1 files changed, 80 insertions, 0 deletions
diff --git a/src/service-worker.ts b/src/service-worker.ts
new file mode 100644
index 00000000..4c15bb6a
--- /dev/null
+++ b/src/service-worker.ts
@@ -0,0 +1,80 @@
+/// <reference lib="webworker" />
+
+import { build, files, version } from '$service-worker';
+
+const worker = self as unknown as ServiceWorkerGlobalScope;
+const FILES = `cache${version}`;
+
+// `build` is an array of all the files generated by the bundler,
+// `files` is an array of everything in the `static` directory
+const to_cache = build.concat(files);
+const staticAssets = new Set(to_cache);
+
+worker.addEventListener('install', (event) => {
+	event.waitUntil(
+		caches
+			.open(FILES)
+			.then((cache) => cache.addAll(to_cache))
+			.then(() => {
+				worker.skipWaiting();
+			})
+	);
+});
+
+worker.addEventListener('activate', (event) => {
+	event.waitUntil(
+		caches.keys().then(async (keys) => {
+			// delete old caches
+			for (const key of keys) {
+				if (key !== FILES) await caches.delete(key);
+			}
+
+			worker.clients.claim();
+		})
+	);
+});
+
+/**
+ * Fetch the asset from the network and store it in the cache.
+ * Fall back to the cache if the user is offline.
+ */
+async function fetchAndCache(request: Request) {
+	const cache = await caches.open(`offline${version}`);
+
+	try {
+		const response = await fetch(request);
+		cache.put(request, response.clone());
+		return response;
+	} catch (err) {
+		const response = await cache.match(request);
+		if (response) return response;
+
+		throw err;
+	}
+}
+
+worker.addEventListener('fetch', (event) => {
+	if (event.request.method !== 'GET' || event.request.headers.has('range')) return;
+
+	const url = new URL(event.request.url);
+
+	// don't try to handle e.g. data: URIs
+	const isHttp = url.protocol.startsWith('http');
+	const isDevServerRequest =
+		url.hostname === self.location.hostname && url.port !== self.location.port;
+	const isStaticAsset = url.host === self.location.host && staticAssets.has(url.pathname);
+	const skipBecauseUncached = event.request.cache === 'only-if-cached' && !isStaticAsset;
+
+	if (isHttp && !isDevServerRequest && !skipBecauseUncached) {
+		event.respondWith(
+			(async () => {
+				// always serve static files and bundler-generated assets from cache.
+				// if your application has other URLs with data that will never change,
+				// set this variable to true for them and they will only be fetched once.
+				const cachedAsset = isStaticAsset && (await caches.match(event.request));
+
+				return cachedAsset || fetchAndCache(event.request);
+			})()
+		);
+	}
+});