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Entwine Test

Part of Entwine – A collection of accessories for Apple's Combine Framework.


About EntwineTest

EntwineTest packages a concise set of tools that are designed to work together to help you test your Combine sequences and operators.

In addition, EntwineTest includes tools to help you to determine whether your publishers are complying with subscriber demand requests (backpressure) so that you can ensure your publisher is behaving like a good Combine citizen before releasing it out in the wild.

Getting Started

The EntwineTest packages consists of three major components – together, they help you write better tests for your Combine sequences.

Let's go through them one by one before finally seeing how they all fit together:


At the heart of Combine is the concept of schedulers. Without them, no work gets done. Essentially they are responsible for both where an action is excuted (main thread, a dipatch queue, or the current thread), and when it is is executed (right now, after the current task has finished, five minutes from now).

Our TestScheduler is a special kind of scheduler that uses 'virtual time' to schedule its tasks on the current thread. Our VirtualTime is really just an Int and its only purpose is to prioritise the order in which tasks are done. However, for testing purposes, we pretend it is actual time, as it helps us to articulate the seqeunce in which we'd like our tests to run.

The best thing about virtual time? It's instantaneous! So we keep our test suites lean and fast.

Here's how you might use the TestScheduler in isolation:

import EntwineTest

let scheduler = TestScheduler()

scheduler.schedule(after: 300) { print("bosh") }
scheduler.schedule(after: 200) { print("bash") }
scheduler.schedule(after: 100) { print("bish") }

scheduler.resume() // the clock is paused until this is called

// outputs:
//  "bish"
//  "bash"
//  "bosh"

Notice that as we've scheduled "bosh" to print at 300, and "bish" to print at 100, when we start the scheduler by calling .resume(), "bish" is printed first.


Now that we have our scheduler, we can think about how we're going to simulate some Combine sequences. If we want to simulate a sequence, we'll need a publisher that lets us define what each element a sequence should be, and when that element should be emitted.

A TestablePublisher is exactly that.

You can generate a TestablePublisher from two factory methods on the TestScheduler. (We do it this way instead of instantiating directly as they depend on the scheduler.)

One, createAbsoluteTestablePublisher(_:), schedules events at exactly the time specified – if the time of an event has passed at the point the publisher is subscribed to, that event won't be fired.

The other, createRelativeTestablePublisher(_:), schedules events at the time specified plus the time the publisher was subscribed to. So an event scheduled at 100 with a subscription at 200 means the event will fire at 300.

import Combine
import EntwineTest

// we'll set the schedulers clock a little forward – at 200

let scheduler = TestScheduler(initialClock: 200)

let relativeTimePublisher: TestablePublisher<String, Never> = scheduler.createRelativeTestablePublisher([
    (020, .input("Mi")),
    (030, .input("Fa")),

let absoluteTimePublisher: TestablePublisher<String, Never> = scheduler.createAbsoluteTestablePublisher([
    (200, .input("Do")),
    (210, .input("Re")),

let relativeSubscription = relativeTimePublisher.sink { element in
    print("time: \( - \(element)")

let absoluteSubscription = absoluteTimePublisher.sink { element in
    print("time: \( - \(element)")


// Outputs:
//    time: 200 - Do
//    time: 210 - Re
//    time: 220 - Mi
//    time: 230 - Fa

Notice how the events events scheduled by the relative publisher fired after the events scheduled by the absolute publisher. As we had set the time of our scheduler to 200 in its initializer, when we subscribed to our relative publisher with the sink(_:) method, our publisher took the current time and added that value to each scheduled event.


The final piece in the jigsaw is the TestableSubscriber. Its role is to gather the output of a publisher so that it can be compared against some expected output. It also depends on the TestScheduler, so to get one we call createTestableSubscriber(_:_:) on our scheduler.

Once we subscribe to a publisher, TestableSubscriber records all the events with their time of arrival and makes them available on its .sequence property ready for us to compare against some expected output. It also records the time the subscription began, as well as its completion (should it end).

import Combine
import EntwineTest

let scheduler = TestScheduler()
let passthroughSubject = PassthroughSubject<String, Never>()

scheduler.schedule(after: 100) { passthroughSubject.send("yippee") }
scheduler.schedule(after: 200) { passthroughSubject.send("ki") }
scheduler.schedule(after: 300) { passthroughSubject.send("yay") }

let subscriber = scheduler.createTestableSubscriber(String.self, Never.self)



let expected: TestSequence<String, Never> = [
    (000, .subscription),
    (100, .input("yippee")),
    (200, .input("ki")),
    (300, .input("yay")),

print("sequences match: \(expected == subscriber.sequence)")

// outputs:
//  sequences match: true

Putting it all together

Now that we have our TestScheduler, TestPublisher, and TestSubscriber let's put them together to test our operators and sequences.

But first, there's one additional method that you should be aware of. That's the start(create:) method on TestScheduler.

The start(create:) method accepts a closure that produces any publisher and then:

  1. Schedules the creation of the publisher (invocation of the passed closure) at 100
  2. Schedules the subscription of the publisher to a TestableSubscriber at 200
  3. Schedules the cancellation of the subscription at 900
  4. Resumes the scheduler clock

These are all configurable by using the start(configuration:create:) method. See the docs for more info.

With that knowledge in place, let's test Combine's map operator. (I'm sure it's fine – but just in case.)

import XCTest
import EntwineTest
func testMap() {
    let testScheduler = TestScheduler(initialClock: 0)
    // creates a publisher that will schedule it's elements relatively, at the point of subscription
    let testablePublisher: TestablePublisher<String, Never> = testScheduler.createRelativeTestablePublisher([
        (100, .input("a")),
        (200, .input("b")),
        (300, .input("c")),
    // a publisher that maps strings to uppercase
    let subjectUnderTest = { $0.uppercased() }
    // uses the method described above (schedules a subscription at 200, to be cancelled at 900)
    let results = testScheduler.start { subjectUnderTest }
    XCTAssertEqual(results.recordedOutput, [
        (200, .subscription),           // subscribed at 200
        (300, .input("A")),             // received uppercased input @ 100 + subscription time
        (400, .input("B")),             // received uppercased input @ 200 + subscription time
        (500, .input("C")),             // received uppercased input @ 300 + subscription time

Hopefully this should be everything you need to get you started with testing your Combine sequences. Don't forget that further information can be found in the docs.


As part of another Swift Package:

  1. Include it in your Package.swift file as both a dependency and a dependency of your target.
import PackageDescription

let package = Package(
    dependencies: [
        .package(url: "", .upToNextMajor(from: "0.0.0")),
    targets: [
        .testTarget(name: "MyTestTarget", dependencies: ["EntwineTest"]),
  1. Then run swift package update from the root directory of your SPM project. If you're using Xcode 11 to edit your SPM project this should happen automatically.

As part of an Xcode 11 or greater project:

  1. Select the File -> Swift Packages -> Add package dependency... menu item.
  2. Enter the repository url and tap next.
  3. Select 'version, 'up to next major', enter 0.0.0, hit next.
  4. Select the EntwineTest library and specify the target you wish to use it with.

n.b. EntwineTest is pre-release software and as such the API may change prior to reaching 1.0. For finer-grained control please use .upToNextMinor(from:) in your SPM dependency declaration


Full documentation for EntwineTest can be found at


EntwineTest is inspired by the great work done in the RxTest library by the contributors to RxSwift.

Copyright and license

Copyright 2019 © Tristan Celder

EntwineTest is made available under the MIT License

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