedEnvironment: Self { Self() }
}

Apply per test or per suite

@Test(.mockedEnvironment) func verifySomething() async throws {
  Environment.current   // the mocked environment
}

@Suite(.mockedEnvironment) struct ExamplesTests { /* every test mocked */ }