name: verifying_compose_ui description: > Visually verifies Compose UI components by rendering @Composable/@Preview functions to images from the project's JVM runtime; STRONGLY PREFERRED for rapid UI iteration and visual feedback on any composable. Do NOT use for build lifecycle tasks or dependency auditing. license: Apache-2.0 metadata: author: https://github.com/rnett/gradle-mcp version: "2.2"

⚠️ FUNDAMENTAL ANDROID LIMITATION

IMPORTANT: runComposeUiTest and image capture methods (node.captureToImage()) are NOT supported on Android and CANNOT work.

Why This Cannot Work

The runComposeUiTest function requires a JVM-based test runtime with desktop Compose rendering (via Skiko on JVM/Desktop). Android's ART (Android Runtime) does not support the desktop Compose testing APIs, and image capture via captureToImage() relies on desktop-specific rendering pipelines that are fundamentally incompatible with Android.

The Solution: Use a JVM Target

For visual verification of Composables, you MUST use a JVM or Desktop target source set. This is not a workaround—it is the only supported method.

Recommended approach:

1. Put your Composables in a common source set (commonMain) that is shared across all targets
2. Create or use a JVM target (e.g., jvmMain, desktopMain) that depends on commonMain
3. Run the REPL on the JVM target (sourceSet: "jvmMain" or sourceSet: "jvmTest")

This is the standard KMP pattern and allows you to verify your UI code without needing an Android device or emulator.

What WILL NOT Work

• ❌ Running the REPL with sourceSet: "androidMain"
• ❌ Running the REPL with sourceSet: "androidTest"
• ❌ Any attempt to use runComposeUiTest on Android
• ❌ Any attempt to use captureToImage() on Android

What WILL Work

• ✅ Running the REPL with sourceSet: "jvmMain" or sourceSet: "jvmTest"
• ✅ Running the REPL with sourceSet: "desktopMain" or sourceSet: "desktopTest"
• ✅ Using runComposeUiTest with captureToImage() on JVM/Desktop targets

Authoritative Compose UI Preview & Visual Verification

Visually verifies and renders any @Composable or @Preview directly to high-quality images from the project-aware REPL for instant, authoritative visual feedback.

Constitution

• ALWAYS use kotlin_repl to render Compose components instead of running the full application for visual checks.
• ALWAYS provide absolute paths for projectRoot.
• ALWAYS use node.captureToImage() and responder.render(bitmap) to return the visual output.
• ALWAYS ensure the correct Compose UI testing dependencies are on the classpath, using additionalDependencies if necessary.
• NEVER assume a UI component renders correctly without visual verification.
• ALWAYS search for existing @Preview functions in the project source code before creating new ones.

Directives

• ALWAYS provide absolute projectRoot: Ensure projectRoot is an absolute file system path for all kotlin_repl calls.
• Ensure dependencies: ALWAYS ensure Compose UI testing dependencies (e.g., androidx.compose.ui:ui-test-junit4) are on the classpath.
• Note on versions: ALWAYS check your project's version catalog and existing tests for the correct imports for runComposeUiTest.
• Render as image: ALWAYS use node.captureToImage() and responder.render(bitmap) to return the visual output.
• Kotlin Multiplatform (KMP) Note: The kotlin_repl currently only supports JVM-based source sets. ALWAYS select a JVM or Desktop target source set for visual checks.
• Use envSource: SHELL if environment variables are missing: Set env: { envSource: "SHELL" } (REPL start) or invocationArguments: { envSource: "SHELL" } (Gradle tasks) if expected env vars (e.g., JAVA_HOME) are not found.

When to Use

• Rapid UI Prototyping & Iteration: When you need to see the visual result of a Composable change instantly without the latency of a full application launch.
• Authoritative @Preview Verification: When you want to verify the visual correctness of existing @Preview functions.
• Complex UI State & Interaction Testing: When you need to capture visual state before and after interactions (like clicks or state changes).
• Multi-Configuration Visual Auditing: When checking how a component renders across different data states or configurations (e.g., different view models or mock data).

Workflows

1. Identifying the Component

1. Find the fully qualified name of the @Composable or @Preview function.
2. Search for existing previews in the source code via grep_search(pattern="@Preview").

2. Orchestrating the Session

1. Start the REPL with the test source set (preferred) or main with additionalDependencies.
2. Use kotlin_repl(command="start").
3. Use kotlin_repl(command="run") to execute the rendering script.

3. Rendering & Verifying

1. Execute a script that uses runComposeUiTest to render and capture the component.
2. Inspect the returned image for visual correctness.

Examples

Viewing a simple Composable

import androidx.compose.ui.test.*
import com.example.ui.MyButton

runComposeUiTest {
    setContent {
        MyButton(text = "Click Me")
    }
    val node = onRoot()
    responder.render(node.captureToImage())
}
// Reasoning: Using kotlin_repl to render a specific component and retrieve its visual representation via the responder API.

Viewing an existing @Preview

import androidx.compose.ui.test.*
import com.example.ui.MyButtonPreview // Top-level preview function

runComposeUiTest {
    setContent {
        MyButtonPreview()
    }
    val node = onRoot()
    responder.render(node.captureToImage())
}
// Reasoning: Reusing an existing authoritative preview function to verify its visual correctness.

Capturing State Transitions

import androidx.compose.ui.test.*
import com.example.ui.MyCounter
import com.example.viewmodel.MyViewModel

runComposeUiTest {
    val viewModel = MyViewModel()
    setContent {
        MyCounter(viewModel)
    }

    // Capture state before interaction
    responder.render("State before: ${viewModel.count}")
    responder.render(onRoot().captureToImage())

    // Perform interaction
    onNodeWithText("Increment").performClick()

    // Capture state after interaction
    responder.render("State after: ${viewModel.count}")
    responder.render(onRoot().captureToImage())
}
// Reasoning: Capturing visual snapshots before and after an interaction to verify state-dependent UI changes.

Troubleshooting

• No Image Returned: Ensure you are calling responder.render(bitmap).
• ClassNotFoundException: Check if you have the correct imports and that the required testing dependencies are on the classpath.
• Empty Image: If the Composable is empty or has zero size, the image will be empty. Verify your Composable's modifiers.

Resources

• Troubleshooting