Automatic @Preview screenshot tests with Showkase and Paparazzi

ScreenshotTests.kt

// Copyright 2022 Google LLC.
// SPDX-License-Identifier: Apache-2.0

import androidx.compose.foundation.layout.Box
import androidx.compose.runtime.Composable
import androidx.compose.runtime.CompositionLocalProvider
import androidx.compose.ui.platform.LocalInspectionMode
import app.cash.paparazzi.DeviceConfig
import app.cash.paparazzi.Paparazzi
import com.airbnb.android.showkase.models.Showkase
import com.airbnb.android.showkase.models.ShowkaseBrowserComponent
import com.google.testing.junit.testparameterinjector.TestParameter
import com.google.testing.junit.testparameterinjector.TestParameter.TestParameterValuesProvider
import com.google.testing.junit.testparameterinjector.TestParameterInjector
import org.junit.Rule
import org.junit.Test
import org.junit.runner.RunWith

@RunWith(TestParameterInjector::class) // Parameterization via https://github.com/google/TestParameterInjector
class ScreenshotTests {

    object PreviewProvider : TestParameterValuesProvider {
        override fun provideValues(): List<ComponentPreview> =
            Showkase.getMetadata().componentList.map(::ComponentPreview)
    }

    enum class BaseDeviceConfig(
        val deviceConfig: DeviceConfig,
    ) {
        NEXUS_5(DeviceConfig.NEXUS_5),
        PIXEL_5(DeviceConfig.PIXEL_5),
        PIXEL_C(DeviceConfig.PIXEL_C),
    }

    @get:Rule
    val paparazzi = Paparazzi(
        maxPercentDifference = 0.0,
    )

    @Test
    fun preview_tests(
        @TestParameter(valuesProvider = PreviewProvider::class) componentPreview: ComponentPreview,
        @TestParameter baseDeviceConfig: BaseDeviceConfig,
    ) {
        paparazzi.snapshot(
            deviceConfig = baseDeviceConfig.deviceConfig.copy(
                softButtons = false,
            )
        ) {
            CompositionLocalProvider(LocalInspectionMode provides true) {
                Box {
                    componentPreview.content()
                }
            }
        }
    }
}

/**
 * A helper class primarily to use the component key as the parameterized test name which uses `toString()`.
 */
private class ComponentPreview(
    private val showkaseBrowserComponent: ShowkaseBrowserComponent
) {
    val content: @Composable () -> Unit = showkaseBrowserComponent.component
    override fun toString(): String = showkaseBrowserComponent.componentKey
}

A couple additional notes about this setup:

The list of components is pulled from Showkase.getMetadata(), and Paparazzi only works in library modules currently, so this test should be a unit test in a library module where a ShowkaseRootModule is configured. The ShowkaseRootModule could only be defined in tests, if using Showkase only to aggregate all previews.

That's impressively simple and effective.