@exponential_app/eui-element: genUI as a custom element

<exponential-surface> paints an A2UI surface in the core catalog (@exponential_app/eui) in any page, with no framework (FEED-130). It does not have a painter of its own. It mounts the Solid renderer (@exponential_app/eui-solid) inside itself, so you get one painter with two entry points. The element uses light DOM and is display: contents through one shared zero-specificity rule define() installs (:where(exponential-surface){display:contents}, so any rule of yours wins), so the surface's .xui-surface root is THE box, in the same place React's div would be. Elements attached in one task mount together: the first at once, the rest in a microtask inside one batch.

The invariant

For every surface, the element paints exactly what the React renderer paints: the same elements, attributes, <style data-xui-style> sheets, layers and runtime themes. src/parity.test.ts compares the element's .xui-surface subtree against @exponential_app/eui-react in jsdom, using the Solid package's serializeDom and renderReact.

<script type="module">
  import "@exponential_app/eui-element/define"   // or the self-contained ./standalone bundle
</script>

<exponential-surface theme="orange" mode="system" src="/ui/stream" post="/ui/events"></exponential-surface>

<script type="module">
  const el = document.querySelector("exponential-surface")
  el.addEventListener("xui-action", (e) => e.detail.waitUntil(fetch("/save", { method: "POST", body: JSON.stringify(e.detail.context) })))
</script>

Without a server, call apply() to feed it A2UI v0.9 messages:

const el = document.createElement("exponential-surface")
document.body.append(el)
el.apply({ createSurface: { surfaceId: "main", catalogId } })
el.apply({ updateComponents: { surfaceId: "main", components } })

API

String properties reflect to their attributes. Setting a property to null removes its attribute. Properties set before the element is attached (or before it is upgraded) still apply.

Read-only properties:

• handle: the SurfaceHandle, or null until the first paint.
• issues: the active store's reduce issues.
• status: the transport status; statusDetail: its detail (an error, or why the URL policy refused src/post).
• surfaceState: the SurfaceState being painted.

Methods:

• apply(message) sends the message to the surface property, else to the host (receive), else to a store created on first use.
• setData(pointer, value) writes the active store. In fixture mode it sets a new data.
• reset() resets the active store.
• run(command), focus(id), announce(text, live?), scrollIntoView(id) and scrollToIndex(id, index, align?) go through the handle. Before the first paint they return false or do nothing. Called without a string id, focus() and scrollIntoView() keep their usual HTMLElement meaning.

Detaching the element disposes the painted DOM. Re-attaching it paints again, and every property survives. Attaching also clears any children the author put inside the element.

The QrScanner camera

A QrScanner paints only the viewfinder; the camera is the page's, through plugin.qrScanner (the Solid renderer's hook, VAPP-162):

el.plugin = {
  qrScanner: {
    // feed: an empty element filling the viewfinder (append a <video>);
    // formats: default ["qr"]; call onValue per decoded string; return the stop.
    start(feed, formats, onValue) {
      const video = feed.appendChild(document.createElement("video"))
      // … getUserMedia + a decoder calling onValue(text) …
      return () => video.srcObject?.getTracks().forEach((t) => t.stop())
    },
  },
}
el.addEventListener("xui-action", (e) => e.detail.event === "scanned" && pair(e.detail.payload.value))

A value fires the node's scanned handler (xui-action, and plugin.onAction) once per distinct value. Without qrScanner (or when start returns false) the scanner shows Camera unavailable.

The Carousel thumbnails

Round 6 (EXP-1495) paints through the Solid painter: a Carousel with counter: true shows the i / n pill and one with thumbnails: [url…] the strip under its pages (two or more pages). A thumbnail press pages like a dot, so the page gets xui-action with event: "change" and payload.page.

Events

Every event is a CustomEvent that bubbles and is composed. When there is a matching plugin callback, both the callback and the event fire. Where a promise is involved, the results are joined: the control stays pending until all of them settle.

HTMLElementEventMap and HTMLElementTagNameMap are augmented, so addEventListener("xui-action", …) and document.createElement("exponential-surface") are typed.

The host API

When src is set and no host property is given, the element builds new ExponentialHost({ transport }) with one of these transports:

• WebSocketTransport when src is a ws:/wss: URL or transport="ws".
• SseTransport when transport="sse".
• JsonlStreamTransport otherwise.

Client messages are POSTed to post (default src). The element connects the host it built when attached, closes it when detached, and rebuilds it when src, post or transport change. A host surface paints nothing until the host has a surface with surface-id. Actions, functions, URLs and media route through the host (hostPlugin). xui-open-url still fires before the host opens a URL.

A host property belongs to the page. The element connects it only if it has a transport and has never connected (status === "closed"), and it never closes it.

src and post pass the page's URL policy (plugin.urls, the one every href passes) BEFORE a transport is built (decideTransport): a private, loopback or link-local host (10.0.0.5, localhost, *.local, 169.254.169.254, ::1, decimal and IPv4-mapped literals) is denied unless plugin.urls.allowPrivate opts it in (the page's own private origin is, so a dev server on localhost streams from itself); a hosts allowlist holds; schemes: ["https"] means https + wss only; javascript:, file:, data: never connect. A ws:/wss: url is decided as its http(s) twin. A denied src or post builds no host and fetches nothing: status is closed, statusDetail says why (src denied by the URL policy (private)) and xui-status fires once. The standalone bundle carries the same filter.

Extensions

import { defineElementExtension } from "@exponential_app/eui-element"

el.extensions = [defineElementExtension({
  catalog,                                    // defineExtension(...) or its JSON
  components: {
    Gauge: (ctx) => {
      const div = document.createElement("div")
      ctx.applyRootProps(div)                   // class, data-xui-id, data-xui-c, data-r-*, aria, dir, style
      div.textContent = `${ctx.props.value}`
      div.append(...ctx.children())             // ctx.slot("header") for a slot
      div.onclick = () => ctx.emit("press")
      return div
    },
  },
})]

A painter is a plain function: ElementPaintContext goes in, and one DOM element comes out. It runs again when the node, props, theme, mode or root attributes change, and the new element replaces the old one. The painted children and slots are the same nodes on every re-paint. registerExtension(ext) registers one for every surface on the page. A components key that names a core native overrides its painter. A host built from src learns the catalogs of el.extensions when it is built: set the extensions before the element attaches (or before src).

Icons for framework-less hosts

el.plugin = { icons: iconsFromSvg((name) => sprite[name]) }   // SVG markup strings

Each markup string becomes an icon component, created once per distinct string. A name with no markup falls back to the renderer's own Lucide glyphs. The wrapper sets on the <svg> root what lucide-react's root carries for the props it receives:

• width/height = width ?? size ?? 24.
• class = the markup's own classes + the painter's.
• aria-hidden="true", unless an aria-*/role/title prop is passed.
• stroke = color, stroke-width = strokeWidth.
• Every other string or number attribute as given (data-icon…).

The markup is trusted, but <script>, foreignObject, on* attributes and javascript: links are stripped anyway.

Entries

Layout

Commands

bun run --filter @exponential_app/eui-element typecheck
bun run --filter @exponential_app/eui-element test