Offline-First Capacitor Apps

Build apps that work seamlessly with or without internet connectivity.

When to Use This Skill

• User needs offline support
• User asks about data sync
• User wants caching
• User needs local database
• User has connectivity issues

Offline-First Architecture

┌─────────────────────────────────────────┐
│              UI Layer                    │
├─────────────────────────────────────────┤
│           Service Layer                  │
│  ┌─────────────┐  ┌─────────────────┐   │
│  │ Online Mode │  │ Offline Mode    │   │
│  └──────┬──────┘  └────────┬────────┘   │
├─────────┼──────────────────┼────────────┤
│         │    Sync Manager  │            │
│         └────────┬─────────┘            │
├──────────────────┼──────────────────────┤
│  ┌───────────────┴───────────────────┐  │
│  │         Local Database            │  │
│  │   (Fast SQL / IndexedDB)          │  │
│  └───────────────────────────────────┘  │
└─────────────────────────────────────────┘

Network Detection

Using Capacitor Network Plugin

npm install @capacitor/network
npx cap sync

import { Network } from '@capacitor/network';

// Check current status
const status = await Network.getStatus();
console.log('Connected:', status.connected);
console.log('Connection type:', status.connectionType);

// Listen for changes
Network.addListener('networkStatusChange', (status) => {
  console.log('Network status changed:', status.connected);

  if (status.connected) {
    // Back online - sync data
    syncManager.syncPendingChanges();
  } else {
    // Offline - show indicator
    showOfflineIndicator();
  }
});

Network-Aware Service

import { Network } from '@capacitor/network';

class NetworkAwareService {
  private isOnline = true;

  constructor() {
    this.init();
  }

  private async init() {
    const status = await Network.getStatus();
    this.isOnline = status.connected;

    Network.addListener('networkStatusChange', (status) => {
      this.isOnline = status.connected;
    });
  }

  async fetch<T>(url: string, options?: RequestInit): Promise<T> {
    if (!this.isOnline) {
      // Return cached data
      return this.getCachedData(url);
    }

    try {
      const response = await fetch(url, options);
      const data = await response.json();

      // Cache the response
      await this.cacheData(url, data);

      return data;
    } catch (error) {
      // Network error - try cache
      return this.getCachedData(url);
    }
  }
}

Local Database with Fast SQL

Installation

npm install @capgo/capacitor-fast-sql
npx cap sync

Before using Fast SQL in production, complete the required platform setup:

• iOS: allow localhost networking for the plugin transport.
• Android: add the localhost cleartext exception required by the plugin.
• Web: install sql.js if the app needs the web fallback.

Use the dedicated sqlite-to-fast-sql skill when you need the full platform checklist.

Database Setup

import { KeyValueStore } from '@capgo/capacitor-fast-sql';

class Database {
  private store: Awaited<ReturnType<typeof KeyValueStore.open>> | null = null;

  async open() {
    if (this.store) return;
    this.store = await KeyValueStore.open({
      database: 'myapp',
      store: 'data',
      encrypted: false,
    });
  }

  async set(key: string, value: any) {
    await this.open();
    await this.store!.set(key, value);
  }

  async get<T>(key: string): Promise<T | null> {
    await this.open();
    return this.store!.get<T>(key);
  }

  async remove(key: string) {
    await this.open();
    await this.store!.remove(key);
  }

  async keys(): Promise<string[]> {
    await this.open();
    return this.store!.keys();
  }
}

Offline Data Repository

interface Entity {
  id: string;
  updatedAt: number;
  syncStatus: 'synced' | 'pending' | 'conflict';
}

class OfflineRepository<T extends Entity> {
  constructor(
    private db: Database,
    private collection: string
  ) {}

  getCollection(): string {
    return this.collection;
  }

  async getAll(): Promise<T[]> {
    const keys = await this.db.keys();
    const items: T[] = [];

    for (const key of keys) {
      if (key.startsWith(`${this.collection}:`)) {
        const item = await this.db.get<T>(key);
        if (item) items.push(item);
      }
    }

    return items;
  }

  async getById(id: string): Promise<T | null> {
    return this.db.get<T>(`${this.collection}:${id}`);
  }

  async save(item: T, options?: { markPending?: boolean }): Promise<void> {
    item.updatedAt = Date.now();
    if (options?.markPending ?? true) {
      item.syncStatus = 'pending';
    }
    await this.db.set(`${this.collection}:${item.id}`, item);
  }

  async delete(id: string): Promise<void> {
    // Soft delete - mark for sync
    const item = await this.getById(id);
    if (item) {
      item.syncStatus = 'pending';
      (item as any).deleted = true;
      await this.db.set(`${this.collection}:${id}`, item);
    }
  }

  async getPending(): Promise<T[]> {
    const all = await this.getAll();
    return all.filter((item) => item.syncStatus === 'pending');
  }

  async markSynced(id: string): Promise<void> {
    const item = await this.getById(id);
    if (item) {
      item.syncStatus = 'synced';
      await this.db.set(`${this.collection}:${id}`, item);
    }
  }
}

Sync Manager

import { Network } from '@capacitor/network';

class SyncManager {
  private isSyncing = false;
  private syncQueue: Array<() => Promise<void>> = [];

  constructor(private repositories: OfflineRepository<any>[]) {
    this.setupNetworkListener();
  }

  private setupNetworkListener() {
    Network.addListener('networkStatusChange', async (status) => {
      if (status.connected) {
        await this.syncAll();
      }
    });
  }

  async syncAll() {
    if (this.isSyncing) return;
    this.isSyncing = true;

    try {
      for (const repo of this.repositories) {
        await this.syncRepository(repo);
      }
    } finally {
      this.isSyncing = false;
    }
  }

  private async syncRepository(repo: OfflineRepository<any>) {
    const pending = await repo.getPending();

    for (const item of pending) {
      try {
        if ((item as any).deleted) {
          await this.deleteRemote(item);
        } else {
          await this.syncToRemote(item);
        }
        await repo.markSynced(item.id);
      } catch (error) {
        console.error('Sync failed for item:', item.id, error);
        // Keep as pending for retry
      }
    }

    // Pull remote changes
    await this.pullRemoteChanges(repo);
  }

  private async syncToRemote(item: any) {
    await fetch(`/api/${item.collection}/${item.id}`, {
      method: 'PUT',
      headers: { 'Content-Type': 'application/json' },
      body: JSON.stringify(item),
    });
  }

  private async deleteRemote(item: any) {
    await fetch(`/api/${item.collection}/${item.id}`, {
      method: 'DELETE',
    });
  }

  private async pullRemoteChanges(repo: OfflineRepository<any>) {
    const lastSync = await this.getLastSyncTime(repo);
    const collection = repo.getCollection();
    const response = await fetch(
      `/api/${collection}?since=${lastSync}`
    );
    const remoteItems = await response.json();

    for (const remoteItem of remoteItems) {
      const localItem = await repo.getById(remoteItem.id);

      if (!localItem) {
        // New item from server
        await repo.save({ ...remoteItem, syncStatus: 'synced' }, { markPending: false });
      } else if (localItem.syncStatus === 'synced') {
        // No local changes - update from server
        await repo.save({ ...remoteItem, syncStatus: 'synced' }, { markPending: false });
      } else {
        // Conflict - local has pending changes
        await this.resolveConflict(localItem, remoteItem, repo);
      }
    }

    await this.setLastSyncTime(repo, Date.now());
  }

  private async resolveConflict(
    local: any,
    remote: any,
    repo: OfflineRepository<any>
  ) {
    // Last-write-wins strategy
    if (local.updatedAt > remote.updatedAt) {
      // Keep local, re-sync to server
      local.syncStatus = 'pending';
      await repo.save(local);
    } else {
      // Server wins
      await repo.save({ ...remote, syncStatus: 'synced' }, { markPending: false });
    }
  }
}

Service Worker Caching

Register Service Worker

// src/main.ts
if ('serviceWorker' in navigator) {
  navigator.serviceWorker.register('/sw.js');
}

Service Worker with Workbox

// public/sw.js
import { precacheAndRoute } from 'workbox-precaching';
import { registerRoute } from 'workbox-routing';
import { StaleWhileRevalidate, CacheFirst, NetworkFirst } from 'workbox-strategies';

// Precache static assets
precacheAndRoute(self.__WB_MANIFEST);

// Cache API responses
registerRoute(
  ({ url }) => url.pathname.startsWith('/api/'),
  new NetworkFirst({
    cacheName: 'api-cache',
    networkTimeoutSeconds: 5,
  })
);

// Cache images
registerRoute(
  ({ request }) => request.destination === 'image',
  new CacheFirst({
    cacheName: 'image-cache',
    plugins: [
      {
        expiration: {
          maxEntries: 100,
          maxAgeSeconds: 7 * 24 * 60 * 60, // 1 week
        },
      },
    ],
  })
);

// Cache fonts
registerRoute(
  ({ request }) => request.destination === 'font',
  new CacheFirst({
    cacheName: 'font-cache',
  })
);

Optimistic UI Updates

class TodoService {
  constructor(
    private repo: OfflineRepository<Todo>,
    private syncManager: SyncManager
  ) {}

  async addTodo(text: string): Promise<Todo> {
    const todo: Todo = {
      id: crypto.randomUUID(),
      text,
      completed: false,
      updatedAt: Date.now(),
      syncStatus: 'pending',
    };

    // Save locally immediately
    await this.repo.save(todo);

    // Trigger sync in background
    this.syncManager.syncAll().catch(console.error);

    return todo;
  }

  async toggleComplete(id: string): Promise<Todo> {
    const todo = await this.repo.getById(id);
    if (!todo) throw new Error('Todo not found');

    todo.completed = !todo.completed;
    await this.repo.save(todo);

    this.syncManager.syncAll().catch(console.error);

    return todo;
  }
}

Queue Failed Requests

class RequestQueue {
  private queue: QueuedRequest[] = [];

  constructor(private storage: Database) {
    this.loadQueue();
  }

  private async loadQueue() {
    this.queue = await this.storage.get<QueuedRequest[]>('requestQueue') || [];
  }

  private async saveQueue() {
    await this.storage.set('requestQueue', this.queue);
  }

  async enqueue(request: QueuedRequest) {
    this.queue.push(request);
    await this.saveQueue();
  }

  async processQueue() {
    const status = await Network.getStatus();
    if (!status.connected) return;

    while (this.queue.length > 0) {
      const request = this.queue[0];

      try {
        await fetch(request.url, {
          method: request.method,
          headers: request.headers,
          body: request.body,
        });

        this.queue.shift();
        await this.saveQueue();
      } catch (error) {
        // Stop processing on failure
        break;
      }
    }
  }
}

Best Practices

1. Show Sync Status

function SyncIndicator() {
  const { isOnline, pendingChanges, isSyncing } = useSyncStatus();

  if (!isOnline) {
    return <Badge color="warning">Offline</Badge>;
  }

  if (isSyncing) {
    return <Badge color="info">Syncing...</Badge>;
  }

  if (pendingChanges > 0) {
    return <Badge color="warning">{pendingChanges} pending</Badge>;
  }

  return <Badge color="success">Synced</Badge>;
}

2. Handle Conflicts Gracefully

async function handleConflict(local: Todo, remote: Todo): Promise<Todo> {
  // Option 1: Last write wins
  return local.updatedAt > remote.updatedAt ? local : remote;

  // Option 2: Merge changes
  return {
    ...remote,
    ...local,
    updatedAt: Math.max(local.updatedAt, remote.updatedAt),
  };

  // Option 3: Ask user
  const choice = await showConflictDialog(local, remote);
  return choice === 'local' ? local : remote;
}

3. Validate Before Sync

function validateTodo(todo: Todo): boolean {
  if (!todo.id || !todo.text) return false;
  if (todo.text.length > 500) return false;
  return true;
}

async function syncTodo(todo: Todo) {
  if (!validateTodo(todo)) {
    throw new Error('Invalid todo');
  }
  // Proceed with sync
}

Resources

• Capacitor Network: https://capacitorjs.com/docs/apis/network
• Workbox: https://developer.chrome.com/docs/workbox
• IndexedDB: https://developer.mozilla.org/docs/Web/API/IndexedDB_API
• Offline First Manifesto: http://offlinefirst.org