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Here's an easy to copy and paste reference on how to write common test assertions for Swift Combine Publishers in XCTest in Xcode.

• How to test Publisher publishes one value then finishes
• How to test Publisher publishes one value then a failure
• How to test Publisher publishes many values then finishes
• How to test Publisher publishes many values then a failure
• How to test Publisher publishes no values then finishes
• How to test Publisher publishes no values then a failure

You can see these snippets in action and experiment with the syntax by checking out the example project.

While researching this post, I found that there is too much boilerplate required to test a Publisher, resulting in code that is harder to read. I started building a Swift Package to try addressing this problem: CombineTestHelpers provides handy one liners for each scenario in this cheat sheet and more.

In the snippets below you'll see some code in the form <# ... #>. That's the source code placeholder Xcode notation and makes it easier to adapt the code to your needs. Xcode will let you jump to those placeholders with Tab.

All the examples in this cheat sheet follow the same pattern: to test a Combine Publisher, define an XCTestExpectation and make the test subscribe to the publisher with sink(receiveCompletion:, receiveValue). Fulfill the expectation in either the value or completion closure, depending on the kind of behavior you want to verify. If necessary, you can run one or more XCTestAssert- assertions on the value and completion you receive.

How to test Publisher publishes one value then finishes

let expectation = XCTestExpectation(description: "Publishes one value then finishes")

var values: [<# Publisher Output type #>] = []

publisher
    .sink(
        receiveCompletion: { completion in
            guard case .finished = completion else { return }
            expectation.fulfill()
        },
        receiveValue: { value in
            guard values.isEmpty else {
                return XCTFail("Expected to receive only one value, got another: (\(value))")
            }
            XCTAssertEqual(value, <# expected value #>)
            values.append(value)
        }
    )
    .store(in: &cancellables)

wait(for: [expectation], timeout: 0.5)

How to test Publisher publishes one value then a failure

let expectation = XCTestExpectation(description: "Publishes one value then fails")

var values: [<# Publisher Output type #>] = []

publisher
    .sink(
        receiveCompletion: { completion in
            guard case .failure(let error) = completion else { return }
            XCTAssertEqual(error, <# expected error #>)
            expectation.fulfill()
        },
        receiveValue: { value in
            guard values.isEmpty else {
                return XCTFail("Expected to receive only one value, got another (\(value))")
            }
            XCTAssertEqual(value, <# expected value #>)
            values.append(value)
        }
    )
    .store(in: &cancellables)

wait(for: [expectation], timeout: 0.5)

How to test Publisher publishes many values then finishes

let expectation = XCTestExpectation(description: "Publishes values then finishes")

var values: [<# Publisher Output type #>] = []

publisher
    .sink(
        receiveCompletion: { completion in
            guard case .finished = completion else { return }
            expectation.fulfill()
        },
        receiveValue: {
            values.append($0)
        }
    )
    .store(in: &cancellables)

wait(for: [expectation], timeout: 0.5)

XCTAssertEqual(values, <# expected values #>)

How to test Publisher publishes many values then a failure

let expectation = XCTestExpectation(description: "Publishes many values then a failure")

var values: [<# Publisher Output type #>] = []

publisher
    .sink(
        receiveCompletion: { completion in
            guard case .failure(let error) = completion else { return }
            XCTAssertEqual(error, <# expected error #>)
            expectation.fulfill()
        },
        receiveValue: {
            values.append($0)
        }
    )
    .store(in: &cancellables)

wait(for: [expectation], timeout: 0.5)

XCTAssertEqual(values, <# expected values #>)

How to test Publisher publishes no values then finishes

let expectation = XCTestExpectation(description: "Finishes without publishing values")

publisher
    .sink(
        receiveCompletion: { completion in
            guard case .finished = completion else { return }
            expectation.fulfill()
        },
        receiveValue: {
            XCTFail("Expected to finish without receiving any value, got \($0)")
        }
    )
    .store(in: &cancellables)

wait(for: [expectation], timeout: 0.5)

How to test Publisher publishes no values then a failure

let expectation = XCTestExpectation(description: "Fails without publishing values")

publisher
    .sink(
        receiveCompletion: { completion in
            guard case .failure(let error) = completion else { return }
            XCTAssertEqual(error, <# expected error #>)
            expectation.fulfill()
        },
        receiveValue: {
            XCTFail("Expected to fail without receiving any value, got \($0)")
        }
    )
    .store(in: &cancellables)

wait(for: [expectation], timeout: 0.5)

You'll find more insights on how to write tests for code using Combine in my book Test-Driven Development in Swift with SwiftUI and Combine.

What are the most common Publisher behavior you test? I'd love to hear from you! Leave a comment below or get in touch on Twitter at @mokagio.

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