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Released documentation snapshot. This immutable manual describes viewcompose-preview-core:0.1.0-alpha02 from source revision 963e114e. For current guidance, open the current module catalog.

Preview Core

viewcompose-preview-core defines the platform-neutral annotation, configuration, discovery, render, worker, and diagnostic protocols shared by ViewCompose's Gradle plugin, Layoutlib workers, Android Studio plugin, tests, and CI. It contains no Android or IDE runtime dependency.

Artifact and stability​

dependencies {
    debugImplementation("com.viewcompose:viewcompose-preview-core:0.1.0-alpha01")
}
  • Stability: Alpha. Annotation source shape is established; the tooling wire protocol requires exact version equality and may advance before stable.
  • Platform: JVM 11; protocol models are platform-neutral.
  • Packaging: apply preview metadata to debuggable source sets. The ViewCompose preview Gradle plugin removes it from non-debuggable Android output.
  • Boundary: this module does not load Android Views, Layoutlib, Gradle, or IDE classes.

Preview entry annotations​

@ViewComposePreview marks a top-level or static DSL function. The compiled method must be public and static, accept exactly one UiTreeBuilder receiver/parameter, and return Unit. Additional parameters remain unsupported even when Kotlin gives them default values, because the worker invokes the exact JVM method rather than Kotlin's synthetic default bridge.

Annotations may repeat or be composed into custom multi-preview annotations. Built-ins cover light/dark, phone/tablet, LTR/RTL, and common font scales. Auto height (-1) begins with a reference viewport and grows within safety limits; use a positive height when clipping or scrolling is the behavior under test.

Deterministic configuration​

PreviewConfiguration resolves width, height, density, font scale, locales, direction, application theme mode, and optional API level without consulting the host system. Configuration matrices form a deterministic Cartesian product. Axes and options preserve declaration order; later axes win if several override the same field.

Stable IDs use lowercase ASCII words joined by -, with __ reserved for matrix composition. They are validated before becoming cache keys or artifact paths.

Build and worker protocol​

The Gradle bridge exports a canonical PreviewBuildManifest, sorted build inputs, and lowercase SHA-256 fingerprints. Project bytecode is separated from Layoutlib-compatible inputs so a warm host can retain the expensive platform runtime while creating a fresh child class loader for each render.

Worker batches contain at most eight commands and execute sequentially in one short-lived host. Response paths must be unique. This amortizes JVM startup without allowing mutable Layoutlib or application state to escape indefinitely.

Protocol version checks require exact equality. Paths remain opaque strings across the boundary; the process that owns a filesystem operation is responsible for resolving and constraining them.

Responses and diagnostics​

Worker failures cross the boundary as structured response data. Successful responses require an image artifact; failures require at least one diagnostic. Optional phase timings use unique names and non-negative durations.

Render snapshots deliberately contain only primitive, string, collection, and serializable protocol values. They expose VNode structure, native View bounds, clipping, common View properties, layout problems, patches, composition scopes, and source call sites without retaining runtime-owned Views, VNodes, class loaders, or exceptions.

JSON compatibility​

PreviewProtocolJson encodes defaults for deterministic artifacts, omits explicit nulls, and uses pretty output. Readers ignore unknown keys to permit additive fields, but still execute model validation and reject unsupported protocol versions or invalid identities.

Testing and operations​

  • Treat protocol model changes as cross-process compatibility changes and update every producer and consumer together.
  • Keep ordered inputs canonical before hashing; do not fingerprint nondeterministic collection order.
  • Validate annotation discovery against compiled JVM signatures, not Kotlin source appearance.
  • Bound externally supplied snapshot size and recursive structures in the consuming process.
  • Test protocol round trips, invalid input rejection, variant ordering, and old/new worker mismatch.

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

Compatibility notes​

The 0.1.0-alpha01 line uses protocol version 1, exact-version negotiation, deterministic JSON and fingerprints, bounded worker batches, auto-height configuration, application-owned theme providers, and source-aware render diagnostics. The wire format is not yet promised stable across alpha lines.