Dropshots is a library and Gradle plugin that makes on-device screenshot testing on Android easy.

Other screenshot testing libraries take screenshots in your instrumentation tests on device, then download those images to your host machine to compare them to reference images, failing at that step. This means that your screenshot assertions aren't run as part of the rest of your test suite, and can't easily be run from within your IDE along with the rest of your tests.

Dropshots makes this process easier by performing your screenshot assertions right in your test, alongside all of your other tests. It's Gradle plugin ensures that your version controlled reference images are available on your test device so that your test screenshots are able to be compared to those reference images right within your test.

Apply the plugin in your module's build.gradle file.

Using the plugins DSL:

// build.gradle(.kts)
plugins {
  id("com.android.application")
  // or id("com.android.library")
  id("com.dropbox.dropshots") version "0.4.1"
}

Note that the plugin is currently published to Maven Central, so you need to add it to the repositories list in settings.gradle.

// settings.gradle(.kts)
pluginsManagement {
  repositories {
    mavenCentral()
    gradlePluginPortal()
  }
}

buildscript {
  repositories {
    mavenCentral()
  }
  dependencies {
    classpath "com.dropbox.dropshots:dropshots-gradle-plugin:0.4.1"
  }
}

apply plugin: "com.android.application"
// or apply plugin: "com.android.library"
apply plugin: "com.dropbox.dropshots"

Once the Dropshots plugin is added to your project, some new tasks will be created to create, validate and manager your screenshot reference images. While you can use the tasks directly, they are also automatically injected into your project's task graph to run as part of your normal testing workflow.

Dropshots screenshot tests are simply standard Android Instrumentation tests which use the runtime library to compare screenshots with reference images. Simply add the Dropshots rule to an instrumentation test, setup the view you'd like to test, and use the Dropshots.assertSnapshot functions to validate them.

class MyTest {
    @get:Rule
    val activityScenarioRule = ActivityScenarioRule(TestActivity::class.java)

    @get:Rule
    val dropshots = Dropshots()

    @Before
    fun setup() {
        // Setup your activity however you like
        activityScenarioRule.scenario.onActivity {
            it.supportFragmentManager.beginTransaction()
                .add(android.R.id.content, ScreenshotTestFragment())
                .commitNow()
        }
    }

    @Test
    fun testMatchesActivityScreenshot() {
        activityScenarioRule.scenario.onActivity {
            // Assert activity snapshots
            dropshots.assertSnapshot(it, "MatchesActivityScreenshot")
        }
    }

    @Test
    fun testMatchesViewScreenshot() {
        activityScenarioRule.scenario.onActivity {
            // or assert view snapshots.
            dropshots.assertSnapshot(
                it.findViewById<View>(android.R.id.content),
                name = "MatchesViewScreenshot",
                path = "views/fullscreen" // optional parameter to set path of stored screenshots
            )
        }
    }
}

With this test in place, any time the connectedAndroidTest task is run the screenshot of the Activity or View will be validated against the reference images stored in the repository. If any screenshots fail to match the reference images (within configurable thresholds), then an image will be written to the test report folder that shows the reference image, the actual image, and the diff of the two. By default, the test report folder is ${project.buildDir}/outputs/androidTest-results/connected.

The first time you create a screenshot test, however, there won't be any reference images, so you'll have to create them...

Updating reference screenshots is as simple as running the tests with a dropshots.record property added to Gradle. This makes it easy to update screenshots in a single step, without requiring you to interact with the emulator or use esoteric adb commands.

Important: Ensure that you record screenshots on an emulator that's been configured in the same way as the emulators on which you'll validate the screenshots.

./gradlew :path:to:module:connectedAndroidTest -Pdropshots.record

After running this command, you'll see that all reference screenshots for the module will have been updated in the src/androidTest/screenshots directory. After that, running connected tests, either from the gradlew CLI or directly from the IDE, will validate the screenshots against the new reference images.

By default Dropshots will fail assertions if the supplied ImageComparator returns any pixels that don't match the reference image. If that is too strict for your use case, then you can supply a custom ResultValidator to specify how comparison results should be validated.

The included CountValidator validates comparison results which contain no more than the specified number of pixel differences. The included ThresholdValidator validates comparison results which contain no more then the specific percentage of pixel differences, based on the entire image size.

Copyright (c) 2022 Dropbox, Inc.

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

    http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.