page-transition-and-rendering-flow

Page Transition and Rendering Flow

Problem

The page transition path in GROWI spans SSR, client-side navigation, URL normalization, Jotai atom hydration, and asynchronous data fetching. Changes in any one of these layers can cause subtle regressions in other layers:

• Forcing global atom updates during render causes "setState during render of a different component" warnings.
• Moving hydration into useEffect without care causes flashes of stale values or hydration mismatches.
• Confusing router.asPath vs props.currentPathname vs currentPathnameAtom leads to inconsistent reads across the transition.

This skill documents the intended flow so that edits preserve invariants.

Key Actors

1. pages/[[...path]]/index.page.tsx — Dynamic route component. Runs on server and client. Hydrates page-data atoms (currentPageDataAtom, etc.) from getServerSideProps output.
2. pages/[[...path]]/use-same-route-navigation.ts — Detects router.asPath changes on the client and triggers fetchCurrentPage. Always attempts fetch; actual skip is decided inside useFetchCurrentPage.
3. states/page/use-fetch-current-page.ts — Single source of truth for page data fetching. Decides whether a fetch is actually needed (guards against duplicate fetches by comparing decoded path / permalink ID with current atom state). Updates page-data atoms atomically on success to avoid intermediate states.
4. pages/[[...path]]/use-shallow-routing.ts — After hydration, compares the browser URL with props.currentPathname (the server-normalized path) and issues a router.replace(..., { shallow: true }) to align them.
5. pages/[[...path]]/server-side-props.ts — getServerSidePropsForInitial calls retrievePageData, performs path normalization (e.g. /user/username → /user/username/), and returns data + normalized currentPathname as props.
6. pages/_app.page.tsx / states/global/hydrate.ts — Hydrates global atoms (currentPathnameAtom, currentUserAtom, isMaintenanceModeAtom) via useHydrateGlobalEachAtoms.

Flow 1: Server-Side Rendering (first load / reload)

1. Request received: server receives request (e.g. /user/username/memo).
2. getServerSideProps runs:
   • getServerSidePropsForInitial executes.
   • retrievePageData normalizes the path and fetches page data from the API.
   • Returns page data and normalized currentPathname as props.
3. Component renders, atoms initialized:
   • [[...path]]/index.page.tsx receives props and initializes page-data atoms (currentPageDataAtom, etc.).
   • PageView and children render on the server.
4. Client-side hydration + URL alignment:
   • Browser receives HTML; React hydrates.
   • useShallowRouting compares browser URL (/user/username/memo) against props.currentPathname (/user/username/memo/).
   • On mismatch, router.replace(..., { shallow: true }) silently rewrites the browser URL to the server-normalized path.

Flow 2: Client-Side Navigation (<Link> click)

1. Navigation start: user clicks <Link href="/new/page">. useRouter detects URL change and [[...path]]/index.page.tsx re-evaluates.
2. useSameRouteNavigation triggers fetch:
   • Its useEffect detects router.asPath change (/new/page).
   • Calls fetchCurrentPage({ path: '/new/page' }). This hook always attempts the call.
3. useFetchCurrentPage decides and executes:
   • 3a. Path preprocessing: decodes the path; detects permalink format (e.g. /65d4...).
   • 3b. Dedup guard: compares preprocessed path / extracted page ID against current Jotai state. If equal, returns without hitting the API.
   • 3c. Loading flag: sets pageLoadingAtom = true.
   • 3d. API call: apiv3Get('/page', ...) with path / pageId / revisionId.
4. Atomic state update:
   • Success: all relevant atoms (currentPageDataAtom, currentPageEntityIdAtom, currentPageEmptyIdAtom, pageNotFoundAtom, pageLoadingAtom, …) are updated together, avoiding intermediate states where pageId is temporarily undefined.
   • Error (e.g. 404): pageErrorAtom set, pageNotFoundAtom = true, pageLoadingAtom = false last.
5. PageView re-renders with the new data.
6. Side effects: after fetchCurrentPage completes, useSameRouteNavigation calls mutateEditingMarkdown to refresh editor state.

Critical Rule: Global Atom Hydration vs Subsequent Sync

Rule: In useHydrateGlobalEachAtoms (and similar hooks that run inside _app.page.tsx), do not use useHydrateAtoms(tuples, { dangerouslyForceHydrate: true }) to keep atoms aligned with commonEachProps across navigations.

Why

• useHydrateAtoms runs during render. With dangerouslyForceHydrate: true, it re-writes atom values on every render — including navigations when props change.
• Those atoms are subscribed by already-mounted components (e.g. PageViewComponent). Writing to them mid-render triggers setState on sibling components during the parent's render, producing:

  Warning: Cannot update a component (PageViewComponent) while rendering a different component (GrowiAppSubstance).

Correct pattern

Split the two concerns:

export const useHydrateGlobalEachAtoms = (commonEachProps: CommonEachProps): void => {
  // 1. Initial hydration only — so children read correct values on first render
  const tuples = [
    createAtomTuple(currentPathnameAtom, commonEachProps.currentPathname),
    createAtomTuple(currentUserAtom, commonEachProps.currentUser),
    createAtomTuple(isMaintenanceModeAtom, commonEachProps.isMaintenanceMode),
  ];
  useHydrateAtoms(tuples); // force NOT enabled

  // 2. Subsequent sync (route transitions) — run after commit to avoid render-time setState
  const setCurrentPathname = useSetAtom(currentPathnameAtom);
  const setCurrentUser = useSetAtom(currentUserAtom);
  const setIsMaintenanceMode = useSetAtom(isMaintenanceModeAtom);

  useEffect(() => {
    setCurrentPathname(commonEachProps.currentPathname);
  }, [commonEachProps.currentPathname, setCurrentPathname]);

  useEffect(() => {
    setCurrentUser(commonEachProps.currentUser);
  }, [commonEachProps.currentUser, setCurrentUser]);

  useEffect(() => {
    setIsMaintenanceMode(commonEachProps.isMaintenanceMode);
  }, [commonEachProps.isMaintenanceMode, setIsMaintenanceMode]);
};

Trade-off accepted by this pattern

On a route transition, currentPathnameAtom is one render behind before the effect commits. This is safe because:

• Data fetching (useSameRouteNavigation, useFetchCurrentPage, useShallowRouting) reads router.asPath or props.currentPathname directly — not currentPathnameAtom.
• useCurrentPagePath uses currentPagePathAtom (page data) as primary and falls back to currentPathname only when the page data is absent.
• Jotai's Object.is comparison means the effect is a no-op when the value hasn't actually changed, so setters don't need manual guards.

Source Reference Map

When to Apply

• Editing any hook under states/global/ that hydrates from commonEachProps / commonInitialProps.
• Modifying useSameRouteNavigation, useFetchCurrentPage, or useShallowRouting.
• Adding new global atoms that must stay aligned with server-side props across navigations.
• Touching _app.page.tsx render order or provider composition.
• Debugging "setState during render of a different component" warnings originating from _app.page.tsx or GrowiAppSubstance.

Common Pitfalls

1. dangerouslyForceHydrate: true for route-sync purposes — breaks the render model. Use useEffect + useSetAtom instead.
2. Moving the initial hydration into useEffect — children reading the atom on first render would see the default (empty) value, causing flashes / hydration mismatches.
3. Using currentPathnameAtom as the trigger for data fetching — the trigger is router.asPath, and the normalized authority is props.currentPathname. The atom is for downstream UI consumers only.
4. Updating page-data atoms one-by-one during a fetch — always update atomically (success block) to avoid intermediate states visible to PageView.
5. Adding a guard like if (new !== old) set(new) for atoms — unnecessary; Jotai already dedupes on Object.is.