add day19

day19/src/lib/router/linkPrefetcher.ts

@@ -0,0 +1,52 @@
+export default (anchorElms: any) => {
+	const prefetchedPages: Array<string> = [];
+
+	function prefetchLink(url: string) {
+		const prefetchElm = document.createElement('link');
+		prefetchElm.rel = 'prefetch';
+		prefetchElm.href = url;
+		prefetchElm.as = 'document';
+
+		if (import.meta.env.VITE_VERBOSE && !import.meta.env.PROD) {
+			prefetchElm.onload = () => { console.log('prefetched url: ' + url); };
+			prefetchElm.onerror = (err) => { console.error('cant prefetch url: ' + url, err); };
+		}
+
+		document.head.appendChild(prefetchElm);
+		prefetchedPages.push(url);
+	}
+
+	if (!('IntersectionObserver' in window)) return;
+	const visibleObserver = new IntersectionObserver((entries, observer) => {
+		entries.forEach((entry) => {
+			const url = entry.target.getAttribute('href');
+			if (!url) return;
+			if (prefetchedPages.includes(url)) {
+				observer.unobserve(entry.target);
+				return;
+			}
+			if (entry.isIntersecting) {
+				prefetchLink(url);
+				observer.unobserve(entry.target);
+			}
+		});
+	});
+
+	anchorElms.forEach((e: HTMLAnchorElement) => {
+		const prefetch = e.getAttribute('client:prefetch');
+		let method;
+		if (prefetch == null) return;
+		if (prefetch) method = prefetch;
+		if (e.href.includes(document.location.origin) && !e.href.includes('#') && e.href !== (document.location.href || document.location.href + '/')) {
+			// page would be a valid prefetch
+			if (method == 'hover') {
+				const url = e.getAttribute('href');
+				if (!url) return;
+				e.addEventListener('pointerenter', () => prefetchLink(url), { once: true });
+			} else {
+				// method is empty, visible, or invalid, either way we so the default of visible
+				visibleObserver.observe(e);
+			}
+		}
+	});
+};