Testing¶

To use the testing support classes, import the extension library:

testImplementation("io.coil-kt.coil3:coil-test:3.1.0")

coil-test includes a FakeImageLoaderEngine, which can be added to your ImageLoader to intercept all incoming ImageRequests and return a custom ImageResult. This is useful for testing as it makes loading images synchronous (from the main thread) and consistent. By using FakeImageLoaderEngine the ImageLoader will avoid all the memory caching, thread jumping, disk/network I/O fetching, and image decoding that's typically done to load an image. Here's an example:

val engine = FakeImageLoaderEngine.Builder()
    .intercept("https://example.com/image.jpg", ColorImage(Color.Red.toArgb()))
    .intercept({ it is String && it.endsWith("test.png") }, ColorImage(Color.Green.toArgb()))
    .default(ColorImage(Color.Blue.toArgb()))
    .build()
val imageLoader = ImageLoader.Builder(context)
    .components { add(engine) }
    .build()

ColorImage is useful for testing as it can draw a colored box or fill the canvas with a color depending on its width/height and it is supported on all platforms.

This strategy works great with Paparazzi to screenshot test UIs without a physical device or emulator:

class PaparazziTest {
    @get:Rule
    val paparazzi = Paparazzi()

    @Before
    fun before() {
        val engine = FakeImageLoaderEngine.Builder()
            .intercept("https://example.com/image.jpg", ColorImage(Color.Red.toArgb()))
            .intercept({ it is String && it.endsWith("test.png") }, ColorImage(Color.Green.toArgb()))
            .default(ColorImage(Color.Blue.toArgb()))
            .build()
        val imageLoader = ImageLoader.Builder(paparazzi.context)
            .components { add(engine) }
            .build()
        SingletonImageLoader.setUnsafe(imageLoader)
    }

    @Test
    fun testContentComposeRed() {
        // Will display a red box.
        paparazzi.snapshot {
            AsyncImage(
                model = "https://example.com/image.jpg",
                contentDescription = null,
            )
        }
    }

    @Test
    fun testContentComposeGreen() {
        // Will display a green box.
        paparazzi.snapshot {
            AsyncImage(
                model = "https://www.example.com/test.png",
                contentDescription = null,
            )
        }
    }

    @Test
    fun testContentComposeBlue() {
        // Will display a blue box.
        paparazzi.snapshot {
            AsyncImage(
                model = "https://www.example.com/default.png",
                contentDescription = null,
            )
        }
    }
}