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Bounded portable behavior through deterministic state-machine traces

Document status: verified. Canonical source.

Summary

Portable behavior should begin with deterministic event traces, not arbitrary host scripts or an immediately universal visual-scripting graph. SCXML provides the mature semantics needed for the first layer: external events, ordered transition selection, guards, actions and run-to-completion execution. KHR_interactivity provides a current asset-format precedent for typed variables, explicit operations, bounded validation and no-op degradation of unsupported optional operations. Its much broader operation graph is still a Release Candidate and is intentionally Turing-complete, so copying it wholesale would expand NUIF’s safety and conformance surface before basic portability is proven.

The best first experiment is therefore a flat, bounded state-machine sidecar that references stable NUIF entities and emits abstract effects as data. The same authored program is executed by independent Rust and JavaScript runtimes; complete traces, not merely final pixels, are the conformance observation.

Evidence

  • SCXML 1.0 is a W3C Recommendation. Its basic model selects transitions from an active state in response to events, permits conditions and resolves multiple matches by document order. Its interpreter principles require causality, deterministic behavior without external processors and run-to-completion before another external event is processed. This supports ordered guarded transitions and one-event-at-a-time traces. Locator: https://www.w3.org/TR/scxml/#Basic, and https://www.w3.org/TR/scxml/#AlgorithmforSCXMLInterpretation, retrieved 2026-08-31.
  • The current KHR_interactivity specification identifies itself as Release Candidate. It describes directed acyclic behavior graphs, strictly typed value sockets, retained custom variables, explicit static/dynamic resource limits and unsupported extension operations demoted to no-ops. It also says arbitrary scripting is not a design goal and acknowledges that its complete execution model is Turing-complete, requiring runtime limits. NUIF profile 0 consequently adopts the type, capability and limit lessons while excluding internal loops, timers and general computation. Locator: https://raw.githubusercontent.com/KhronosGroup/glTF/refs/heads/main/extensions/2.0/Khronos/KHR_interactivity/Specification.adoc, sections Introduction, Concepts, Unsupported Operations and Limits, retrieved 2026-08-31.
  • Khronos submitted KHR_interactivity for ratification on 16 July 2026. The announcement describes event/control/data/state nodes, custom variables, property writes and extension-defined capabilities; unavailable companion operations degrade to no-ops. Submission is not ratification, so NUIF must retain the Release Candidate status in its decision record. Locator: https://www.khronos.org/news/press/gltf-interactivity-extension-submitted-for-ratification, retrieved 2026-08-31.
  • The Khronos interactivity test-asset repository currently reports 149 self-checking cases and 831 sub-tests and recommends both manual and automated engine execution. This supports generated fixtures and machine-readable traces rather than prose-only behavior claims. Locator: https://github.com/KhronosGroup/glTF-Test-Assets-Interactivity/blob/main/Tests/Interactivity/README.md, retrieved 2026-08-31.

Mechanism

nuif-behavior-state-machine-0 admits activate events only from stable NUIF entities carrying an activatable semantic role. The active state’s transitions are evaluated in authored order. The first exact event and equality-guard match executes at most 64 sequential actions, then reaches its target state. There is no internal event generation, recursion or asynchronous continuation, so every accepted external event terminates under a statically known action budget.

The value surface is Boolean plus bounded string. State actions set or toggle those values. Effects are abstract visibility(Boolean) and announcement(String) records addressed to stable entities. A host declares the effect capabilities it implements. Missing required capabilities reject runtime construction; unavailable optional_noop effects are skipped and recorded. This is an explicit fallback policy, not silent loss.

The Rust gate produces a five-event fixture and expected full/required-only traces. A separately written Node interpreter validates and executes the same program, then compares every selected transition, state, variable, effect and skipped optional operation. The initial trial passes both capability runs and the required-capability refusal probe on local Node 26.7.0; CI separately pins Node 24.20.0 and records its hosted result rather than inferring it from the workflow.

NUIF relevance

Behavior should be a modular layer over stable document identity. It should not make the canonical document an event log, grant scripts authority, or force every reader to implement timers, networking and host business logic. The sidecar experiment established semantics and test vectors before a wire decision. RFC 0012 now selects one canonical-CBOR, content-addressed package resource as the first experimental transport. This reuses the existing package manifest and hash without adding behavior to the canonical semantic document. Generic readers preserve inert bytes; explicit behavior loading revalidates the program against the package document before any separately authorized runtime can be constructed.

The abstract-effect boundary also keeps target fidelity honest. A web adapter now maps the bounded subset to native activation, DOM visibility and one status region under nuif-web-behavior-0; a presentation runtime may map it to scene visibility; a device profile may reject announcements. Those adapters share a trace contract but retain independent capability and host-observation reports.

Open questions

  • What multi-host evidence would justify moving a later behavior profile from a package resource into the canonical semantic model?
  • Which additional event kinds can be grounded in portable semantics rather than vendor input APIs?
  • What numeric type and JSON encoding can preserve exact cross-language values without JavaScript precision loss?
  • Which internal events, timers and animation triggers admit a static termination/budget rule strong enough for the next profile?
  • What native UI adapter can observe the same effects without confusing host agreement with visual or assistive-technology equivalence?