events: Add mechanism to enqueue events.

The implementation of the respective functions will be done through
subsequent PRs.
Also added the AppEvent type.

backend/eventRecipients.ts

@@ -1,18 +1,39 @@
 // this module is responsible for handling the clients currently connected to the server.
-// It stores the clients in a Map object, where the key is the client's user_id and the value is the client's http response object.
+// It stores the clients in a Map object, where the key is the client id and the value is the
+// client's http response object.
+// When sending an event, we first push the event to a queue. We need to use a queue because it
+// is possible that the client hasn't sent the next polling request yet, i.e, we don't have a response
+// object for that client yet. When the client sends the next polling request, we will send the events
+// from the queue to the client.
 
 import { Response } from 'express';
 
-const clients = new Map<string, Response>();
+type ClientId = string;
 
-export function addClient(userId: string, res: Response) {
-    clients.set(userId, res);
+const clients = new Map<ClientId, Response>();
+const eventQueue = new Map<ClientId, AppEvent[]>();
+
+export function addClient(clientId: ClientId, res: Response) {
+    // todo: We can immediately send any events that are in the queue.
+    clients.set(clientId, res);
+    eventQueue.set(clientId, []);
+}
+
+export function removeClient(clientId: ClientId) {
+    clients.delete(clientId);
+    eventQueue.delete(clientId);
+}
+
+export function getClient(clientId: ClientId) {
+    return clients.get(clientId);
 }
 
-export function removeClient(userId: string) {
-    clients.delete(userId);
+// eslint-disable-next-line @typescript-eslint/no-unused-vars
+export function scheduleSend(clientId: ClientId, event: AppEvent) {
+    //todo: Enqueue the event for sending.
 }
 
-export function getClient(userId: string) {
-    return clients.get(userId);
+// eslint-disable-next-line @typescript-eslint/no-unused-vars
+export function doSendEvent(clientId: ClientId) {
+    //todo: Send all the events in the queue to the client, only if the response object is available.
 }

backend/src/types.d.ts

@@ -44,4 +44,7 @@ type GameEvent =
               card: UNOCard;
           };
       };
+
+// Represent all the events that can be sent to the client
+type AppEvent = GameEvent;
 //todo: Add more events