Can you use async/await in swift along with a test scheduler?

Question

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We use the Pointfree Combine Schedulers , specifically theTestScheduler , extensively in our unit tests.

Is there a way to use this same library, or a similar test scheduler library, with the new async/await in Swift? Basically, can you programmatically move and advance through time in the reactive streams produced by async/await?

Why might we want this? As Pointfree describes:

This scheduler is useful for testing how the flow of time effects publishers that use asynchronous operators, such as debounce, throttle, delay, timeout, receive(on:), subscribe(on:) and more

It appears that Swift has recently introduced Swift Async Algorithms that include support for many of these operators. How does the community test these operators?

A similar question was asked a few months back, but the solution seems to propose waiting for the actual amount of time. Obviously if you have a 10 or 30 second timeout, one does not want to literally wait 10 or 30 seconds in their test.

Answer 1

If I understand your question correctly, you are looking into a way to test async/await code. If that's correct, let's image you have the following DataDownloader which will execute an async function to download something:

struct DataDownloader {
    func downloadData() async throws -> Data {
        ...
    }
}

In order to call that function we will need to use await , which would basically mark that the calls need to happen in a concurrency supported context. Fortunately Apple's XCTest has been upgraded to support that. We don't need to wrap it in a Task . So the trick is that in your test you can mark any function as async and you will be use the await . A simple example below how would you call the Downloader async function in a test context:

class DataDownloaderTests: XCTestCase {
    func testDataDownload() async throws {
        let dataDownloader = DataDownloader()
        let downloadedData = try await dataDownloader.downloadData()

        XCTAssertNil(resizedImage.size)
    }
}

Answer 2

func AsyncFunction() async -> Int {
  let result = await AsyncOperation()
  return result
}

let scheduler = TestScheduler()
let result = try scheduler.start {
  return try AsyncFunction().wait()
}

XCTAssertEqual(result, 42)