2 files changed, 36 insertions, 12 deletions

diff --git a/packages/main/src/index.ts b/packages/main/src/index.ts
index 4a51518..b134002 100644
--- a/packages/main/src/index.ts
+++ b/packages/main/src/index.ts
@@ -219,8 +219,8 @@ function createWindow(): Promise<unknown> {
     webContents.send(MainToRendererIpcMessage.SharedStorePatch, patch);
   });
 
-  ipcMain.on(ServiceToMainIpcMessage.ApiExposedInMainWorld, (event) => {
-    event.returnValue = event.sender.id == browserView.webContents.id
+  ipcMain.handle(ServiceToMainIpcMessage.ApiExposedInMainWorld, (event) => {
+    return event.sender.id == browserView.webContents.id
       ? serviceInject
       : null;
   });
@@ -229,7 +229,8 @@ function createWindow(): Promise<unknown> {
     try {
       switch (channel) {
         case ServiceToMainIpcMessage.ApiExposedInMainWorld:
-          // Synchronous message must be handled with `ipcMain.on`
+          // Asynchronous message with reply must be handled in `ipcMain.handle`,
+          // otherwise electron emits a no handler registered warning.
           break;
         case ServiceToMainIpcMessage.SetUnreadCount:
           console.log('Unread count:', unreadCount.parse(args[0]));
diff --git a/packages/service-preload/src/index.ts b/packages/service-preload/src/index.ts
index e42c406..d1ea13c 100644
--- a/packages/service-preload/src/index.ts
+++ b/packages/service-preload/src/index.ts
@@ -27,13 +27,36 @@ if (webFrame.parent === null) {
   webFrame.insertCSS('html { background-color: #fff; }');
 }
 
-const injectSource = webSource.safeParse(ipcRenderer.sendSync(ServiceToMainIpcMessage.ApiExposedInMainWorld));
-if (injectSource.success) {
-  webFrame.executeJavaScriptInIsolatedWorld(0, [
-    injectSource.data,
-  ]).catch((err) => {
-    console.log('Failed to inject source:', err);
-  });
-} else {
-  console.log('Invalid source to inject:', injectSource.error);
+/**
+ * Fetches and executes the service inject script in the isolated world.
+ *
+ * The service inject script relies on exposed APIs, so this function can only
+ * be called after APIs have been exposed via `contextBridge` to the main world.
+ *
+ * We have to call `executeJavaScriptInIsolatedWorld` from the service preload script,
+ * beause there is no way currently (electron 16) to execute a script on a
+ * `WebFrameMain` in the main process by specifying a `WebSource`.
+ * Calling `executeJavaScriptInInsolatedWorld` on a `WebContents` in the main process
+ * will always inject the script into the _top-level_ frame, but here we
+ * are injecting into the _current_ frame instead.
+ * As a tradeoff, the promise returned by `executeJavaScriptInIsolatedWorld`
+ * will resolve to `unknown` (instead of rejecting) even if the injected script fails,
+ * because chromium doesn't dispatch main world errors to isolated worlds.
+ *
+ * @return A promise that only rejects if we fail to fetch the inject script.
+ * @see https://www.electronjs.org/docs/latest/api/web-frame#webframeexecutejavascriptinisolatedworldworldid-scripts-usergesture-callback
+ * @see https://www.electronjs.org/docs/latest/api/web-frame-main#frameexecutejavascriptcode-usergesture
+ * @see https://www.electronjs.org/docs/latest/api/web-contents#contentsexecutejavascriptinisolatedworldworldid-scripts-usergesture
+ */
+async function fetchAndExecuteInjectScript(): Promise<void> {
+  const apiExposedResponse = await ipcRenderer.invoke(
+    ServiceToMainIpcMessage.ApiExposedInMainWorld,
+  );
+  const injectSource = webSource.parse(apiExposedResponse);
+  // Isolated world 0 is the main world.
+  return webFrame.executeJavaScriptInIsolatedWorld(0, [injectSource]);
 }
+
+fetchAndExecuteInjectScript().catch((err) => {
+  console.log('Failed to fetch inject source:', err);
+});