PetService.kt

class PetService(private val pets: PetsDao, private val images: ThirdPartyImageClient) {

    fun get(id: Long): Pet? {
        return pets[id]
    }

    fun create(name: String): Pet {
        val id = pets.create(name)
        return pets[id] ?: throw IllegalStateException("Pet was not created")
    }

    fun uploadImage(petId: Long, contentType: String, content: InputStream): Pet? {
        pets[petId] ?: return null

        val url = images.uploadImage(contentType, content) ?: return null
        pets.addImage(petId, url)

        return pets[petId]
    }
}

PetServiceTest.kt

class PetServiceTest {

    private val petsDao = PetsDao.mock()
    private val thirdPartyImageBackend = FakeThirdPartyImageBackend()

    private val testObj = PetService(
        pets = petsDao,
        images = ThirdPartyImageClient(thirdPartyImageBackend)
    )

    @Test
    fun `get missing pet`() {
        testObj.get(123) shouldBe null
    }

    @Test
    fun `get pet`() {
        val id = petsDao.create("Tigger")

        testObj.get(id) shouldBe Pet(
            id = id,
            name = "Tigger",
            photoUrls = emptyList()
        )
    }

    @Test
    fun `create pet`() {
        val result = testObj.create("Tigger")

        result.name shouldBe "Tigger"
        result.photoUrls.shouldBeEmpty()
    }

    @Test
    fun `add image`() {
        val id = petsDao.create("Tigger")

        val content = "foobarbaz".toByteArray()
        val updated = content.inputStream().use { data ->
            testObj.uploadImage(id, "image/png", data)
        }

        // test service result
        updated shouldBe Pet(
            id = id,
            name = "Tigger",
            photoUrls = listOf("http://images.fake/image0")
        )
        
        // test side-effect on the pets dao
        petsDao[id].shouldNotBeNull()
            .photoUrls.shouldContainExactly("http://images.fake/image0")

        // test side-effect on the image repo
        thirdPartyImageBackend.images.shouldContainExactly(
            ThirdPartyImageDto(
                id = "image0",
                url = "http://images.fake/image0",
                contentType = "image/png",
                size = content.size
            )
        )
    }
}