web: ESBuild performance + Live reload (#13026)

* web: Silence ESBuild warning.

* web: Flesh out live reload. Tidy ESBuild.

---------

Signed-off-by: Teffen Ellis <592134+GirlBossRush@users.noreply.github.com>

web/build-observer-plugin.mjs

@@ -0,0 +1,141 @@
+import * as http from "http";
+import path from "path";
+
+/**
+ * Serializes a custom event to a text stream.
+ * a
+ * @param {Event} event
+ * @returns {string}
+ */
+export function serializeCustomEventToStream(event) {
+    // @ts-ignore
+    const data = event.detail ?? {};
+
+    const eventContent = [`event: ${event.type}`, `data: ${JSON.stringify(data)}`];
+
+    return eventContent.join("\n") + "\n\n";
+}
+
+/**
+ * Options for the build observer plugin.
+ *
+ * @typedef {Object} BuildObserverOptions
+ *
+ * @property {URL} serverURL
+ * @property {string} logPrefix
+ * @property {string} relativeRoot
+ */
+
+/**
+ * Creates a plugin that listens for build events and sends them to a server-sent event stream.
+ *
+ * @param {BuildObserverOptions} options
+ * @returns {import('esbuild').Plugin}
+ */
+export function buildObserverPlugin({ serverURL, logPrefix, relativeRoot }) {
+    const timerLabel = `[${logPrefix}] Build`;
+    const endpoint = serverURL.pathname;
+    const dispatcher = new EventTarget();
+
+    const eventServer = http.createServer((req, res) => {
+        res.setHeader("Access-Control-Allow-Origin", "*");
+        res.setHeader("Access-Control-Allow-Methods", "GET");
+        res.setHeader("Access-Control-Allow-Headers", "Content-Type");
+
+        if (req.url !== endpoint) {
+            console.log(`🚫 Invalid request to ${req.url}`);
+            res.writeHead(404);
+            res.end();
+            return;
+        }
+
+        console.log("🔌 Client connected");
+
+        res.writeHead(200, {
+            "Content-Type": "text/event-stream",
+            "Cache-Control": "no-cache",
+            "Connection": "keep-alive",
+        });
+
+        /**
+         * @param {Event} event
+         */
+        const listener = (event) => {
+            const body = serializeCustomEventToStream(event);
+
+            res.write(body);
+        };
+
+        dispatcher.addEventListener("esbuild:start", listener);
+        dispatcher.addEventListener("esbuild:error", listener);
+        dispatcher.addEventListener("esbuild:end", listener);
+
+        req.on("close", () => {
+            console.log("🔌 Client disconnected");
+
+            clearInterval(keepAliveInterval);
+
+            dispatcher.removeEventListener("esbuild:start", listener);
+            dispatcher.removeEventListener("esbuild:error", listener);
+            dispatcher.removeEventListener("esbuild:end", listener);
+        });
+
+        const keepAliveInterval = setInterval(() => {
+            console.timeStamp("🏓 Keep-alive");
+
+            res.write("event: keep-alive\n\n");
+            res.write(serializeCustomEventToStream(new CustomEvent("esbuild:keep-alive")));
+        }, 15_000);
+    });
+
+    return {
+        name: "build-watcher",
+        setup: (build) => {
+            eventServer.listen(parseInt(serverURL.port, 10), serverURL.hostname);
+
+            build.onDispose(() => {
+                eventServer.close();
+            });
+
+            build.onStart(() => {
+                console.time(timerLabel);
+
+                dispatcher.dispatchEvent(
+                    new CustomEvent("esbuild:start", {
+                        detail: new Date().toISOString(),
+                    }),
+                );
+            });
+
+            build.onEnd((buildResult) => {
+                console.timeEnd(timerLabel);
+
+                if (!buildResult.errors.length) {
+                    dispatcher.dispatchEvent(
+                        new CustomEvent("esbuild:end", {
+                            detail: new Date().toISOString(),
+                        }),
+                    );
+
+                    return;
+                }
+
+                console.warn(`Build ended with ${buildResult.errors.length} errors`);
+
+                dispatcher.dispatchEvent(
+                    new CustomEvent("esbuild:error", {
+                        detail: buildResult.errors.map((error) => ({
+                            ...error,
+                            location: error.location
+                                ? {
+                                      ...error.location,
+                                      file: path.resolve(relativeRoot, error.location.file),
+                                  }
+                                : null,
+                        })),
+                    }),
+                );
+            });
+        },
+    };
+}