Replace eventMessageChannel with clearer implementation

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);
+    }
+};