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# Event Bus (`@repo/core-events`)

The Global Pub/Sub & Hardware Integration Blueprint.

> This module provides a strictly-typed, global event bus for the monorepo ecosystem. It decouples cross-component communication and manages real-time hardware signals (such as printers and POS peripherals), ensuring a reactive and memory-safe architecture across all applications.

---

## 🧠 System Overview

`@repo/core-events` is the **decoupled Nervous System** of the ERP. It provides a highly performant, strictly-typed Event Bus powered by `mitt` and custom React hooks.

**This package is a pure tool.** It ships zero application-specific events. Each consuming app (`apps/web`, `apps/desktop`, etc.) registers its own events autonomously using **TypeScript Declaration Merging** — the exact same Inversion of Control (IoC) pattern utilized by our `@repo/core-api` factory and `@repo/core-storage` engine.

### Architectural Topology

### 1. Conceptual Topology: The Pub/Sub Data Flow
This diagram illustrates the high-level concept of our decoupled architecture, demonstrating how application-specific types merge into the core bus.

```mermaid
graph LR
    %% ─── Styling Definitions (Dark-Mode Friendly Enterprise Palette) ───
    classDef appEntity fill:#dbeafe,stroke:#3b82f6,stroke-width:2px,color:#1e3a8a
    classDef coreEntity fill:#e2e8f0,stroke:#64748b,stroke-width:2px,color:#0f172a
    classDef busEntity fill:#10b981,stroke:#047857,stroke-width:2px,color:#ffffff

    %% ─── Nodes ───
    TYPES[[App-Specific Event Types]]
    PUB([Publisher Component])
    BUS{Global Event Bus 'mitt'}
    SUB([Subscriber Component])

    %% ─── Flow ───
    TYPES -.->|Declaration Merging| BUS
    PUB ===>|emit 'event', payload| BUS
    BUS ===>|useAppEvent 'event'| SUB

    %% ─── Apply Styles ───
    class TYPES,PUB,SUB appEntity;
    class BUS busEntity;
```

### 2. System Architecture: Core Engine vs. App Autonomy
This detailed diagram shows the exact boundaries between the @repo/core-events engine and the consuming application, highlighting real-world publishers (e.g., Cashier UI) and subscribers.

```mermaid
graph TD
    %% ─── Styling Definitions (Dark-Mode Friendly Enterprise Palette) ───
    classDef appComponent fill:#dbeafe,stroke:#3b82f6,stroke-width:2px,color:#1e3a8a
    classDef injection fill:#f59e0b,stroke:#b45309,stroke-width:2px,color:#ffffff
    classDef registry fill:#10b981,stroke:#047857,stroke-width:2px,color:#ffffff
    classDef coreBus fill:#6366f1,stroke:#4338ca,stroke-width:2px,color:#ffffff

    %% ─── Subgraphs ───
    subgraph Core ["@repo/core-events (Pure Tool)"]
        R[AppEventRegistry Empty Interface]
        T[AppEvents Mapped Type]
        E((Global Event Bus mitt))
        H[Hooks: useAppEvent / usePublishEvent]
    end

    subgraph Apps ["apps/web (App Autonomy)"]
        D[[events.d.ts Declaration Merging]]
        
        %% Publishers
        A([Cashier UI])
        B([Profile Settings])
        C([WebSocket Client])
        
        %% Subscribers
        X([Electron IPC Bridge])
        Y([IndexedDB Sync])
        Z([Stock Grid Row])
    end

    %% ─── Flow & Relationships ───
    D -.->|Augments| R
    R ---> T ---> E
    E ---> H

    %% Emitting Events
    A ===>|DEVICE:PRINT_RECEIPT| E
    B ===>|AUTH:PROFILE_UPDATED| E
    C ===>|WS:STOCK_UPDATE| E

    %% Subscribing to Events
    E -.->|Triggers| X
    E -.->|Triggers| Y
    E -.->|Triggers| Z

    %% ─── Apply Styles ───
    class A,B,C,X,Y,Z appComponent;
    class D injection;
    class R,T registry;
    class E,H coreBus;

    %% ─── Subgraph Backgrounds (Transparent for Native GitHub Support) ───
    style Core fill:transparent,stroke:#94a3b8,stroke-width:2px,stroke-dasharray: 5 5
    style Apps fill:transparent,stroke:#3b82f6,stroke-width:2px,stroke-dasharray: 5 5
```

### Core Value Proposition

By routing communication through this centralized event bus, we achieve:

* **App Autonomy**: The core defines the engine. The app defines the contract. There is zero circular dependency.
* **Zero Coupling**: Publishers and subscribers do not need to import, reference, or know about each other's existence.
* **Extreme Performance**: Components can subscribe to high-frequency data streams (like WebSockets or hardware signals) and update their own local state *without* triggering massive React tree re-renders.
* **Memory Safety**: The provided `useAppEvent` hook automatically handles subscription cleanup on component unmount, proactively preventing the most common source of memory leaks in Single Page Architectures (SPAs).


---

## Defining Events (Module Augmentation)

> [!IMPORTANT]
> **Do NOT add application events to `packages/core-events/src/events.registry.ts`.**
> The core registry is intentionally empty. Each app owns its own event contract.

The core exports an open `AppEventRegistry` interface. Apps extend it using TypeScript's `declare module` syntax — the same pattern used for `@types/*` across the JS ecosystem.

### Step 1: Create an augmentation file in your app

> [!WARNING]
> The `import type {}` line is **mandatory**. Without it, TypeScript treats `declare module` as an ambient module declaration that **replaces** the module's types instead of merging into them. All actual exports (`useAppEvent`, `publish`, etc.) would become invisible.

```typescript
// apps/web/src/types/events.d.ts

// This import makes this file a module augmentation (merge)
// instead of an ambient declaration (replace).
import type {} from '@repo/core-events';

declare module '@repo/core-events' {
  // Define your payload shapes
  interface OrderPayload {
    orderId: string;
    total: number;
    items: Array<{ sku: string; qty: number }>;
  }

  // Extend the registry
  interface AppEventRegistry {
    'STORE:ORDER_PLACED': OrderPayload;
    'STORE:ORDER_CANCELLED': { orderId: string; reason: string };
    'UI:SIDEBAR_TOGGLED': { collapsed: boolean };
    
    // Explicit payloads for the examples below:
    'DEVICE:PRINT_RECEIPT': { receiptId: string; items: any[]; total: number; cashierName: string; timestamp: number };
    'WS:STOCK_UPDATE': { id: string; price: number };
    'AUTH:PROFILE_UPDATED': { id: string; name: string; email: string; avatar: string; updatedAt: number };
    'SYSTEM:ERROR': { source: string; error: Error };
  }
}
```

### Step 2: Use it — autocomplete works immediately

```tsx
import { usePublishEvent, useAppEvent } from '@repo/core-events';

function CheckoutButton() {
  const publish = usePublishEvent();
  // ✅ 'STORE:ORDER_PLACED' autocompletes.
  // ✅ Payload shape is enforced by TypeScript.
  publish('STORE:ORDER_PLACED', { orderId: '123', total: 99, items: [] });
}

function OrderTracker() {
  // ✅ payload is fully typed as OrderPayload
  useAppEvent('STORE:ORDER_PLACED', (payload) => {
    console.log(payload.orderId); // string
  });
}
```

### Why this pattern?

| Concern | Old (Hardcoded) | New (Module Augmentation) |
|---|---|---|
| Core knows about app events? | ❌ Yes — violates IoC | ✅ No — core is a pure tool |
| Adding events requires editing core? | ❌ Yes | ✅ No — edit your app's `.d.ts` only |
| Multiple apps share the same registry? | ❌ Collision risk | ✅ Each app has its own `.d.ts` |
| Type safety / autocomplete | ✅ Works | ✅ Works identically |

---

## Usage Outside React (Vanilla TS)

For utility files, API interceptors, Web Workers, or vanilla functions where React hooks cannot be used, import the raw `eventBus` instance directly.

```ts
import { eventBus } from '@repo/core-events';

// Publishing
eventBus.publish('STORE:ORDER_CANCELLED', { orderId: '123', reason: 'Out of stock' });

// Subscribing
const handler = (payload) => {
  console.log('Order cancelled:', payload.orderId);
};

eventBus.subscribe('STORE:ORDER_CANCELLED', handler);

// CRITICAL: Always unsubscribe when done to prevent memory leaks in non-React contexts!
eventBus.unsubscribe('STORE:ORDER_CANCELLED', handler);
```

---

## Usage Examples

Here are three real-world architectural patterns powered by the Event Bus. All event types below are registered in `apps/web/src/types/events.d.ts`, **not** in the core package.

### Example 1: Hardware Abstraction (Cross-Platform)

**Problem**: The web app needs to print receipts. If running in a browser, it should use `window.print()`. If running in the Electron wrapper, it must use the secure IPC bridge (`window.electronAPI.print()`). We don't want the UI components cluttered with platform-detection logic.

**Solution**: The UI publishes a blind event. A headless listener handles the platform routing.

**Publisher (Cashier UI)**:
```tsx
import { usePublishEvent } from '@repo/core-events';

export function CashierUI() {
  const publish = usePublishEvent();

  const handlePrint = () => {
    // Fire and forget. Zero knowledge of how printing actually happens.
    publish('DEVICE:PRINT_RECEIPT', {
      receiptId: 'RCP-123',
      items: [],
      total: 45.00,
      cashierName: 'Firman',
      timestamp: Date.now(),
    });
  };

  return <Button onClick={handlePrint}>Print Receipt</Button>;
}
```

**Subscriber (Headless Listener)**:
```tsx
import { useAppEvent } from '@repo/core-events';

export function PrinterListener() {
  useAppEvent('DEVICE:PRINT_RECEIPT', (payload) => {
    const isElectron = typeof window !== 'undefined' && !!window.electronAPI;

    if (isElectron) {
      // Route via secure Electron IPC bridge
      window.electronAPI.print({ silent: true });
    } else {
      // Fallback to standard browser print dialog
      window.print();
    }
  });

  return null; // Renders nothing
}
```

---

### Example 2: Extreme Performance (High-Frequency Data)

**Problem**: A massive data grid (1,000+ rows) receives 50 WebSocket updates per second. If the parent grid holds the state and passes it down via props, React will attempt to re-render all 1,000 rows 50 times a second, crushing the browser.

**Solution**: The parent grid renders empty rows. Each row subscribes to the event bus and filters updates so it only re-renders when its specific data changes.

**Parent Grid (Never re-renders)**:
```tsx
export function LiveStockGrid() {
  // Generates 1000 IDs once. No stock data is stored here!
  const stockIds = generateStockIds(1000); 

  return (
    <table>
      <tbody>
        {stockIds.map((id) => (
          <StockRow key={id} stockId={id} />
        ))}
      </tbody>
    </table>
  );
}
```

**Child Row (Targeted Updates)**:
```tsx
import { memo, useState } from 'react';
import { useAppEvent } from '@repo/core-events';

export const StockRow = memo(function StockRow({ stockId }) {
  const [data, setData] = useState(null);

  useAppEvent('WS:STOCK_UPDATE', (payload) => {
    // CRITICAL: Filter out events for other rows.
    // 999 out of 1000 rows will exit here instantly without causing a re-render.
    if (payload.id !== stockId) return;
    
    // Only the targeted row updates its local state
    setData(payload);
  });

  return (
    <tr>
      <td>{stockId}</td>
      <td>{data?.price}</td>
    </tr>
  );
});
```

---

### Example 3: Background Sync (Auth to IndexedDB)

**Problem**: When a user updates their profile, we need to persist it to the secure local IndexedDB. We don't want to tightly couple our UI forms to the `@repo/core-storage` package.

**Solution**: The UI form announces the profile update. A dedicated storage listener persists it in the background, properly escalating errors if the storage fails.

**Publisher (Profile UI)**:
```tsx
import { usePublishEvent } from '@repo/core-events';

export function ProfileSettingsUI() {
  const publish = usePublishEvent();

  const handleSave = () => {
    publish('AUTH:PROFILE_UPDATED', {
      id: 'user-1',
      name: 'Firman',
      email: 'firman@eigen.co.id',
      avatar: '[https://example.com/avatar.png](https://example.com/avatar.png)',
      updatedAt: Date.now(),
    });
  };

  return <Button onClick={handleSave}>Save Profile</Button>;
}
```

**Subscriber (Storage Sync Listener)**:
```tsx
import { useAppEvent, usePublishEvent } from '@repo/core-events';
import { secureIndexedDB } from '@repo/core-storage';

export function StorageSyncListener() {
  const publish = usePublishEvent();

  useAppEvent('AUTH:PROFILE_UPDATED', (payload) => {
    // Automatically encrypted at rest because 'user_profile' 
    // is defined in ENCRYPTED_KEYS in @repo/core-storage
    secureIndexedDB.setItem('user_profile', payload).catch((error) => {
      // Escalate to global error handler instead of swallowing it
      publish('SYSTEM:ERROR', { source: 'StorageSyncListener', error });
    });
  });

  return null;
}
```