Combine Expectations

Utilities for tests that wait for Combine publishers.

Latest release: version 0.10.0 (August 11, 2021) • Release Notes

Requirements: iOS 13+, macOS 10.15+, and tvOS 13+ require Swift 5.1+ or Xcode 11+. watchOS 7.4+ requires Swift 5.4+ or Xcode 12.5+.

Contact: Report bugs and ask questions in Github issues.

Testing Combine publishers with XCTestExpectation often requires setting up a lot of boilerplate code.

CombineExpectations aims at streamlining those tests. It defines an XCTestCase method which waits for publisher expectations.

• Usage
• Installation
• Publisher Expectations: availableElements, completion, elements, finished, last, next(), next(count), prefix(maxLength), recording, single

Usage

Waiting for Publisher Expectations allows your tests to look like this:

import XCTest
import CombineExpectations

class PublisherTests: XCTestCase {
    func testElements() throws {
        // 1. Create a publisher
        let publisher = ...
        
        // 2. Start recording the publisher
        let recorder = publisher.record()
        
        // 3. Wait for a publisher expectation
        let elements = try wait(for: recorder.elements, timeout: ..., description: "Elements")
        
        // 4. Test the result of the expectation
        XCTAssertEqual(elements, ["Hello", "World!"])
    }
}

When you wait for a publisher expectation:

• The test fails if the expectation is not fulfilled within the specified timeout.
• An error is thrown if the expected value can not be returned. For example, waiting for recorder.elements throws the publisher error if the publisher completes with a failure.
• The wait method returns immediately if the expectation has already reached the waited state.

You can wait multiple times for a publisher:

class PublisherTests: XCTestCase {
    func testPublisher() throws {
        let publisher = ...
        let recorder = publisher.record()
        
        // Wait for first element
        _ = try wait(for: recorder.next(), timeout: ...)
        
        // Wait for second element
        _ = try wait(for: recorder.next(), timeout: ...)
        
        // Wait for successful completion
        try wait(for: recorder.finished, timeout: ...)
    }
}

Not all tests have to wait, because some publishers expectations are fulfilled right away. In this case, prefer the synchronous get() method over wait(for:timeout:), as below:

class PublisherTests: XCTestCase {
    func testSynchronousPublisher() throws {
        // 1. Create a publisher
        let publisher = ...
        
        // 2. Start recording the publisher
        let recorder = publisher.record()
        
        // 3. Grab the expected result
        let elements = try recorder.elements.get()
        
        // 4. Test the result of the expectation
        XCTAssertEqual(elements, ["Hello", "World!"])
    }
}

Just like wait(for:timeout:), the get() method can be called multiple times:

class PublisherTests: XCTestCase {
    // SUCCESS: no error
    func testPassthroughSubjectSynchronouslyPublishesElements() throws {
        let publisher = PassthroughSubject<String, Never>()
        let recorder = publisher.record()
        
        publisher.send("foo")
        try XCTAssertEqual(recorder.next().get(), "foo")
        
        publisher.send("bar")
        try XCTAssertEqual(recorder.next().get(), "bar")
    }
}

Installation

Add a dependency for CombineExpectations to your Swift Package test targets:

 import PackageDescription
 
 let package = Package(
     dependencies: [
+        .package(url: "https://github.com/groue/CombineExpectations.git", ...)
     ],
     targets: [
         .testTarget(
             dependencies: [
+                "CombineExpectations"
             ])
     ]
 )

Publisher Expectations

There are various publisher expectations. Each one waits for a specific publisher aspect:

• availableElements: all published elements until timeout expiration
• completion: the publisher completion
• elements: all published elements until successful completion
• finished: the publisher successful completion
• last: the last published element
• next(): the next published element
• next(count): the next N published elements
• prefix(maxLength): the first N published elements
• recording: the full recording of publisher events
• single: the one and only published element

availableElements

recorder.availableElements waits for the expectation to expire, or the recorded publisher to complete.

When waiting for this expectation, the publisher error is thrown if the publisher fails before the expectation has expired.

✅ Otherwise, an array of all elements published before the expectation has expired is returned.

Related expectations: elements, prefix(maxLength).

Unlike other expectations, availableElements does not make a test fail on timeout expiration. It just returns the elements published so far.

Example:

// SUCCESS: no timeout, no error
func testTimerPublishesIncreasingDates() throws {
    let publisher = Timer.publish(every: 0.01, on: .main, in: .common).autoconnect()
    let recorder = publisher.record()
    let dates = try wait(for: recorder.availableElements, timeout: ...)
    XCTAssertEqual(dates.sorted(), dates)
}

completion

recorder.completion waits for the recorded publisher to complete.

When waiting for this expectation, a RecordingError.notCompleted is thrown if the publisher does not complete on time.

✅ Otherwise, a Subscribers.Completion is returned.

Related expectations: finished, recording.

Example:

// SUCCESS: no timeout, no error
func testArrayPublisherCompletesWithSuccess() throws {
    let publisher = ["foo", "bar", "baz"].publisher
    let recorder = publisher.record()
    let completion = try wait(for: recorder.completion, timeout: ...)
    if case let .failure(error) = completion {
        XCTFail("Unexpected error \(error)")
    }
}

// SUCCESS: no error
func testArrayPublisherSynchronouslyCompletesWithSuccess() throws {
    let publisher = ["foo", "bar", "baz"].publisher
    let recorder = publisher.record()
    let completion = try recorder.completion.get()
    if case let .failure(error) = completion {
        XCTFail("Unexpected error \(error)")
    }
}

// FAIL: Asynchronous wait failed
// FAIL: Caught error RecordingError.notCompleted
func testCompletionTimeout() throws {
    let publisher = PassthroughSubject<String, Never>()
    let recorder = publisher.record()
    let completion = try wait(for: recorder.completion, timeout: ...)
}

elements

recorder.elements waits for the recorded publisher to complete.

When waiting for this expectation, a RecordingError.notCompleted is thrown if the publisher does not complete on time, and the publisher error is thrown if the publisher fails.

✅ Otherwise, an array of published elements is returned.

Related expectations: availableElements, last, prefix(maxLength), recording, single.

Example:

// SUCCESS: no timeout, no error
func testArrayPublisherPublishesArrayElements() throws {
    let publisher = ["foo", "bar", "baz"].publisher
    let recorder = publisher.record()
    let elements = try wait(for: recorder.elements, timeout: ...)
    XCTAssertEqual(elements, ["foo", "bar", "baz"])
}

// SUCCESS: no error
func testArrayPublisherSynchronouslyPublishesArrayElements() throws {
    let publisher = ["foo", "bar", "baz"].publisher
    let recorder = publisher.record()
    let elements = try recorder.elements.get()
    XCTAssertEqual(elements, ["foo", "bar", "baz"])
}

// FAIL: Asynchronous wait failed
// FAIL: Caught error RecordingError.notCompleted
func testElementsTimeout() throws {
    let publisher = PassthroughSubject<String, Never>()
    let recorder = publisher.record()
    let elements = try wait(for: recorder.elements, timeout: ...)
}

// FAIL: Caught error MyError
func testElementsError() throws {
    let publisher = PassthroughSubject<String, MyError>()
    let recorder = publisher.record()
    publisher.send(completion: .failure(MyError()))
    let elements = try wait(for: recorder.elements, timeout: ...)
}

finished

recorder.finished waits for the recorded publisher to complete.

When waiting for this expectation, the publisher error is thrown if the publisher fails.

Related expectations: completion, recording.

Example:

// SUCCESS: no timeout, no error
func testArrayPublisherFinishesWithoutError() throws {
    let publisher = ["foo", "bar", "baz"].publisher
    let recorder = publisher.record()
    try wait(for: recorder.finished, timeout: ...)
}

// SUCCESS: no error
func testArrayPublisherSynchronouslyFinishesWithoutError() throws {
    let publisher = ["foo", "bar", "baz"].publisher
    let recorder = publisher.record()
    try recorder.finished.get()
}

// FAIL: Asynchronous wait failed
func testFinishedTimeout() throws {
    let publisher = PassthroughSubject<String, Never>()
    let recorder = publisher.record()
    try wait(for: recorder.finished, timeout: ...)
}

// FAIL: Caught error MyError
func testFinishedError() throws {
    let publisher = PassthroughSubject<String, MyError>()
    let recorder = publisher.record()
    publisher.send(completion: .failure(MyError()))
    try wait(for: recorder.finished, timeout: ...)
}

recorder.finished can be inverted:

// SUCCESS: no timeout, no error
func testPassthroughSubjectDoesNotFinish() throws {
    let publisher = PassthroughSubject<String, Never>()
    let recorder = publisher.record()
    try wait(for: recorder.finished.inverted, timeout: ...)
}

// FAIL: Fulfilled inverted expectation
// FAIL: Caught error MyError
func testInvertedFinishedError() throws {
    let publisher = PassthroughSubject<String, MyError>()
    let recorder = publisher.record()
    publisher.send(completion: .failure(MyError()))
    try wait(for: recorder.finished.inverted, timeout: ...)
}

last

recorder.last waits for the recorded publisher to complete.

When waiting for this expectation, a RecordingError.notCompleted is thrown if the publisher does not complete on time, and the publisher error is thrown if the publisher fails.

✅ Otherwise, the last published element is returned, or nil if the publisher completes before it publishes any element.

Related expectations: elements, single.

Example:

// SUCCESS: no timeout, no error
func testArrayPublisherPublishesLastElementLast() throws {
    let publisher = ["foo", "bar", "baz"].publisher
    let recorder = publisher.record()
    if let element = try wait(for: recorder.last, timeout: ...) {
        XCTAssertEqual(element, "baz")
    } else {
        XCTFail("Expected one element")
    }
}

// SUCCESS: no error
func testArrayPublisherSynchronouslyPublishesLastElementLast() throws {
    let publisher = ["foo", "bar", "baz"].publisher
    let recorder = publisher.record()
    if let element = try recorder.last.get() {
        XCTAssertEqual(element, "baz")
    } else {
        XCTFail("Expected one element")
    }
}

// FAIL: Asynchronous wait failed
// FAIL: Caught error RecordingError.notCompleted
func testLastTimeout() throws {
    let publisher = PassthroughSubject<String, Never>()
    let recorder = publisher.record()
    let element = try wait(for: recorder.last, timeout: ...)
}

// FAIL: Caught error MyError
func testLastError() throws {
    let publisher = PassthroughSubject<String, MyError>()
    let recorder = publisher.record()
    publisher.send(completion: .failure(MyError()))
    let element = try wait(for: recorder.last, timeout: ...)
}

next()

recorder.next() waits for the recorded publisher to emit one element, or to complete.

When waiting for this expectation, a RecordingError.notEnoughElements is thrown if the publisher does not publish one element after last waited expectation. The publisher error is thrown if the publisher fails before publishing the next element.

✅ Otherwise, the next published element is returned.

Related expectations: next(count), single.

Example:

// SUCCESS: no timeout, no error
func testArrayOfTwoElementsPublishesElementsInOrder() throws {
    let publisher = ["foo", "bar"].publisher
    let recorder = publisher.record()
    
    var element = try wait(for: recorder.next(), timeout: ...)
    XCTAssertEqual(element, "foo")
    
    element = try wait(for: recorder.next(), timeout: ...)
    XCTAssertEqual(element, "bar")
}

// SUCCESS: no error
func testArrayOfTwoElementsSynchronouslyPublishesElementsInOrder() throws {
    let publisher = ["foo", "bar"].publisher
    let recorder = publisher.record()
    
    var element = try recorder.next().get()
    XCTAssertEqual(element, "foo")
    
    element = try recorder.next().get()
    XCTAssertEqual(element, "bar")
}

// FAIL: Asynchronous wait failed
// FAIL: Caught error RecordingError.notEnoughElements
func testNextTimeout() throws {
    let publisher = PassthroughSubject<String, Never>()
    let recorder = publisher.record()
    let element = try wait(for: recorder.next(), timeout: ...)
}

// FAIL: Caught error MyError
func testNextError() throws {
    let publisher = PassthroughSubject<String, MyError>()
    let recorder = publisher.record()
    publisher.send(completion: .failure(MyError()))
    let element = try wait(for: recorder.next(), timeout: ...)
}

// FAIL: Caught error RecordingError.notEnoughElements
func testNextNotEnoughElementsError() throws {
    let publisher = PassthroughSubject<String, Never>()
    let recorder = publisher.record()
    publisher.send(completion: .finished)
    let element = try wait(for: recorder.next(), timeout: ...)
}

recorder.next() can be inverted:

// SUCCESS: no timeout, no error
func testPassthroughSubjectDoesNotPublishAnyElement() throws {
    let publisher = PassthroughSubject<String, Never>()
    let recorder = publisher.record()
    try wait(for: recorder.next().inverted, timeout: ...)
}

// FAIL: Fulfilled inverted expectation
func testInvertedNextTooEarly() throws {
    let publisher = PassthroughSubject<String, Never>()
    let recorder = publisher.record()
    publisher.send("foo")
    try wait(for: recorder.next().inverted, timeout: ...)
}

// FAIL: Fulfilled inverted expectation
// FAIL: Caught error MyError
func testInvertedNextError() throws {
    let publisher = PassthroughSubject<String, MyError>()
    let recorder = publisher.record()
    publisher.send(completion: .failure(MyError()))
    try wait(for: recorder.next().inverted, timeout: ...)
}

next(count)

recorder.next(count) waits for the recorded publisher to emit count elements, or to complete.

When waiting for this expectation, a RecordingError.notEnoughElements is thrown if the publisher does not publish count elements after last waited expectation. The publisher error is thrown if the publisher fails before publishing the next count elements.

✅ Otherwise, an array of exactly count elements is returned.

Related expectations: next(), prefix(maxLength).

Example:

// SUCCESS: no timeout, no error
func testArrayOfThreeElementsPublishesTwoThenOneElement() throws {
    let publisher = ["foo", "bar", "baz"].publisher
    let recorder = publisher.record()
    
    var elements = try wait(for: recorder.next(2), timeout: ...)
    XCTAssertEqual(elements, ["foo", "bar"])
    
    elements = try wait(for: recorder.next(1), timeout: ...)
    XCTAssertEqual(elements, ["baz"])
}

// SUCCESS: no error
func testArrayOfThreeElementsSynchronouslyPublishesTwoThenOneElement() throws {
    let publisher = ["foo", "bar", "baz"].publisher
    let recorder = publisher.record()
    
    var elements = try recorder.next(2).get()
    XCTAssertEqual(elements, ["foo", "bar"])
    
    elements = try recorder.next(1).get()
    XCTAssertEqual(elements, ["baz"])
}

// FAIL: Asynchronous wait failed
// FAIL: Caught error RecordingError.notEnoughElements
func testNextCountTimeout() throws {
    let publisher = PassthroughSubject<String, Never>()
    let recorder = publisher.record()
    publisher.send("foo")
    let elements = try wait(for: recorder.next(2), timeout: ...)
}

// FAIL: Caught error MyError
func testNextCountError() throws {
    let publisher = PassthroughSubject<String, MyError>()
    let recorder = publisher.record()
    publisher.send("foo")
    publisher.send(completion: .failure(MyError()))
    let elements = try wait(for: recorder.next(2), timeout: ...)
}

// FAIL: Caught error RecordingError.notEnoughElements
func testNextCountNotEnoughElementsError() throws {
    let publisher = PassthroughSubject<String, Never>()
    let recorder = publisher.record()
    publisher.send("foo")
    publisher.send(completion: .finished)
    let elements = try wait(for: recorder.next(2), timeout: ...)
}

prefix(maxLength)

recorder.prefix(maxLength) waits for the recorded publisher to emit maxLength elements, or to complete.

When waiting for this expectation, the publisher error is thrown if the publisher fails before maxLength elements are published.

✅ Otherwise, an array of received elements is returned, containing at most maxLength elements, or less if the publisher completes early.

Related expectations: availableElements, elements, next(count).

Example:

// SUCCESS: no timeout, no error
func testArrayOfThreeElementsPublishesTwoFirstElementsWithoutError() throws {
    let publisher = ["foo", "bar", "baz"].publisher
    let recorder = publisher.record()
    let elements = try wait(for: recorder.prefix(2), timeout: ...)
    XCTAssertEqual(elements, ["foo", "bar"])
}

// SUCCESS: no error
func testArrayOfThreeElementsSynchronouslyPublishesTwoFirstElementsWithoutError() throws {
    let publisher = ["foo", "bar", "baz"].publisher
    let recorder = publisher.record()
    let elements = try recorder.prefix(2).get()
    XCTAssertEqual(elements, ["foo", "bar"])
}

// FAIL: Asynchronous wait failed
func testPrefixTimeout() throws {
    let publisher = PassthroughSubject<String, Never>()
    let recorder = publisher.record()
    publisher.send("foo")
    let elements = try wait(for: recorder.prefix(2), timeout: ...)
}

// FAIL: Caught error MyError
func testPrefixError() throws {
    let publisher = PassthroughSubject<String, MyError>()
    let recorder = publisher.record()
    publisher.send("foo")
    publisher.send(completion: .failure(MyError()))
    let elements = try wait(for: recorder.prefix(2), timeout: ...)
}

recorder.prefix(maxLength) can be inverted:

// SUCCESS: no timeout, no error
func testPassthroughSubjectPublishesNoMoreThanSentValues() throws {
    let publisher = PassthroughSubject<String, Never>()
    let recorder = publisher.record()
    publisher.send("foo")
    publisher.send("bar")
    let elements = try wait(for: recorder.prefix(3).inverted, timeout: ...)
    XCTAssertEqual(elements, ["foo", "bar"])
}

// FAIL: Fulfilled inverted expectation
func testInvertedPrefixTooEarly() throws {
    let publisher = PassthroughSubject<String, Never>()
    let recorder = publisher.record()
    publisher.send("foo")
    publisher.send("bar")
    publisher.send("baz")
    let elements = try wait(for: recorder.prefix(3).inverted, timeout: ...)
}

// FAIL: Fulfilled inverted expectation
// FAIL: Caught error MyError
func testInvertedPrefixError() throws {
    let publisher = PassthroughSubject<String, MyError>()
    let recorder = publisher.record()
    publisher.send("foo")
    publisher.send(completion: .failure(MyError()))
    let elements = try wait(for: recorder.prefix(3).inverted, timeout: ...)
}

recording

recorder.recording waits for the recorded publisher to complete.

When waiting for this expectation, a RecordingError.notCompleted is thrown if the publisher does not complete on time.

✅ Otherwise, a Record.Recording is returned.

Related expectations: completion, elements, finished.

Example:

// SUCCESS: no timeout, no error
func testArrayPublisherRecording() throws {
    let publisher = ["foo", "bar", "baz"].publisher
    let recorder = publisher.record()
    let recording = try wait(for: recorder.recording, timeout: ...)
    XCTAssertEqual(recording.output, ["foo", "bar", "baz"])
    if case let .failure(error) = recording.completion {
        XCTFail("Unexpected error \(error)")
    }
}

// SUCCESS: no error
func testArrayPublisherSynchronousRecording() throws {
    let publisher = ["foo", "bar", "baz"].publisher
    let recorder = publisher.record()
    let recording = try recorder.recording.get()
    XCTAssertEqual(recording.output, ["foo", "bar", "baz"])
    if case let .failure(error) = recording.completion {
        XCTFail("Unexpected error \(error)")
    }
}

// FAIL: Asynchronous wait failed
// FAIL: Caught error RecordingError.notCompleted
func testRecordingTimeout() throws {
    let publisher = PassthroughSubject<String, Never>()
    let recorder = publisher.record()
    let recording = try wait(for: recorder.recording, timeout: ...)
}

single

recorder.single waits for the recorded publisher to complete.

When waiting for this expectation, a RecordingError is thrown if the publisher does not complete on time, or does not publish exactly one element before it completes. The publisher error is thrown if the publisher fails.

✅ Otherwise, the single published element is returned.

Related expectations: elements, last, next().

Example:

// SUCCESS: no timeout, no error
func testJustPublishesExactlyOneElement() throws {
    let publisher = Just("foo")
    let recorder = publisher.record()
    let element = try wait(for: recorder.single, timeout: ...)
    XCTAssertEqual(element, "foo")
}

// SUCCESS: no error
func testJustSynchronouslyPublishesExactlyOneElement() throws {
    let publisher = Just("foo")
    let recorder = publisher.record()
    let element = try recorder.single.get()
    XCTAssertEqual(element, "foo")
}

// FAIL: Asynchronous wait failed
// FAIL: Caught error RecordingError.notCompleted
func testSingleTimeout() throws {
    let publisher = PassthroughSubject<String, Never>()
    let recorder = publisher.record()
    let element = try wait(for: recorder.single, timeout: ...)
}

// FAIL: Caught error MyError
func testSingleError() throws {
    let publisher = PassthroughSubject<String, MyError>()
    let recorder = publisher.record()
    publisher.send(completion: .failure(MyError()))
    let element = try wait(for: recorder.single, timeout: ...)
}

// FAIL: Caught error RecordingError.tooManyElements
func testSingleTooManyElementsError() throws {
    let publisher = PassthroughSubject<String, Never>()
    let recorder = publisher.record()
    publisher.send("foo")
    publisher.send("bar")
    publisher.send(completion: .finished)
    let element = try wait(for: recorder.single, timeout: ...)
}

// FAIL: Caught error RecordingError.notEnoughElements
func testSingleNotEnoughElementsError() throws {
    let publisher = PassthroughSubject<String, Never>()
    let recorder = publisher.record()
    publisher.send(completion: .finished)
    let element = try wait(for: recorder.single, timeout: ...)
}