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Preview Worker Host

viewcompose-preview-worker-host is the standalone JDK 17+ process boundary that owns mutable Layoutlib state outside Gradle and Android Studio. The Gradle plugin resolves it on the dedicated worker configuration; applications must not place it on an Android runtime classpath.

fun executeWorkerCommandSample(commandJsonFile: File): PreviewRenderResponse {
    return PreviewWorkerHost.execute(commandJsonFile)
}

Process and isolation contract​

One-shot mode accepts one command JSON file; a top-level command collection selects a bounded, sequential batch. Each command validates protocol, module/variant/fingerprint, Layoutlib roots, and exported build inputs, prepares one Paparazzi/Layoutlib SDK, invokes the runner reflectively, and tears down in finally.

Reloadable project code enters a fresh child URLClassLoader for each command. It becomes the thread context loader only during that render, then the previous loader is restored and the child is closed on success or failure. Permanent host/Layoutlib classpaths contain no application code. Non-fatal failures become structured responses after decoding; malformed JSON, filesystem publication failure, thread death, and out-of-memory conditions may terminate the process.

Server mode uses an ephemeral loopback socket plus a random token, exact protocol version, and Layoutlib compatibility fingerprint. It retires after 120 seconds idle, 24 commands, 768 MiB used heap, any failed render/invalid request, or explicit shutdown. Endpoint replacement is token-safe. Each batched render still receives a fresh application class loader and response file.

  • Stability: Alpha executable tooling infrastructure.
  • Treat endpoint files as credentials and require loopback plus token.
  • Test class-loader restoration/closure, atomic response publication, compatibility fingerprints, idle/count/heap/failure retirement, and explicit shutdown with :viewcompose-preview-worker-host:test.

See Preview Core, Gradle Plugin, and the generated API reference.