[HTML5] Add PWA support to the editor page.

This allows to install it as an app, and provide offline support (after
the first run).
Practically, this boils down to adding a JSON file as a manifest, an
offline page to be displayed when the cached files are not avaialble,
and a JS file to cache resources and return them.

The reason for the "first run requirements" is that some browsers, will
emit an "install" by just visiting the page (to see if the JS code is
compatibile), and we do not want to force casual visitors to just
download the 10 MiB+ compressed editor WebAssembly file without pressing
the start button.

Special thanks to Hugo Locurcio (Calinou) for the initial work.

1 files changed, 84 insertions, 0 deletions

diff --git a/misc/dist/html/service-worker.js b/misc/dist/html/service-worker.js
new file mode 100644
index 0000000000..d4eaed2b17
--- /dev/null
+++ b/misc/dist/html/service-worker.js
@@ -0,0 +1,84 @@
+// This service worker is required to expose an exported Godot project as a
+// Progressive Web App. It provides an offline fallback page telling the user
+// that they need an Internet conneciton to run the project if desired.
+// Incrementing CACHE_VERSION will kick off the install event and force
+// previously cached resources to be updated from the network.
+const CACHE_VERSION = "@GODOT_VERSION@";
+const CACHE_NAME = "@GODOT_NAME@-cache";
+const OFFLINE_URL = "offline.html";
+// Files that will be cached on load.
+const CACHED_FILES = [
+	"godot.tools.html",
+	"offline.html",
+	"godot.tools.js",
+	"godot.tools.worker.js",
+	"godot.tools.audio.worklet.js",
+	"logo.svg",
+	"favicon.png",
+];
+
+// Files that we might not want the user to preload, and will only be cached on first load.
+const CACHABLE_FILES = [
+	"godot.tools.wasm",
+];
+const FULL_CACHE = CACHED_FILES.concat(CACHABLE_FILES);
+
+self.addEventListener("install", (event) => {
+	event.waitUntil(async function () {
+		const cache = await caches.open(CACHE_NAME);
+		// Clear old cache (including optionals).
+		await Promise.all(FULL_CACHE.map(path => cache.delete(path)));
+		// Insert new one.
+		const done = await cache.addAll(CACHED_FILES);
+		return done;
+	}());
+});
+
+self.addEventListener("activate", (event) => {
+	event.waitUntil(async function () {
+		if ("navigationPreload" in self.registration) {
+			await self.registration.navigationPreload.enable();
+		}
+	}());
+	// Tell the active service worker to take control of the page immediately.
+	self.clients.claim();
+});
+
+self.addEventListener("fetch", (event) => {
+	const isNavigate = event.request.mode === "navigate";
+	const url = event.request.url || "";
+	const referrer = event.request.referrer || "";
+	const base = referrer.slice(0, referrer.lastIndexOf("/") + 1);
+	const local = url.startsWith(base) ? url.replace(base, "") : "";
+	const isCachable = FULL_CACHE.some(v => v === local) || (base === referrer && base.endsWith(CACHED_FILES[0]));
+	if (isNavigate || isCachable) {
+		event.respondWith(async function () {
+			try {
+				// Use the preloaded response, if it's there
+				let request = event.request.clone();
+				let response = await event.preloadResponse;
+				if (!response) {
+					// Or, go over network.
+					response = await fetch(event.request);
+				}
+				if (isCachable) {
+					// Update the cache
+					const cache = await caches.open(CACHE_NAME);
+					cache.put(request, response.clone());
+				}
+				return response;
+			} catch (error) {
+				const cache = await caches.open(CACHE_NAME);
+				if (event.request.mode === "navigate") {
+					// Check if we have full cache.
+					const cached = await Promise.all(FULL_CACHE.map(name => cache.match(name)));
+					const missing = cached.some(v => v === undefined);
+					const cachedResponse = missing ? await caches.match(OFFLINE_URL) : await caches.match(CACHED_FILES[0]);
+					return cachedResponse;
+				}
+				const cachedResponse = await caches.match(event.request);
+				return cachedResponse;
+			}
+		}());
+	}
+});