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[2.0 rc.14] Docs: what a pending <Loading> subtree computes, commits and runs, so a view can be preloaded by rendering it

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Assessment

Difficulty
3/5
Estimated time
Half a day
Newbie friendliness
32/100
Issue type
Documentation
Clarity
Clearly specified
Activity status
Active
Tech stack
typescript
Domain
documentation

Research direction

The request is about the boundaries documentation page, which the issue says covers only when <Loading> shows its fallback. Start by reading that page, then run the issue's reproduction (the gated and plain buttons) to confirm the three behaviours: children keep computing, a pending boundary never commits or runs effects, and a never-settling promise memo keeps it pending. Done means the page either documents those three points as supported with the preloading case as an example, or states which are not guaranteed. The maintainers need to decide which, so check with them before writing the text.

Written by the indexing model from the issue text.

Description

Summary

A view's data can be preloaded without listing what it reads: render it into a detached element inside a <Loading> held pending by a promise that never settles. Every read in its tree then starts, level by level as data arrives, while nothing is committed and no effect runs. This works on rc.14, but the docs don't describe any of the behaviour it depends on, so it may change without notice. This asks for that behaviour to be documented as supported, or for the docs to say it isn't.

Reproduction

A page with a list and one detail per row. run(true) renders it into a detached element behind a never-settling gate; run(false) renders the same tree without the gate, as a control.

import { render } from "@solidjs/web";
import {
  createEffect,
  createMemo,
  For,
  getOwner,
  Loading,
  onSettled,
  runWithOwner,
} from "solid-js";

const t0 = performance.now();
const log: string[] = [];
const stamp = (entry: string) => log.push(`${Math.round(performance.now() - t0)}ms ${entry}`);
const delay = (ms: number) => new Promise((resolve) => setTimeout(resolve, ms));
async function load<T>(name: string, value: T) {
  stamp(`load ${name}`);
  await delay(100);
  return value;
}

function Detail(props: { id: string }) {
  const detail = createMemo(() => load(`detail:${props.id}`, `detail ${props.id}`));
  return <p ref={() => stamp(`element detail:${props.id}`)}>{detail()}</p>;
}

function Page() {
  const list = createMemo(() => load("list", ["a", "b"]));
  createEffect(
    () => list(),
    () => {
      stamp("effect ran");
    },
  );
  onSettled(() => {
    stamp("onSettled ran");
  });
  return (
    <ul ref={() => stamp("element list")}>
      <For each={list()}>
        {(id) => (
          <li>
            <Detail id={id} />
          </li>
        )}
      </For>
    </ul>
  );
}

function run(gated: boolean) {
  log.length = 0;
  const host = document.createElement("div");
  const dispose = runWithOwner(owner, () =>
    render(() => {
      const gate = createMemo(() => new Promise<never>(() => {}));
      return (
        <Loading fallback={null}>
          {gated ? gate() : null}
          <Page />
        </Loading>
      );
    }, host),
  );
  setTimeout(() => {
    console.log(log, host.innerHTML);
    dispose();
  }, 600);
}

let owner: ReturnType<typeof getOwner> = null;
export default function App() {
  owner = getOwner();
  return (
    <main>
      <h1>preview</h1>
      <button type="button" onClick={() => run(true)}>
        gated
      </button>
      <button type="button" onClick={() => run(false)}>
        plain
      </button>
    </main>
  );
}

Measured in headless Chromium under vite dev (times from run()):

Loads started Elements created (ref ran) Committed to the host createEffect / onSettled Console
gated list at 0 ms, detail:a/detail:b at 103 ms ul, both p no (host empty) never ran nothing
plain list at 0 ms, detail:a/detail:b at 105 ms ul, both p yes ran at 208 ms nothing
What is undocumented

The boundaries page describes only when a <Loading> shows its fallback. The pattern above depends on three things it doesn't say:

  1. The children of a pending <Loading> keep computing, and reads discovered as data arrives start too.
  2. A pending boundary never commits its children or runs their effects (createEffect, onSettled), however long it stays pending.
  3. A memo whose value is a promise that never settles is an acceptable way to keep a boundary pending, without a diagnostic now or later.
Request

Document these three points as supported behaviour, for example on the boundaries page with this preloading use as the example, or say which of them is not guaranteed.

A primitive that does the same without building the DOM is requested in #3975.

(submitted by Claude Opus 5.5 on behalf of rvlzzr)

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