MVP-Testing-ErrorTestPath.kt

import io.mockk.every
import io.mockk.mockk
import io.mockk.verify
import io.reactivex.Single
import io.reactivex.schedulers.TestScheduler
import org.junit.Test

    @Test
    fun `GIVEN unsuccessful data fetch WHEN screen opens THEN display error`() {
        val view: MainScreenContract.View = mockk(relaxed = true)

        val fetchCurrencyUseCase: FetchCurrencyUseCase = mockk {
            every { execute() } returns Single.error(Throwable("error occurred"))
        }

        val presenter = MainScreenPresenter(
            fetchCurrencyUseCase = fetchCurrencyUseCase,
            schedulerProvider = TrampolineTestSchedulers()
        )

        presenter.attach(view)

        verify {
            view.displayError(any())
        }
    }