Add service worker for offline compatibility (#115)

For the moment this is opt in via flag=sw on the URL so existing embedders won't be affected.

The main motivation here is to experiment with PWA support in micro:bit Python Editor which is being worked on separately.

.github/workflows/build.yml

@@ -26,9 +26,9 @@ jobs:
     steps:
       # Note: This workflow will not run on forks without modification; we're open to making steps
       #       that rely on our deployment infrastructure conditional. Please open an issue.
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v4
       - name: Configure node
-        uses: actions/setup-node@v3
+        uses: actions/setup-node@v4
         with:
           node-version: 20.x
           cache: "npm"

Makefile

@@ -8,7 +8,7 @@ build:
 
 dist: build
 	mkdir -p $(BUILD)/build
-	cp -r $(SRC)/*.html $(SRC)/term.js src/examples $(BUILD)
+	cp -r $(SRC)/*.html $(SRC)/term.js src/examples $(SRC)/build/sw.js $(BUILD)
 	cp $(SRC)/build/firmware.js $(SRC)/build/simulator.js $(SRC)/build/firmware.wasm  $(BUILD)/build/
 
 watch: dist

src/Makefile

@@ -146,7 +146,8 @@ $(BUILD)/micropython.js: $(OBJ) jshal.js simulator-js
 	$(Q)emcc $(LDFLAGS) -o $(BUILD)/firmware.js $(OBJ) $(JSFLAGS)
 
 simulator-js:
-	npx esbuild ./simulator.ts --bundle --outfile=$(BUILD)/simulator.js --loader:.svg=text
+	npx esbuild '--define:process.env.STAGE="$(STAGE)"' ./simulator.ts --bundle --outfile=$(BUILD)/simulator.js --loader:.svg=text
+	npx esbuild --define:process.env.VERSION="$$(node -e 'process.stdout.write(`"` + require("../package.json").version + `"`)')" ./sw.ts --bundle --outfile=$(BUILD)/sw.js
 
 include $(TOP)/py/mkrules.mk
 

src/environment.ts

@@ -0,0 +1,3 @@
+export type Stage = "local" | "REVIEW" | "STAGING" | "PRODUCTION";
+
+export const stage = (process.env.STAGE || "local") as Stage;

src/flags.ts

@@ -0,0 +1,56 @@
+import { Stage, stage as stageFromEnvironment } from "./environment";
+
+/**
+ * A union of the flag names (alphabetical order).
+ */
+export type Flag =
+  /**
+   * Enables service worker registration.
+   *
+   * Registers the service worker and enables offline use.
+   */
+  "sw";
+
+interface FlagMetadata {
+  defaultOnStages: Stage[];
+  name: Flag;
+}
+
+const allFlags: FlagMetadata[] = [{ name: "sw", defaultOnStages: [] }];
+
+type Flags = Record<Flag, boolean>;
+
+const flagsForParams = (stage: Stage, params: URLSearchParams) => {
+  const enableFlags = new Set(params.getAll("flag"));
+  const allFlagsDefault = enableFlags.has("none")
+    ? false
+    : enableFlags.has("*")
+    ? true
+    : undefined;
+  return Object.fromEntries(
+    allFlags.map((f) => [
+      f.name,
+      isEnabled(f, stage, allFlagsDefault, enableFlags.has(f.name)),
+    ])
+  ) as Flags;
+};
+
+const isEnabled = (
+  f: FlagMetadata,
+  stage: Stage,
+  allFlagsDefault: boolean | undefined,
+  thisFlagOn: boolean
+): boolean => {
+  if (thisFlagOn) {
+    return true;
+  }
+  if (allFlagsDefault !== undefined) {
+    return allFlagsDefault;
+  }
+  return f.defaultOnStages.includes(stage);
+};
+
+export const flags: Flags = (() => {
+  const params = new URLSearchParams(window.location.search);
+  return flagsForParams(stageFromEnvironment, params);
+})();

src/simulator.ts

@@ -7,6 +7,7 @@ import {
   createMessageListener,
   Notifications,
 } from "./board";
+import { flags } from "./flags";
 
 declare global {
   interface Window {
@@ -15,6 +16,48 @@ declare global {
   }
 }
 
+function initServiceWorker() {
+  window.addEventListener("load", () => {
+    navigator.serviceWorker.register("sw.js").then(
+      (registration) => {
+        console.log("Simulator service worker registration successful");
+        // Reload the page when a new service worker is installed.
+        registration.onupdatefound = function () {
+          const installingWorker = registration.installing;
+          if (installingWorker) {
+            installingWorker.onstatechange = function () {
+              if (
+                installingWorker.state === "installed" &&
+                navigator.serviceWorker.controller
+              ) {
+                window.location.reload();
+              }
+            };
+          }
+        };
+      },
+      (error) => {
+        console.error(`Simulator service worker registration failed: ${error}`);
+      }
+    );
+  });
+}
+
+if ("serviceWorker" in navigator) {
+  if (flags.sw) {
+    initServiceWorker();
+  } else {
+    navigator.serviceWorker.getRegistrations().then((registrations) => {
+      if (registrations.length > 0) {
+        // We should only have one service worker to unregister.
+        registrations[0].unregister().then(() => {
+          window.location.reload();
+        });
+      }
+    });
+  }
+}
+
 const fs = new FileSystem();
 const board = createBoard(new Notifications(window.parent), fs);
 window.addEventListener("message", createMessageListener(board));

src/sw.ts

@@ -0,0 +1,51 @@
+/// <reference lib="WebWorker" />
+// Empty export required due to --isolatedModules flag in tsconfig.json
+export type {};
+declare const self: ServiceWorkerGlobalScope;
+declare const clients: Clients;
+
+const assets = ["simulator.html", "build/simulator.js", "build/firmware.js", "build/firmware.wasm"];
+const cacheName = `simulator-${process.env.VERSION}`;
+
+self.addEventListener("install", (event) => {
+  console.log("Installing simulator service worker...");
+  self.skipWaiting();
+  event.waitUntil(
+    (async () => {
+      const cache = await caches.open(cacheName);
+      await cache.addAll(assets);
+    })()
+  );
+});
+
+self.addEventListener("activate", (event) => {
+  console.log("Activating simulator service worker...");
+  event.waitUntil(
+    (async () => {
+      const names = await caches.keys();
+      await Promise.all(
+        names.map((name) => {
+          if (/^simulator-/.test(name) && name !== cacheName) {
+            return caches.delete(name);
+          }
+        })
+      );
+      await clients.claim();
+    })()
+  );
+});
+
+self.addEventListener("fetch", (event) => {
+  event.respondWith(
+    (async () => {
+      const cachedResponse = await caches.match(event.request);
+      if (cachedResponse) {
+        return cachedResponse;
+      }
+      const response = await fetch(event.request);
+      const cache = await caches.open(cacheName);
+      cache.put(event.request, response.clone());
+      return response;
+    })()
+  );
+});

tsconfig.json

@@ -1,7 +1,7 @@
 {
   "compilerOptions": {
     "target": "es2019",
-    "lib": ["dom", "dom.iterable", "esnext"],
+    "lib": ["dom", "dom.iterable", "esnext", "WebWorker"],
     "allowJs": true,
     "skipLibCheck": true,
     "esModuleInterop": true,