BDD style tests in Swift

context("given instruction cards should be shown") {
    var testObs: TestObserver<PSSMatchViewModel.Event, Never>!

    beforeEach {
        testObs = TestObserver<PSSMatchViewModel.Event, Never>()
        viewModel.events.signal.observe(testObs.observer)
    }

    it("then swipe right card shown first") {
        guard case .started(let context) = viewModel.state.value else {
            XCTFail("Invalid state")
            return
        }

        guard case .instructions(let rating) = context.cards.first else {
            XCTFail("Invalid card")
            return
        }

        expect(rating).to(equal(PSSMatchRating.interested))
    }

    context("when swiping past first") {
        beforeEach {
            viewModel.swipe(rating: .interested)
        }

        it("then app event is recorded") {
            expect((appEventLogger as! AppEventLoggerFixture).lastEvent).to(beAppEventLoggable(PSSMatchAppEvent.swipedInstructions(instructionRating: .interested, actualRating: .interested)))
        }

        it("then swipe left shown next") {
            guard case .started(let context) = viewModel.state.value else {
                XCTFail("Invalid state")
                return
            }

            guard case .instructions(let rating) = context.cards.first else {
                XCTFail("Invalid card")
                return
            }

            expect(rating).to(equal(PSSMatchRating.notInterested))
        }
        
        <snip/>