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Released documentation snapshot. This immutable manual describes viewcompose-preview-worker-host:0.1.0-alpha03 from source revision 143b09ac. For current guidance, open the current module catalog.

Preview Worker Host

viewcompose-preview-worker-host is the standalone JVM process boundary that owns Layoutlib for ViewCompose static previews. It keeps mutable platform rendering state outside Gradle and Android Studio, validates protocol files, isolates reloadable application classes, and publishes structured responses atomically.

Artifact and stability​

dependencies {
    runtimeOnly("com.viewcompose:viewcompose-preview-worker-host:0.1.0-alpha03")
}
  • Stability: Alpha. The executable protocol is internal tooling infrastructure.
  • Runtime: JDK 17 or newer.
  • Normal installation: the ViewCompose preview Gradle plugin resolves this artifact; application code should not add it to an Android runtime classpath.
  • Boundary: the host depends on preview-core and the platform Layoutlib bridge, not application UI modules at compile time.

One-shot execution​

The main entry point accepts one worker-command JSON path. A top-level commands field selects a bounded batch; otherwise the file is one command. Commands validate protocol version, module path, variant, build fingerprint, Layoutlib roots, and every exported build input before rendering.

Each command configures a Paparazzi/Layoutlib SDK from the canonical manifest, prepares it, invokes the Android runner reflectively, and tears it down in finally. Setup, runner, export, and teardown timings are preserved in the response.

Class-loader and failure isolation​

Reloadable project bytecode enters a fresh child URLClassLoader per command. The host installs it as the thread context loader only for that render, then restores the previous loader and closes the child even when rendering fails. Layoutlib and host classes remain in the parent process classpath.

Once a request is decoded, non-fatal validation, setup, runner, and export failures become a source-aware RenderFailure response. Thread death and out-of-memory errors escape. Malformed command/request JSON and filesystem publication failures may fail the process before a response is available. Response files use temporary-file replacement so clients never observe partial JSON.

Warm server lifecycle​

Server mode binds an ephemeral loopback-only socket and atomically publishes an endpoint containing protocol version, process ID, random token, port, and compatibility fingerprint. Every client must present the matching token and protocol version.

The default server retires after 120 seconds idle, 24 processed commands, 768 MiB used heap, any failed render, an invalid client request, or an explicit shutdown. The endpoint file is deleted only if it still carries that server's token, preventing an old process from deleting a replacement server's endpoint.

Batch behavior​

Commands in one protocol batch execute sequentially. Each render still gets its own reloadable class loader and response file; the shared process only amortizes JVM and retained Layoutlib startup. Batch size is bounded by preview-core before execution. A structured render failure is returned for that command and causes a persistent server to retire before accepting more work.

Testing and operations​

  • Never place application bytecode on the permanent worker process classpath.
  • Verify the compatibility fingerprint whenever retained Layoutlib inputs change.
  • Test context-class-loader restoration and closure on success and every failure phase.
  • Treat endpoint files as credentials: require loopback transport and the random token together.
  • Exercise idle, command-count, heap-pressure, render-failure, and explicit-shutdown retirement.
  • Keep worker stdout/stderr diagnostic-only; protocol results belong in response files.

The complete generated reference is available in the viewcompose-preview-worker-host API tree.

Compatibility notes​

The 0.1.0-alpha02 line establishes one-shot and loopback server modes, exact protocol/token checks, fresh reloadable child class loaders, deterministic Layoutlib teardown, atomic responses, and bounded retirement. These process-level limits may be tuned across alpha releases.