Plugin Recipes

Copy-paste plugins for common cross-cutting concerns. Each recipe is a self-contained BlacPlugin that you can drop into your app and tweak. All recipes use only @blac/core imports so they work without any extra dependencies unless noted.

For the plugin authoring API that every recipe is built on, see Plugin Authoring. For the official first-party plugins see Plugin Overview.

Compose order and throw semantics

Plugins execute in install order. Keep these rules in mind when you compose several recipes together:

• Persistence first. If you layer a logging plugin on top of a persistence plugin, the logger will see the post-hydration state. That is usually what you want — logging will then record the initial restored value rather than the constructor default.
• Sinks last. Send-to-server plugins (Sentry, audit log) should be installed after any transforming or filtering plugins so they see the final view of the state.
• Hook throws are not caught. If onStateChange throws, the exception propagates synchronously out of the flush and the remaining plugins in the chain are skipped. Guard any I/O inside a try/catch in your hooks rather than letting errors bubble.

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1. localStorage / debounced-save adapter

A hand-rolled localStorage-backed persistence plugin is the same onCreated + onStateChange shape as the official persistence plugin, just backed by localStorage instead of IndexedDB and without the .persist() registration API. Rather than re-deriving that adapter here, see:

• Persistence: custom storage adapter for the adapter interface, or reach for @blac/plugin-persist with a localStorage-backed IndexedDbPersistAdapter directly.
• The debounced-save recipe below if you want a hand-rolled plugin (no .persist() API) that batches writes.

PII caveat

Any client-side storage (localStorage, IndexedDB) is readable by scripts on the same origin. Never persist passwords, tokens, or health data without an encryption layer.

---

2. Debounced-save adapter

Wraps any synchronous write with a debounce so rapid state changes only trigger one write per quiet window. Useful when state changes on every keystroke and the save target (IndexedDB, a remote API) is slow.

import {
  getPluginManager,
  type BlacPlugin,
  type PathSet,
  ALL_PATHS,
} from '@blac/core';

// --- Recipe: debounced-save wrapper ---

type SaveFn = (key: string, state: unknown) => void;

interface DebouncedSaveOptions {
  /**
   * Synchronous or async function that does the actual write.
   * Called once per quiet window per instance.
   */
  save: SaveFn;
  /** Milliseconds of inactivity before flushing. Default: 300. */
  debounceMs?: number;
  /** Key prefix. Default: "blac". */
  prefix?: string;
  /** Optional set of class names to include. Omit to include all. */
  include?: string[];
}

function createDebouncedSavePlugin(opts: DebouncedSaveOptions): BlacPlugin {
  const debounceMs = opts.debounceMs ?? 300;
  const prefix = opts.prefix ?? 'blac';
  const includeSet = opts.include ? new Set(opts.include) : null;
  const timers = new Map<string, ReturnType<typeof setTimeout>>();

  return {
    name: 'debounced-save',
    version: '1.0.0',

    onStateChange(ctx, _prev, next, _paths: PathSet) {
      const c = ctx.container;
      if (!c) return;
      if (includeSet && !includeSet.has(c.$blac.name)) return;

      const key = `${prefix}:${c.$blac.name}:${c.$blac.id}`;

      const existing = timers.get(key);
      if (existing !== undefined) clearTimeout(existing);

      timers.set(
        key,
        setTimeout(() => {
          timers.delete(key);
          opts.save(key, next);
        }, debounceMs),
      );
    },

    onDestroyed(ctx) {
      const c = ctx.container;
      if (!c) return;
      const key = `${prefix}:${c.$blac.name}:${c.$blac.id}`;
      const existing = timers.get(key);
      if (existing !== undefined) {
        // Flush immediately on disposal — don't lose the last state.
        clearTimeout(existing);
        timers.delete(key);
        opts.save(key, c.state);
      }
    },
  };
}

// Example: write to localStorage with a 500 ms debounce.
const debouncedPlugin = createDebouncedSavePlugin({
  save: (key, state) => localStorage.setItem(key, JSON.stringify(state)),
  debounceMs: 500,
  include: ['CartCubit', 'DraftCubit'],
});

// Install once at app startup.
// getPluginManager().install(debouncedPlugin);

Compose order

Install the debounced-save plugin after any persistence plugin if you also use the official @blac/plugin-persist. The official plugin handles hydration; this recipe only handles writes.

---

3. Cross-tab sync

Broadcasts state changes to other browser tabs via BroadcastChannel and applies incoming updates through the hydration API. Use this when multiple tabs show the same app and you want them to stay in sync (e.g. a shopping cart or an auth session).

PII caveat

BroadcastChannel messages are visible to all same-origin tabs for the duration of the browser session. Strip or redact any sensitive fields (tokens, PII) in the serialize option before broadcasting.

import {
  getPluginManager,
  type BlacPlugin,
  type PathSet,
  ALL_PATHS,
  type StateContainerConstructor,
} from '@blac/core';

// --- Recipe: cross-tab sync via BroadcastChannel ---

interface CrossTabSyncOptions<S extends object> {
  /** Classes to sync across tabs. */
  targets: StateContainerConstructor<S>[];
  /** BroadcastChannel name. Default: "blac-sync". */
  channelName?: string;
  /**
   * Optional transform applied before broadcasting.
   * Use this to strip PII or tokens before the message leaves the tab.
   */
  serialize?: (state: S) => unknown;
  /** Optional transform applied when receiving a message. */
  deserialize?: (payload: unknown) => S;
}

interface SyncMessage {
  type: 'state-update';
  className: string;
  instanceId: string;
  state: unknown;
}

function createCrossTabSyncPlugin<S extends object>(
  opts: CrossTabSyncOptions<S>,
): BlacPlugin {
  const channelName = opts.channelName ?? 'blac-sync';
  const targetNames = new Set(opts.targets.map((T) => T.name));

  let channel: BroadcastChannel | null = null;
  // Track which instance keys are receiving a remote update so we don't
  // echo the incoming state back out to other tabs.
  const receiving = new Set<string>();

  return {
    name: 'cross-tab-sync',
    version: '1.0.0',

    onInstall(ctx) {
      if (typeof BroadcastChannel === 'undefined') return;
      channel = new BroadcastChannel(channelName);

      channel.onmessage = (event: MessageEvent<SyncMessage>) => {
        const msg = event.data;
        if (msg?.type !== 'state-update') return;
        if (!targetNames.has(msg.className)) return;

        const instances = ctx.queryInstances(
          // Use the metadata helper to match by name since we only have strings.
          // queryInstances needs the constructor — skip if class not in registry.
          opts.targets.find(
            (T) => T.name === msg.className,
          ) as StateContainerConstructor<S>,
        );

        for (const inst of instances) {
          if (inst.$blac.id !== msg.instanceId) continue;

          const key = `${msg.className}:${msg.instanceId}`;
          receiving.add(key);
          const next = opts.deserialize
            ? opts.deserialize(msg.state)
            : (msg.state as S);
          ctx.startHydration(inst);
          ctx.applyHydratedState(inst, next);
          ctx.finishHydration(inst);
          // Remove after the microtask so onStateChange fires first.
          Promise.resolve().then(() => receiving.delete(key));
        }
      };
    },

    onUninstall() {
      channel?.close();
      channel = null;
    },

    onStateChange(ctx, _prev, next, _paths: PathSet) {
      const c = ctx.container;
      if (!c || !channel) return;
      if (!targetNames.has(c.$blac.name)) return;

      const key = `${c.$blac.name}:${c.$blac.id}`;
      if (receiving.has(key)) return; // avoid echo

      const payload = opts.serialize
        ? opts.serialize(next as unknown as S)
        : next;

      const msg: SyncMessage = {
        type: 'state-update',
        className: c.$blac.name,
        instanceId: c.$blac.id,
        state: payload,
      };
      channel.postMessage(msg);
    },
  };
}

// Install once at app startup.
// getPluginManager().install(createCrossTabSyncPlugin({ targets: [] }));

---

4. Sentry breadcrumb sink

Adds a Sentry breadcrumb for each state change. Because breadcrumbs are sent with every event, this gives you a state-change timeline attached to error reports — similar to Redux DevTools’ action history but visible in Sentry.

PII caveat

State is included verbatim in Sentry breadcrumbs and uploaded to Sentry’s servers. Redact or omit any fields that contain passwords, tokens, health data, or other PII using the sanitize option below. Failure to do so may violate GDPR, HIPAA, or your own privacy policy.

No @sentry/browser in twoslash scope

The Sentry SDK is not installed in the docs type-checker. The snippet below uses a plain ts fence. Copy it into a project where @sentry/browser (or @sentry/react) is installed.

import { getPluginManager } from '@blac/core';
import type { BlacPlugin, PathSet } from '@blac/core';
import * as Sentry from '@sentry/browser';

// --- Recipe: Sentry breadcrumb sink ---

interface SentryPluginOptions {
  /** Classes to include. Omit to include all blocs. */
  include?: string[];
  /** Classes to exclude. */
  exclude?: string[];
  /**
   * Strip sensitive fields from state before attaching to the breadcrumb.
   * Called with the full next state; return the shape you want Sentry to see.
   * REQUIRED if your state contains PII, tokens, or health data.
   */
  sanitize?: (state: unknown, className: string) => unknown;
}

function createSentryPlugin(opts: SentryPluginOptions = {}): BlacPlugin {
  const includeSet = opts.include ? new Set(opts.include) : null;
  const excludeSet = opts.exclude ? new Set(opts.exclude) : null;

  return {
    name: 'sentry-breadcrumbs',
    version: '1.0.0',

    onStateChange(ctx, _prev, next, _paths: PathSet) {
      const c = ctx.container;
      if (!c) return;
      if (includeSet && !includeSet.has(c.$blac.name)) return;
      if (excludeSet && excludeSet.has(c.$blac.name)) return;

      const data = opts.sanitize ? opts.sanitize(next, c.$blac.name) : next;

      Sentry.addBreadcrumb({
        category: 'blac.state',
        message: `${c.$blac.name} changed`,
        level: 'info',
        data: data as Record<string, unknown>,
      });
    },
  };
}

// Install once at app startup. Gate to production so dev overhead stays low.
// getPluginManager().install(createSentryPlugin({
//   sanitize: (state, name) => {
//     if (name === 'AuthCubit') {
//       const s = state as { user: unknown; token: unknown };
//       return { user: s.user, token: '[redacted]' };
//     }
//     return state;
//   },
// }), { environment: 'production' });

---

5. Audit log

Writes a structured log entry for every state change, creation, and disposal. Useful for compliance or debugging — you can pipe entries to your existing logging infrastructure, a remote endpoint, or a circular buffer.

PII caveat

The audit log captures the full prev and next states. If any bloc holds PII (user profiles, payment details, health data), provide a sanitize function to strip sensitive fields before they are stored. Do not ship raw state to external endpoints without first sanitizing it.

import {
  getPluginManager,
  type BlacPlugin,
  type PathSet,
  ALL_PATHS,
} from '@blac/core';

// --- Recipe: structured audit log ---

type AuditEventKind = 'created' | 'state-changed' | 'disposed';

interface AuditEntry {
  kind: AuditEventKind;
  className: string;
  instanceId: string;
  timestamp: number;
  prev?: unknown;
  next?: unknown;
  /** Human-readable changed paths, or "(all)" for a full-replace. */
  paths?: string[];
}

interface AuditLogOptions {
  /**
   * Called with each new entry. Wire to console.log, a remote endpoint,
   * or push into a circular buffer.
   */
  onEntry: (entry: AuditEntry) => void;
  /** Optional per-entry sanitizer. Strip PII before logging externally. */
  sanitize?: (state: unknown, className: string) => unknown;
  /** Optional allowlist of class names to audit. */
  include?: string[];
  /** Optional denylist of class names to skip. */
  exclude?: string[];
}

function createAuditLogPlugin(opts: AuditLogOptions): BlacPlugin {
  const includeSet = opts.include ? new Set(opts.include) : null;
  const excludeSet = opts.exclude ? new Set(opts.exclude) : null;

  const allowed = (name: string) => {
    if (includeSet && !includeSet.has(name)) return false;
    if (excludeSet && excludeSet.has(name)) return false;
    return true;
  };

  const san = (state: unknown, name: string) =>
    opts.sanitize ? opts.sanitize(state, name) : state;

  return {
    name: 'audit-log',
    version: '1.0.0',

    onCreated(ctx) {
      const c = ctx.container;
      if (!c || !allowed(c.$blac.name)) return;
      opts.onEntry({
        kind: 'created',
        className: c.$blac.name,
        instanceId: c.$blac.id,
        timestamp: Date.now(),
        next: san(c.state, c.$blac.name),
      });
    },

    onStateChange(ctx, prev, next, paths: PathSet) {
      const c = ctx.container;
      if (!c || !allowed(c.$blac.name)) return;

      const changedPaths =
        paths === ALL_PATHS
          ? ['(all)']
          : [...paths].map((id) => c.interner.lookup(id));

      opts.onEntry({
        kind: 'state-changed',
        className: c.$blac.name,
        instanceId: c.$blac.id,
        timestamp: Date.now(),
        prev: san(prev, c.$blac.name),
        next: san(next, c.$blac.name),
        paths: changedPaths,
      });
    },

    onDestroyed(ctx) {
      const c = ctx.container;
      if (!c || !allowed(c.$blac.name)) return;
      opts.onEntry({
        kind: 'disposed',
        className: c.$blac.name,
        instanceId: c.$blac.id,
        timestamp: Date.now(),
        prev: san(c.state, c.$blac.name),
      });
    },
  };
}

// Example: log to console with a circular buffer of the last 100 entries.
const auditEntries: AuditEntry[] = [];

getPluginManager().install(
  createAuditLogPlugin({
    onEntry(entry) {
      if (auditEntries.length >= 100) auditEntries.shift();
      auditEntries.push(entry);
      console.debug('[audit]', entry.kind, entry.className, entry.paths);
    },
    sanitize(state, name) {
      if (name === 'AuthCubit') {
        const s = state as { user: unknown; token?: string };
        return { ...s, token: '[redacted]' };
      }
      return state;
    },
  }),
  { environment: 'development' },
);

---

See also

• Plugin Authoring — the BlacPlugin interface and hook reference
• Plugin Overview — first-party plugin catalog
• Persistence — official IndexedDB persistence, including a custom adapter API
• Logging — official structured console logging plugin