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Unit Tests

Testing custom operators

RxSwift uses RxTest for all operator tests, located in the AllTests-* target inside the project Rx.xcworkspace.

This is an example of a typical RxSwift operator unit test:

func testMap_Range() {
    // Initializes test scheduler.
    // Test scheduler implements virtual time that is
    // detached from local machine clock.
    // This enables running the simulation as fast as possible
    // and proving that all events have been handled.
    let scheduler = TestScheduler(initialClock: 0)
    // Creates a mock hot observable sequence.
    // The sequence will emit events at designated
    // times, no matter if there are observers subscribed or not.
    // (that's what hot means).
    // This observable sequence will also record all subscriptions
    // made during its lifetime (`subscriptions` property).
    let xs = scheduler.createHotObservable([
        .next(150, 1),  // first argument is virtual time, second argument is element value
        .next(210, 0),
        .next(220, 1),
        .next(230, 2),
        .next(240, 4),
        .completed(300) // virtual time when completed is sent
    // `start` method will by default:
    // * Run the simulation and record all events
    //   using observer referenced by `res`.
    // * Subscribe at virtual time 200
    // * Dispose subscription at virtual time 1000
    let res = scheduler.start { { $0 * 2 } }
    let correctMessages =
        .next(210, 0 * 2),
        .next(220, 1 * 2),
        .next(230, 2 * 2),
        .next(240, 4 * 2),
    let correctSubscriptions = [
        Subscription(200, 300)
    XCTAssertEqual(, correctMessages)
    XCTAssertEqual(xs.subscriptions, correctSubscriptions)

In the case of non-terminating sequences where you don't necessarily care about the event times, You may also use RxTest's XCTAssertRecordedElements to assert specific elements have been emitted. A terminating stop event (e.g. completed or error) will cause the test to fail.

func testElementsEmitted() {
    let scheduler = TestScheduler(initialClock: 0)

    let xs = scheduler.createHotObservable([
        .next(210, "RxSwift"),
        .next(220, "is"),
        .next(230, "pretty"),
        .next(240, "awesome")

    let res = scheduler.start { xs.asObservable() }

    XCTAssertRecordedElements(, ["RxSwift", "is", "pretty", "awesome"])

Testing operator compositions (view models, components)

Examples of how to test operator compositions are contained inside Rx.xcworkspace > RxExample-iOSTests target.

It's easy to define RxTest extensions so you can write your tests in a readable way. Provided examples inside RxExample-iOSTests are just suggestions on how you can write those extensions, but there are a lot of possibilities on how to write those tests.

    // expected events and test data
    let (

    ) = (
        scheduler.parseEventsAndTimes("e---u1----u2-----u3-----------------", values: stringValues).first!,
        scheduler.parseEventsAndTimes("e----------------------p1-----------", values: stringValues).first!,
        scheduler.parseEventsAndTimes("e---------------------------p2---p1-", values: stringValues).first!,
        scheduler.parseEventsAndTimes("------------------------------------", values: events).first!,

        scheduler.parseEventsAndTimes("e---v--f--v--f---v--o----------------", values: validations).first!,
        scheduler.parseEventsAndTimes("f--------------------------------t---", values: booleans).first!

Integration tests

It is also possible to write integration tests by using RxBlocking operators.

Using RxBlocking's toBlocking() method, you can block the current thread and wait for the sequence to complete, allowing you to synchronously access its result.

A simple way to test the result of your sequence is using the toArray method. It will return an array of all elements emitted once a sequence has completed successfully, or throw if an error caused the sequence to terminate.

let result = try fetchResource(location)

XCTAssertEqual(result, expectedResult)

Another option would be to use the materialize operator which lets you more granularly examine your sequence. It will return a MaterializedSequenceResult enumeration that could be either .completed along with the emitted elements if the sequence completed successfully, or failed if the sequence terminated with an error, along with the emitted error.

let result = try fetchResource(location)

// For testing the results or error in the case of terminating with error
switch result {
        case .completed:
            XCTFail("Expected result to complete with error, but result was successful.")
        case .failed(let elements, let error):
            XCTAssertEqual(elements, expectedResult)
            XCTAssertErrorEqual(error, expectedError)

// For testing the results in the case of termination with completion
switch result {
        case .completed(let elements):
            XCTAssertEqual(elements, expectedResult)
        case .failed(_, let error):
            XCTFail("Expected result to complete without error, but received \(error).")
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