feat(notifications): added notification system w/ interwoven refactoring

server/internal/notifications/index.ts

@@ -0,0 +1,88 @@
+/*
+The notification system handles the recieving, creation and sending of notifications in Drop
+
+Design goals:
+1. Nonce-based notifications; notifications should only be created once
+2. Real-time; use websocket listeners to keep clients up-to-date
+*/
+
+import { Notification } from "@prisma/client";
+import prisma from "../db/database";
+
+export type NotificationCreateArgs = Pick<
+  Notification,
+  "title" | "description" | "actions" | "nonce"
+>;
+
+class NotificationSystem {
+  private listeners: {
+    [key: string]: Map<string, (notification: Notification) => any>;
+  } = {};
+
+  listen(
+    userId: string,
+    id: string,
+    callback: (notification: Notification) => any
+  ) {
+    this.listeners[userId] ??= new Map();
+    this.listeners[userId].set(id, callback);
+
+    this.catchupListener(userId, id);
+  }
+
+  unlisten(userId: string, id: string) {
+    this.listeners[userId].delete(id);
+  }
+
+  private async catchupListener(userId: string, id: string) {
+    const callback = this.listeners[userId].get(id);
+    if (!callback)
+      throw new Error("Failed to catch-up listener: callback does not exist");
+    const notifications = await prisma.notification.findMany({
+      where: { userId: userId },
+      orderBy: {
+        created: "asc", // Oldest first, because they arrive in reverse order
+      },
+    });
+    for (const notification of notifications) {
+      await callback(notification);
+    }
+  }
+
+  private async pushNotification(userId: string, notification: Notification) {
+    for (const listener of this.listeners[userId] ?? []) {
+      await listener[1](notification);
+    }
+  }
+
+  async push(userId: string, notificationCreateArgs: NotificationCreateArgs) {
+    const notification = await prisma.notification.create({
+      data: {
+        userId: userId,
+        ...notificationCreateArgs,
+      },
+    });
+
+    await this.pushNotification(userId, notification);
+  }
+
+  async pushAll(notificationCreateArgs: NotificationCreateArgs) {
+    const users = await prisma.user.findMany({
+      where: { id: { not: "system" } },
+      select: {
+        id: true,
+      },
+    });
+
+    for (const user of users) {
+      await this.push(user.id, notificationCreateArgs);
+    }
+  }
+
+  async systemPush(notificationCreateArgs: NotificationCreateArgs) {
+    return await this.push("system", notificationCreateArgs);
+  }
+}
+
+export const notificationSystem = new NotificationSystem();
+export default notificationSystem;