ActorIsolated

This type is most useful when writing tests for when you want to inspect what happens inside an async operation.

For example, suppose you have a feature such that when a button is tapped you track some analytics:

struct AnalyticsClient {
  var track: (String) async -> Void
}

class FeatureModel: ObservableObject {
  let analytics: AnalyticsClient
  // ...
  func buttonTapped() {
    // ...
    await self.analytics.track("Button tapped")
  }
}

Then, in tests we can construct an analytics client that appends events to a mutable array rather than actually sending events to an analytics server. However, in order to do this in a safe way we should use an actor, and ActorIsolated makes this easy:

func testAnalytics() async {
  let events = ActorIsolated<[String]>([])
  let analytics = AnalyticsClient(
    track: { event in await events.withValue { $0.append(event) } }
  )
  let model = FeatureModel(analytics: analytics)
  model.buttonTapped()
  await events.withValue {
    XCTAssertEqual($0, ["Button tapped"])
  }
}

To synchronously isolate a value, see LockIsolated.