init(_:)

Useful as a kind of type eraser for live AsyncSequence-based dependencies.

For example, your feature may want to subscribe to screenshot notifications. You can model this as a dependency client that returns an AsyncStream:

struct ScreenshotsClient {
  var screenshots: () -> AsyncStream<Void>
  func callAsFunction() -> AsyncStream<Void> { self.screenshots() }
}

The “live” implementation of the dependency can supply a stream by erasing the appropriate NotificationCenter.Notifications async sequence:

extension ScreenshotsClient {
  static let live = Self(
    screenshots: {
      AsyncStream(
        NotificationCenter.default
          .notifications(named: UIApplication.userDidTakeScreenshotNotification)
          .map { _ in }
      )
    }
  )
}

While your tests can use AsyncStream.makeStream to spin up a controllable stream for tests:

func testScreenshots() {
  let screenshots = AsyncStream.makeStream(of: Void.self)

  let model = withDependencies {
    $0.screenshots = { screenshots.stream }
  } operation: {
    FeatureModel()
  }

  XCTAssertEqual(model.screenshotCount, 0)
  screenshots.continuation.yield()  // Simulate a screenshot being taken.
  XCTAssertEqual(model.screenshotCount, 1)
}