Compose Preview Screenshot Testing
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Screenshot testing is an effective way to verify how your UI looks to users. The Compose Preview Screenshot Testing tool combines the simplicity and features of composable previews with the productivity gains of running host-side screenshot tests. Compose Preview Screenshot Testing is designed to be as easy to use as composable previews.

A screenshot test is an automated test that takes a screenshot of a piece of UI and then compares it against a previously approved reference image. If the images don't match, the test fails and produces an HTML report to help you compare and find the differences.

With the Compose Preview Screenshot Testing tool, you can:

Use @PreviewTest to create screenshot tests for existing or new composable previews.
Generate reference images from those composable previews.
Generate an HTML report that identifies changes to those previews after you make code changes.
Use @Preview parameters, such as uiMode or fontScale, and multi-previews to help you scale your tests.
Modularize your tests with the new screenshotTest source set.

Requirements

To use Compose Preview Screenshot Testing, you need the following:

Android Gradle plugin 8.5.0-beta01 or higher.
Kotlin 1.9.20 or higher. We recommend using Kotlin 2.0 or higher so you can use the Compose Compiler Gradle plugin.
Compose enabled for your project. We recommend enabling Compose using the Compose Compiler Gradle plugin.

Note: If you can't use the Compose Compiler Gradle plugin, you can enable Compose by declaring a dependency on the Compose Compiler directly. Make sure you use kotlinCompilerExtensionVersion version 1.5.4 or higher.

Setup

To enable the tool, follow these steps:

Add the com.android.compose.screenshot plugin, version 0.0.1-alpha10 to your project.

Add the plugin to your version catalogs file:

[versions]
agp = "8.11.0-alpha06"
kotlin = "2.1.20"
screenshot = "0.0.1-alpha10"

[plugins]
screenshot = { id = "com.android.compose.screenshot", version.ref = "screenshot"}

In your module-level build.gradle.kts file, add the plugin in the plugins {} block:
```
plugins {
    alias(libs.plugins.screenshot)
}
```

Enable the experimental property in your project's gradle.properties file.
```
android.experimental.enableScreenshotTest=true
```
In the android {} block of your module-level build.gradle.kts file, enable the experimental flag to use the screenshotTest source set.
```
android {
    experimentalProperties["android.experimental.enableScreenshotTest"] = true
}
```

Add the screenshot-validation-api and ui-tooling dependencies.

Add them to your version catalogs:

[libraries]
screenshot-validation-api = { group = "com.android.tools.screenshot", name = "screenshot-validation-api", version.ref = "screenshot"}
androidx-ui-tooling = { group = "androidx.compose.ui", name = "ui-tooling"}

Add them to your module-level build.gradle.kts file:

dependencies {
  screenshotTestImplementation(libs.screenshot.validation.api)
  screenshotTestImplementation(libs.androidx.ui.tooling)
}

Designate composable previews to use for screenshot tests

To designate the composable previews you want to use for screenshot tests, mark the previews with the @PreviewTest annotation. The previews must be located in the new screenshotTest source set, for example app/src/screenshotTest/kotlin/com/example/yourapp/ExamplePreviewScreenshotTest.kt.

You can add more composables and/or previews, including multi-previews, in this file or other files created in the same source set.

package com.example.yourapp

import androidx.compose.runtime.Composable
import androidx.compose.ui.tooling.preview.Preview
import com.android.tools.screenshot.PreviewTest
import com.example.yourapp.ui.theme.MyApplicationTheme

@PreviewTest
@Preview(showBackground = true)
@Composable
fun GreetingPreview() {
    MyApplicationTheme {
        Greeting("Android!")
    }
}

Generate reference images

After you set up a test class, you need to generate reference images for each preview. These reference images are used to identify changes later, after you make code changes. To generate reference images for your composable preview screenshot tests, run the following Gradle task:

Linux and macOS: ./gradlew updateDebugScreenshotTest (./gradlew :{module}:update{Variant}ScreenshotTest)
Windows: gradlew updateDebugScreenshotTest (gradlew :{module}:update{Variant}ScreenshotTest)

After the task completes, find the reference images in app/src/screenshotTestDebug/reference ({module}/src/screenshotTest{Variant}/reference).

Generate a test report

Once the reference images exist, run the validate task to take a new screenshot and compare it with the reference image:

Linux and macOS: ./gradlew validateDebugScreenshotTest (./gradlew :{module}:validate{Variant}ScreenshotTest)
Windows: gradlew validateDebugScreenshotTest (gradlew :{module}:validate{Variant}ScreenshotTest)

The verification task creates an HTML report at {module}/build/reports/screenshotTest/preview/{variant}/index.html.

Known issues

You can find the current list of known issues in the tool's issue tracker component. Report any other feedback and issues through the issue tracker.

Release updates

Release notes and changes for ongoing versions.

0.0.1-alpha10

This release introduces:

From this version, you need to mark all of your preview functions with the @PreviewTest annotation. Previews without the annotation won't be executed.
Reference image directory is changed from {module}/src/{variant}/screenshotTest/reference to {module}/src/screenshotTest{Variant}/reference. This is to make sure those generated reference images won't be part of the production code, and to be aligned with the directory structure of other test types.
The {variant}PreviewScreenshotRender task is removed. Image rendering is migrated into JUnit Test Engine.
The update{Variant}ScreenshotTest task will compare new rendering images to reference images before updating. It will only update images that have differences greater than a specified threshold. --updateFilter commandline flag was removed.

0.0.1-alpha06