Using argument captors

captors.kt

import org.junit.Assert.assertEquals
import org.junit.Assert.assertNotNull
import org.junit.Test
import org.mockito.kotlin.*

class RepositoryImplTest {
    @Test
    fun `without validating prefix`() {
        // setup
        val mockApi: Api = mock()
        val repository = RepositoryImpl("TEST", mockApi)
        var result: String? = null
        val onResult: Callback = { result = it }


        // test
        repository.fetch("foo", onResult)


        // verify
        verify(mockApi).fetch(eq("foo"), any())
        
        // but how do we assert the prefix was used correctly ???
    }

    @Test
    fun `validating prefix using a fake`() {
        // setup
        class FakeApi : Api {
            lateinit var data: String
            lateinit var onResult: Callback
            override fun fetch(data: String, onResult: Callback) {
                this.data = data
                this.onResult = onResult
            }
        }

        val fakeApi = FakeApi()
        val repository = RepositoryImpl("TEST", fakeApi)
        var result: String? = null
        val onResult: Callback = { result = it }


        // test
        repository.fetch("foo", onResult)


        // verify
        assertEquals("foo", fakeApi.data)
        fakeApi.onResult.invoke("bar")
        assertNotNull(result)
        assertEquals("[TEST] bar", result)
    }

    @Test
    fun `validating prefix using a captor`() {
        // setup
        val mockApi: Api = mock()
        val repository = RepositoryImpl("TEST", mockApi)
        val captor = argumentCaptor<Callback>()
        var result: String? = null
        val onResult: Callback = { result = it }


        // test
        repository.fetch("foo", onResult)


        // verify
        verify(mockApi).fetch(eq("foo"), captor.capture())

        val callback = captor.firstValue
        callback.invoke("bar")
        assertNotNull(result)
        assertEquals("[TEST] bar", result)
    }

    @Test
    fun `validating prefix using an inline captor`() {
        // setup
        val mockApi: Api = mock()
        val repository = RepositoryImpl("TEST", mockApi)
        var result: String? = null
        val onResult: Callback = { result = it }


        // test
        repository.fetch("foo", onResult)


        // verify
        val captor = argumentCaptor<Callback> {
            verify(mockApi).fetch(eq("foo"), capture())
        }

        val callback = captor.firstValue
        callback.invoke("bar")
        assertNotNull(result)
        assertEquals("[TEST] bar", result)
    }
}

typealias Callback = (String) -> Unit

interface Repository {
    fun fetch(data: String, onResult: Callback)
}

class RepositoryImpl(private val prefix: String, private val api: Api) : Repository {
    override fun fetch(data: String, onResult: Callback) {
        api.fetch(data) { onResult.invoke("[$prefix] $it") }
    }
}

interface Api {
    fun fetch(data: String, onResult: Callback)
}