TestableSubscriber, ever wondered how to test the publisher? This is a simple solution to test the emitted values by any publisher in Test target without calling the actual subscriber inside business logic.The provided XCTest case demonstrates how to use this TestableSubscriber for testing by subscribing to a publisher and asserting values and c…

TestableSubscriber.swift

final class TestableSubscriber<Input, Failure: Error>: Subscriber {
    typealias Input = Input
    typealias Failure = Failure
    
    private var subscription: Subscription?
    private var values: [Input] = []
    private var completion: Subscribers.Completion<Failure>?
   
    private var demand: Subscribers.Demand {
        didSet { subscription?.request(demand) }
    }
    
    init(demand: Subscribers.Demand = .unlimited) {
        self.demand = demand
    }
    
    func receive(_ input: Input) -> Subscribers.Demand {
        values.append(input) // Emitted Values
        return demand
    }
    

    func receive(completion: Subscribers.Completion<Failure>) {
        self.completion = completion
        cancel()
    }
    
    func receive(subscription: Subscription) {
        self.subscription = subscription
        subscription.request(.unlimited)
    }
    
    func cancel() {
        subscription?.cancel()
        subscription = nil
    }
    
    var emittedValues: [Input] {
        values
    }
    
    var completionState: Subscribers.Completion<Failure>? {
        completion
    }
}


// Extension for publisher to connect TestableSubscriber
extension Publisher {
    func testableSubscriber() -> TestableSubscriber<Output, Failure> {
        let testSubscriber = TestableSubscriber<Output,Failure>()
        subscribe(testSubscriber)
        return testSubscriber
    }
}

// Usuage in Test target.
final class SomeViewModelTests: XCTestCase {
    func testPublisher() {
        let publisher = Just("Hello, World!") // Test with any publisher, e.g viewModel.someDataPublisher
        let testableSubscriber = publisher.testableSubscriber()
        XCTAssertEqual(testableSubscriber.emittedValues.last, "Hello, World!")
        XCTAssertEqual(testableSubscriber.completionState, .finished)
    }
}