Replace eventMessageChannel with clearer implementation

ext/src/cast/contentBridge.ts

@@ -1,21 +1,25 @@
 "use strict";
 
-import { onMessageResponse, sendMessage } from "./eventMessageChannel";
+import eventMessaging from "./eventMessaging";
 
 import messaging, { Message } from "../messaging";
 
 // Message port to background script
 export const backgroundPort = messaging.connect({ name: "cast" });
 
-const forwardToCast = (message: Message) => sendMessage(message);
-const forwardToMain = (message: Message) => backgroundPort.postMessage(message);
+const forwardToPage = (message: Message) => {
+    eventMessaging.extension.sendMessage(message);
+};
+const forwardToMain = (message: Message) => {
+    backgroundPort.postMessage(message);
+};
 
 // Add message listeners
-backgroundPort.onMessage.addListener(forwardToCast);
-const listener = onMessageResponse(forwardToMain);
+backgroundPort.onMessage.addListener(forwardToPage);
+eventMessaging.extension.addListener(forwardToMain);
 
 // Remove listeners
 backgroundPort.onDisconnect.addListener(() => {
-    backgroundPort.onMessage.removeListener(forwardToCast);
-    listener.disconnect();
+    backgroundPort.onMessage.removeListener(forwardToPage);
+    eventMessaging.extension.addListener(forwardToMain);
 });

ext/src/cast/eventMessageChannel.ts

@@ -1,79 +0,0 @@
-"use strict";
-
-import { Message } from "../messaging";
-
-type ListenerFunc = (message: Message) => void;
-
-export interface ListenerObject {
-    disconnect(): void;
-}
-
-export function onMessage(listener: ListenerFunc): ListenerObject {
-    function on__castMessage(ev: CustomEvent) {
-        listener(JSON.parse(ev.detail));
-
-        /**
-         * TODO:
-         * Figure out a way to handle and stop propagation of this
-         * event to hide it from page scripts.
-         * Currently the event handler is set after the page loads the
-         * cast API, allowing pages set handlers before this script,
-         * intercept the event, and cancel it.
-         */
-        ev.stopPropagation();
-    }
-
-    // @ts-ignore
-    document.addEventListener("__castMessage", on__castMessage, true);
-
-    return {
-        disconnect() {
-            // @ts-ignore
-            document.removeEventListener(
-                "__castMessage",
-                on__castMessage,
-                true
-            );
-        }
-    };
-}
-
-export function sendMessageResponse(message: Message) {
-    const event = new CustomEvent("__castMessageResponse", {
-        detail: JSON.stringify(message)
-    });
-
-    document.dispatchEvent(event);
-}
-
-export function onMessageResponse(listener: ListenerFunc): ListenerObject {
-    function on__castMessageResponse(ev: CustomEvent) {
-        listener(JSON.parse(ev.detail));
-    }
-
-    // @ts-ignore
-    document.addEventListener(
-        "__castMessageResponse",
-        on__castMessageResponse,
-        true
-    );
-
-    return {
-        disconnect() {
-            // @ts-ignore
-            document.removeEventListener(
-                "__castMessageResponse",
-                on__castMessageResponse,
-                true
-            );
-        }
-    };
-}
-
-export function sendMessage(message: Message) {
-    const event = new CustomEvent("__castMessage", {
-        detail: JSON.stringify(message)
-    });
-
-    document.dispatchEvent(event);
-}

ext/src/cast/eventMessaging.ts

@@ -0,0 +1,93 @@
+"use strict";
+
+import logger from "../lib/logger";
+import { TypedEventTarget } from "../lib/TypedEventTarget";
+import { Message } from "../messaging";
+
+type EventMessengerListener = (message: Message) => void;
+
+/**
+ * Messenger class for cross-context messages via CustomEvent.
+ *
+ * Supplied with an incoming and outgoing event name, it provides a
+ * message channel from content scripts to page scripts provided that
+ * the opposite event names are used with instances on either side.
+ *
+ * Note:
+ * Extending EventTarget seems to cause issues with dispatching custom
+ * events in WebExtension content scripts (sandbox issue?), so custom
+ * addListener/removeListener methods are used instead.
+ */
+abstract class EventMessenger {
+    private listeners = new Set<EventMessengerListener>();
+
+    constructor(
+        private incomingMessageEventName: string,
+        private outgoingMessageEventName: string
+    ) {
+        // @ts-ignore
+        document.addEventListener(
+            this.incomingMessageEventName,
+            (ev: CustomEvent<string>) => {
+                for (const listener of this.listeners) {
+                    listener(JSON.parse(ev.detail));
+                }
+            }
+        );
+    }
+
+    addListener(listener: EventMessengerListener) {
+        this.listeners.add(listener);
+    }
+    removeListener(listener: EventMessengerListener) {
+        this.listeners.delete(listener);
+    }
+
+    sendMessage(message: Message) {
+        document.dispatchEvent(
+            new CustomEvent<string>(this.outgoingMessageEventName, {
+                detail: JSON.stringify(message)
+            })
+        );
+    }
+}
+
+const EV_TO_PAGE = "__castMessage";
+const EV_FROM_PAGE = "__castMessageResponse";
+
+export class PageEventMessenger extends EventMessenger {
+    constructor() {
+        super(EV_TO_PAGE, EV_FROM_PAGE);
+    }
+}
+export class ExtensionEventMessenger extends EventMessenger {
+    constructor() {
+        super(EV_FROM_PAGE, EV_TO_PAGE);
+    }
+}
+
+// Ensure only one instance of the type initially created is used
+let messenger: EventMessenger;
+function getMessenger(messengerType: { new (): EventMessenger }) {
+    if (!messenger) {
+        messenger = new messengerType();
+    } else if (!(messenger instanceof messengerType)) {
+        throw logger.error(
+            "Requested messenger does not match type of instantiated messenger!"
+        );
+    }
+
+    return messenger;
+}
+
+export default {
+    /** Event messenger for page scripts. */
+    get page() {
+        return getMessenger(PageEventMessenger);
+    },
+
+    /** Event messenger for extension content scripts. */
+    get extension() {
+        return getMessenger(ExtensionEventMessenger);
+    }
+};

ext/src/cast/export.ts

@@ -1,18 +1,20 @@
 "use strict";
 
-import { Message } from "../messaging";
+import messaging, { Message } from "../messaging";
 
 import { BridgeInfo } from "../lib/bridge";
 import { TypedMessagePort } from "../lib/TypedMessagePort";
 
-import {
-    onMessage,
-    onMessageResponse,
-    sendMessage
-} from "./eventMessageChannel";
-
 import CastSDK from "./sdk";
 
+import { PageEventMessenger, ExtensionEventMessenger } from "./eventMessaging";
+
+// Create messengers manually instead of relying on getters
+const eventMessaging = {
+    page: new PageEventMessenger(),
+    extension: new ExtensionEventMessenger()
+};
+
 let initializedBridgeInfo: BridgeInfo;
 let initializedBackgroundPort: MessagePort;
 
@@ -57,14 +59,14 @@ export function ensureInit(): Promise<TypedMessagePort<Message>> {
                 const message = ev.data as Message;
 
                 // Send message to cast instance
-                sendMessage(message);
+                eventMessaging.extension.sendMessage(message);
                 handleIncomingMessageToCast(message);
             };
 
             // cast instance -> bridge
-            onMessageResponse(message => {
-                channel.port1.postMessage(message);
-            });
+            eventMessaging.extension.addListener(message =>
+                channel.port1.postMessage(message)
+            );
         } else {
             /**
              * Import reference to message port created by contentBridge.
@@ -85,7 +87,9 @@ export function ensureInit(): Promise<TypedMessagePort<Message>> {
             };
 
             // Handle cast messages
-            onMessage(handleIncomingMessageToCast);
+            eventMessaging.page.addListener(message =>
+                handleIncomingMessageToCast(message)
+            );
         }
 
         function handleIncomingMessageToCast(message: Message) {

ext/src/cast/index.ts

@@ -4,7 +4,7 @@ import logger from "../lib/logger";
 
 import { loadScript } from "../lib/utils";
 import { CAST_FRAMEWORK_SCRIPT_URL } from "./endpoints";
-import { onMessage } from "./eventMessageChannel";
+import eventMessaging from "./eventMessaging";
 
 import CastSDK from "./sdk";
 
@@ -40,7 +40,7 @@ if (document.currentScript) {
     }
 }
 
-onMessage(async message => {
+eventMessaging.page.addListener(async message => {
     switch (message.subject) {
         case "cast:initialized": {
             bridgeInfo = message.data;

ext/src/cast/sdk/Session.ts

@@ -4,7 +4,7 @@ import { v4 as uuid } from "uuid";
 
 import logger from "../../lib/logger";
 
-import { sendMessageResponse } from "../eventMessageChannel";
+import eventMessaging from "../eventMessaging";
 
 import {
     ErrorCallback,
@@ -201,7 +201,7 @@ export default class Session {
         return new Promise<void>((resolve, reject) => {
             const messageId = uuid();
 
-            sendMessageResponse({
+            eventMessaging.page.sendMessage({
                 subject: "bridge:sendCastReceiverMessage",
                 data: {
                     sessionId: this.sessionId,
@@ -271,7 +271,7 @@ export default class Session {
     ) {
         const messageId = uuid();
 
-        sendMessageResponse({
+        eventMessaging.page.sendMessage({
             subject: "bridge:sendCastSessionMessage",
             data: {
                 sessionId: this.sessionId,

ext/src/cast/sdk/index.ts

@@ -8,7 +8,7 @@ import {
 } from "../../types";
 import { ErrorCallback, SuccessCallback } from "../types";
 
-import { onMessage, sendMessageResponse } from "../eventMessageChannel";
+import eventMessaging from "../eventMessaging";
 
 import {
     AutoJoinPolicy,
@@ -125,7 +125,7 @@ export default class {
     timeout = new Timeout();
 
     constructor() {
-        onMessage(this.#onMessage.bind(this));
+        eventMessaging.page.addListener(this.#onMessage.bind(this));
     }
 
     #sendSessionRequest(
@@ -136,7 +136,7 @@ export default class {
             listener(createReceiver(receiverDevice), ReceiverAction.CAST);
         }
 
-        sendMessageResponse({
+        eventMessaging.page.sendMessage({
             subject: "bridge:createCastSession",
             data: {
                 appId: sessionRequest.appId,
@@ -158,7 +158,7 @@ export default class {
              */
             case "cast:sessionCreated": {
                 // Notify background to close UI
-                sendMessageResponse({
+                eventMessaging.page.sendMessage({
                     subject: "main:closeReceiverSelector"
                 });
 
@@ -397,7 +397,7 @@ export default class {
 
         this.#apiConfig = apiConfig;
 
-        sendMessageResponse({
+        eventMessaging.page.sendMessage({
             subject: "main:initializeCast",
             data: { appId: this.#apiConfig.sessionRequest.appId }
         });
@@ -465,7 +465,7 @@ export default class {
             }
         } else {
             // Open receiver selector UI
-            sendMessageResponse({
+            eventMessaging.page.sendMessage({
                 subject: "main:selectReceiver",
                 data: { sessionRequest: this.#sessionRequest }
             });

ext/src/lib/TypedMessagePort.ts

@@ -5,5 +5,5 @@
  */
 export interface TypedMessagePort<T> extends MessagePort {
     postMessage(message: T, transfer: Transferable[]): void;
-    postMessage(message: T, options?: PostMessageOptions): void;
+    postMessage(message: T, options?: StructuredSerializeOptions): void;
 }