add stream caching ("offline mode")

src/lib/streamCache.svelte.ts

@@ -0,0 +1,66 @@
+import { browser } from '$app/environment';
+import { SvelteSet, SvelteMap } from 'svelte/reactivity';
+import type { StreamSummary } from './types.ts';
+
+const STORAGE_KEY = 'cachedStreams';
+
+export const cached = new SvelteSet<string>(
+	browser ? JSON.parse(localStorage.getItem(STORAGE_KEY) || '[]') : []
+);
+export const downloading = new SvelteMap<string, number>();
+
+if (browser) {
+	$effect.root(() => {
+		$effect(() => {
+			localStorage.setItem(STORAGE_KEY, JSON.stringify([...cached]));
+		});
+	});
+}
+
+export async function download(stream: StreamSummary) {
+	const res = await fetch(`/media/tracks/${stream.filename}`);
+	if (!res.ok || !res.body) throw new Error('Download failed');
+
+	const total = Number(res.headers.get('content-length') || 0);
+	const reader = res.body.getReader();
+	const chunks: Uint8Array[] = [];
+	let received = 0;
+
+	downloading.set(stream.id, 0);
+
+	while (true) {
+		const { done, value } = await reader.read();
+		if (done) break;
+		chunks.push(value);
+		received += value.length;
+		if (total) downloading.set(stream.id, Math.round((received / total) * 100));
+	}
+
+	const root = await navigator.storage.getDirectory();
+	const handle = await root.getFileHandle(stream.id, { create: true });
+	const writable = await handle.createWritable();
+	await writable.write(new Blob(chunks));
+	await writable.close();
+
+	downloading.delete(stream.id);
+	cached.add(stream.id);
+}
+
+export async function remove(streamId: string) {
+	const root = await navigator.storage.getDirectory();
+	await root.removeEntry(streamId);
+	cached.delete(streamId);
+}
+
+export async function getUrl(streamId: string): Promise<string | null> {
+	if (!cached.has(streamId)) return null;
+	try {
+		const root = await navigator.storage.getDirectory();
+		const handle = await root.getFileHandle(streamId);
+		const file = await handle.getFile();
+		return URL.createObjectURL(file);
+	} catch {
+		cached.delete(streamId);
+		return null;
+	}
+}