In iOS development using Swift, what options do I have for testing code that utilizes the Combine framework? Specifically, I'm looking to test a function that returns an AnyPublisher
asynchronously. I prefer not to use expectations or any approaches that require specifying a waiting time. Are there methodologies similar to async/await code testing that I can use?
Here's an example of the function I need to test:
import Combine
func fetchData() -> AnyPublisher<Data, Error> {
// Assume this is some asynchronous network call returning AnyPublisher
someAsyncCall.
.map(\.data)
.eraseToAnyPublisher()
}
I am interested in understanding how to test the fetchData
function effectively using best practices for Combine
To test Combine code in Swift without relying on XCTestExpectation or fixed waiting times, you can use Combine’s .sink and Swift’s async/await support with the new XCTest measure functions, which allows you to seamlessly integrate Combine into an asynchronous testing flow.
Here's how to test fetchData asynchronously:
import XCTest
import Combine
class FetchDataTests: XCTestCase {
var cancellables = Set<AnyCancellable>()
func testFetchData() async throws {
// Given
let expectedData = Data() // Assume we expect some data here
let fetcher = YourFetcher() // Your class or struct that holds fetchData
// When
let data = try await fetcher.fetchData().firstValue()
// Then
XCTAssertEqual(data, expectedData, "Fetched data should match expected data")
}
}
extension Publisher {
/// Converts a Publisher to an async sequence and returns the first value emitted.
func firstValue() async throws -> Output {
try await withCheckedThrowingContinuation { continuation in
var cancellable: AnyCancellable?
cancellable = first()
.sink(receiveCompletion: { completion in
if case .failure(let error) = completion {
continuation.resume(throwing: error)
}
cancellable?.cancel()
}, receiveValue: { value in
continuation.resume(returning: value)
cancellable?.cancel()
})
}
}
}