1 files changed, 59 insertions, 0 deletions
diff --git a/src/lib/auto-refresh.ts b/src/lib/auto-refresh.ts
new file mode 100644
index 0000000..72bcdb9
--- /dev/null
+++ b/src/lib/auto-refresh.ts
@@ -0,0 +1,59 @@
+import { invalidateAll } from '$app/navigation';
+import { navigating, page } from '$app/stores';
+import { get } from 'svelte/store';
+import { shouldAutoRefresh } from '$lib/shared/refresh-policy';
+
+/** Away for at least this long before refocus re-runs the page load. */
+const MIN_AWAY_MS = 10_000;
+
+/** Poll cadence: near-live for a shared list, cheap for one open tab. */
+const POLL_INTERVAL_MS = 15_000;
+
+/**
+ * Keeps the current page near-live: polls the server while the tab is visible,
+ * and refetches on refocus after an absence. Call from `onMount`; the returned
+ * function tears the listeners and timer down.
+ */
+export function startAutoRefresh(): () => void {
+	// When the tab stopped being the active one; null while it is active.
+	let deactivatedAt: number | null = null;
+
+	const conditions = () => ({
+		routeId: get(page).route.id,
+		visible: document.visibilityState === 'visible',
+		online: navigator.onLine,
+		navigating: get(navigating) !== null,
+	});
+
+	const deactivate = () => {
+		deactivatedAt ??= Date.now();
+	};
+
+	const activate = () => {
+		if (deactivatedAt === null) return;
+		const awayMs = Date.now() - deactivatedAt;
+		deactivatedAt = null;
+		if (awayMs >= MIN_AWAY_MS && shouldAutoRefresh(conditions()))
+			void invalidateAll();
+	};
+
+	const onVisibilityChange = () => {
+		if (document.visibilityState === 'hidden') deactivate();
+		else activate();
+	};
+
+	const timer = setInterval(() => {
+		if (shouldAutoRefresh(conditions())) void invalidateAll();
+	}, POLL_INTERVAL_MS);
+
+	document.addEventListener('visibilitychange', onVisibilityChange);
+	window.addEventListener('blur', deactivate);
+	window.addEventListener('focus', activate);
+
+	return () => {
+		clearInterval(timer);
+		document.removeEventListener('visibilitychange', onVisibilityChange);
+		window.removeEventListener('blur', deactivate);
+		window.removeEventListener('focus', activate);
+	};
+}