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Swift µFramework for synchronous testing of asynchronous Combine code.
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.swiftpm/xcode/package.xcworkspace Initial commit. Aug 9, 2019


Thresher is a Swift µFramework for synchronously testing asynchronous code using Apple's Combine framework.


Swift Package Manager (requires Xcode 11 or higher)

File -> Swift Packages -> Add Package Dependency and use the repository URL (

Manual Installation

Copy the TestScheduler.swift file into your project.


Normally, Publisher values are delivered to a Subscriber on the thread on which they are sent. However, you can specify an alternate Scheduler on which a Publisher's values should be sent to the Subscriber using receive(on:). For example, if you want the results of a network request to be delivered to subscribers on the main thread:

func loadImage(from url: URL) -> AnyPublisher<UIImage?, Never> {
    return URLSession.shared.dataTaskPublisher(for: url)
        .catch { _ in Empty<UIImage?, Never>() }
        .receive(on: DispatchQueue.main)

DispatchQueue conforms to the Combine Scheduler protocol.

Asynchronous code like the above snippet is fiddly to test. You have to use XCTestExpectation, dispatch your test code asynchronously and then wait for your expectation to be fulfilled:

func test_image_loading() {
    let url = URL(fileURLWithPath: "test-image.png")
    let expectation = XCTestExpectation()

    DispatchQueue.main.async {
        _ = loadImage(from: url)
            .sink { image in

    waitForExpectations(timeout: 2)

TestScheduler allows you to test asynchronous code in a synchronous way, which makes it much easier to write tests.

You need to add a receive(on:) call in your test code, so your testing function becomes:

func test_image_loading() {
    let url = URL(fileURLWithPath: "test-image.png")
    let scheduler = TestScheduler()
    var image: UIImage? = nil

    let _ = loadImage(from: url)
        .receive(on: scheduler)
        .sink { image = $0 }

    // schedule the subscription

    // wait for URLSession dataTaskPublisher to do its thing
    wait(seconds: 2)

    // process subscription results

func wait(seconds: TimeInterval) {
    let expectation = XCTestExpectation(description: "waiting") .now() + seconds) {
    wait(for: [expectation], timeout: seconds + 1)

Whilst this code is actually more verbose than the async code it is written in an synchronous manner:

  • create the publisher and setup the subcription
  • advance the scheduler so the subscription closure runs
  • check the results

To illustrate this synchronous nature, here is a test from ThresherTests:

func test_passthrough_subject_single_values() {
    let scheduler = TestScheduler()
    let subject = PassthroughSubject<Int, Never>()

    var ints: [Int] = []

    let _ = subject
        .receive(on: scheduler)
        .sink { value in

    // the act of subscription is also scheduled on the scheduler
    // so we need to advance the scheduler to make sure the subription
    // has occured

    XCTAssert(ints == [1])

    XCTAssert(ints == [1, 2])

We can individually advance the scheduler after each send and check the result of the subscription at each stage. These examples are rather contrived, but when testing more complex Combine code this level of fine grained control is very useful.

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