cinny/src/app/hooks/createAccountDataListStore.ts

import { useEffect, useState } from 'react';
import { ClientEvent, ClientEventHandlerMap, MatrixClient } from 'matrix-js-sdk';
import { AccountDataEvent } from '../../types/matrix/accountData';
import { getAccountData, setAccountData } from '../utils/accountData';

// Generic, concurrency-safe store for a single global account-data event that
// holds a mutable collection (a list or a record). It is the shared engine
// behind useBookmarks / useReminders / useUserNotes.
//
// Why a module-scoped singleton (per store, per client):
//
// These hooks are mounted by many components at once (e.g. useBookmarks() lives
// on every message row). A per-instance snapshot + write queue would only
// serialize writes within a single instance, so two edits issued from different
// instances before the server echo lands would each compute from a stale
// snapshot and clobber each other — setAccountData replaces the whole content
// with no server-side merge. We therefore keep ONE shared `latest` snapshot and
// ONE write queue per store, keyed off the active MatrixClient, and serialize
// every write through that queue (the lost-update fix). Do not weaken this.

// How the differing payload shapes are parameterized:
// - `read` maps the raw account-data content (C | undefined) to the in-memory
//   value T (e.g. `content?.bookmarks ?? []`, or `content ?? {}` for notes).
// - `write` maps the in-memory value T back to the content object to persist
//   (e.g. `{ bookmarks: value }`, or just `value` for the flat notes record).
export type AccountDataListStoreConfig<T, C> = {
  eventType: AccountDataEvent | string;
  read: (content: C | undefined) => T;
  write: (value: T) => object;
};

export type AccountDataListStore<T> = {
  /** React hook: subscribe to the store and return the latest value. */
  useValue: (mx: MatrixClient) => T;
  /** Read the current shared snapshot without subscribing. */
  getLatest: (mx: MatrixClient) => T;
  /**
   * Serialize a write. `compute` receives the latest snapshot and returns the
   * next value; writes run one at a time so none is computed from stale data.
   */
  enqueueWrite: (mx: MatrixClient, compute: (current: T) => T) => Promise<void>;
};

export function createAccountDataListStore<T, C = unknown>({
  eventType,
  read,
  write,
}: AccountDataListStoreConfig<T, C>): AccountDataListStore<T> {
  type ModuleState = {
    mx: MatrixClient;
    latest: T;
    writeQueue: Promise<unknown>;
    listeners: Set<(value: T) => void>;
    onAccountData: ClientEventHandlerMap[ClientEvent.AccountData];
  };

  let moduleState: ModuleState | null = null;

  const readLatest = (mx: MatrixClient): T => read(getAccountData<C>(mx, eventType));

  // Lazily initialize the shared state for the given client. On a client change
  // (login/logout swaps the MatrixClient) we tear down the old subscription and
  // re-initialize against the new client so we never leak or double-subscribe.
  function ensureModuleState(mx: MatrixClient): ModuleState {
    if (moduleState && moduleState.mx === mx) {
      return moduleState;
    }

    if (moduleState) {
      moduleState.mx.removeListener(ClientEvent.AccountData, moduleState.onAccountData);
    }

    const state: ModuleState = {
      mx,
      latest: readLatest(mx),
      writeQueue: Promise.resolve(),
      listeners: new Set(),
      // Reassigned below once `state` is captured.
      onAccountData: () => undefined,
    };

    state.onAccountData = (evt) => {
      if (evt.getType() === eventType) {
        const value = read(evt.getContent() as C);
        state.latest = value;
        state.listeners.forEach((listener) => listener(value));
      }
    };

    mx.on(ClientEvent.AccountData, state.onAccountData);
    moduleState = state;
    return state;
  }

  function enqueueWrite(mx: MatrixClient, compute: (current: T) => T): Promise<void> {
    const state = ensureModuleState(mx);
    const run = state.writeQueue.then(async () => {
      const next = compute(state.latest);
      state.latest = next;
      state.listeners.forEach((listener) => listener(next));
      await setAccountData(mx, eventType, write(next));
    });
    // Keep the chain alive even if one write rejects, but propagate the
    // rejection to this caller so it can react (e.g. retry).
    state.writeQueue = run.catch(() => undefined);
    return run;
  }

  function useValue(mx: MatrixClient): T {
    const [value, setValue] = useState<T>(() => ensureModuleState(mx).latest);

    // Subscribe to the shared module state. A single AccountData listener is
    // installed per client (in ensureModuleState); each hook instance only
    // registers a local setter and unregisters it on unmount / client change.
    useEffect(() => {
      const state = ensureModuleState(mx);
      setValue(state.latest);
      state.listeners.add(setValue);
      return () => {
        state.listeners.delete(setValue);
      };
    }, [mx]);

    return value;
  }

  const getLatest = (mx: MatrixClient): T => ensureModuleState(mx).latest;

  return { useValue, getLatest, enqueueWrite };
}