Progressive Web Apps - service workers and offline support

                  // service-worker.js

const CACHE_NAME = 'my-pwa-cache-v1';
const urlsToCache = [
  '/',
  '/index.html',
  '/styles.css',
  '/app.js',
  '/manifest.json',
  '/icon-192.png',
  '/icon-512.png',
];

// Install event - cache resources
self.addEventListener('install', (event) => {
  console.log('Service Worker installing');

  event.waitUntil(
    caches.open(CACHE_NAME)
      .then((cache) => {
        console.log('Caching app shell');
        return cache.addAll(urlsToCache);
      })
      .then(() => self.skipWaiting()) // Activate immediately
  );
});

// Activate event - clean old caches
self.addEventListener('activate', (event) => {
  console.log('Service Worker activating');

  event.waitUntil(
    caches.keys().then((cacheNames) => {
      return Promise.all(
        cacheNames.map((cacheName) => {
          if (cacheName !== CACHE_NAME) {
            console.log('Deleting old cache:', cacheName);
            return caches.delete(cacheName);
          }
        })
      );
    }).then(() => self.clients.claim()) // Take control of all pages
  );
});

// Fetch event - serve from cache, fallback to network
self.addEventListener('fetch', (event) => {
  event.respondWith(
    caches.match(event.request)
      .then((response) => {
        // Cache hit - return response
        if (response) {
          return response;
        }

        // Clone the request
        const fetchRequest = event.request.clone();

        return fetch(fetchRequest).then((response) => {
          // Check if valid response
          if (!response || response.status !== 200 || response.type !== 'basic') {
            return response;
          }

          // Clone the response
          const responseToCache = response.clone();

          caches.open(CACHE_NAME).then((cache) => {
            cache.put(event.request, responseToCache);
          });

          return response;
        });
      })
      .catch(() => {
        // Return offline page if available
        return caches.match('/offline.html');
      })
  );
});

// Background sync
self.addEventListener('sync', (event) => {
  if (event.tag === 'sync-messages') {
    event.waitUntil(syncMessages());
  }
});

async function syncMessages() {
  const db = await openDB();
  const messages = await db.getAll('outbox');

  for (const message of messages) {
    try {
      await fetch('/api/messages', {
        method: 'POST',
        body: JSON.stringify(message),
      });
      await db.delete('outbox', message.id);
    } catch (error) {
      console.error('Sync failed:', error);
    }
  }
}

// Push notifications
self.addEventListener('push', (event) => {
  const data = event.data.json();

  const options = {
    body: data.body,
    icon: '/icon-192.png',
    badge: '/badge-72.png',
    vibrate: [100, 50, 100],
    data: {
      dateOfArrival: Date.now(),
      primaryKey: data.id,
    },
    actions: [
      { action: 'explore', title: 'View' },
      { action: 'close', title: 'Close' },
    ],
  };

  event.waitUntil(
    self.registration.showNotification(data.title, options)
  );
});

// Notification click
self.addEventListener('notificationclick', (event) => {
  event.notification.close();

  if (event.action === 'explore') {
    event.waitUntil(
      clients.openWindow('/notifications')
    );
  }
});

// Caching strategies
// 1. Cache First (good for static assets)
self.addEventListener('fetch', (event) => {
  if (event.request.destination === 'image') {
    event.respondWith(
      caches.match(event.request).then((response) => {
        return response || fetch(event.request).then((response) => {
          return caches.open(CACHE_NAME).then((cache) => {
            cache.put(event.request, response.clone());
            return response;
          });
        });
      })
    );
  }
});

// 2. Network First (good for API calls)
self.addEventListener('fetch', (event) => {
  if (event.request.url.includes('/api/')) {
    event.respondWith(
      fetch(event.request)
        .then((response) => {
          const responseClone = response.clone();
          caches.open(CACHE_NAME).then((cache) => {
            cache.put(event.request, responseClone);
          });
          return response;
        })
        .catch(() => caches.match(event.request))
    );
  }
});

// 3. Stale While Revalidate (good for frequently updated content)
self.addEventListener('fetch', (event) => {
  event.respondWith(
    caches.open(CACHE_NAME).then((cache) => {
      return cache.match(event.request).then((response) => {
        const fetchPromise = fetch(event.request).then((networkResponse) => {
          cache.put(event.request, networkResponse.clone());
          return networkResponse;
        });
        return response || fetchPromise;
      });
    })
  );
});