refactor: reimplement realtime-communication

This commit refactors a lot of things that are not easy to separate.
It replaces the binary protocol of y-protocols with json.
It introduces event based message processing.
It implements our own code mirror plugins for synchronisation of content and remote cursors

Signed-off-by: Tilman Vatteroth <git@tilmanvatteroth.de>

backend/src/realtime/realtime-note/realtime-connection.ts

@@ -0,0 +1,76 @@
+/*
+ * SPDX-FileCopyrightText: 2022 The HedgeDoc developers (see AUTHORS file)
+ *
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+import { MessageTransporter, YDocSyncServerAdapter } from '@hedgedoc/commons';
+import { Logger } from '@nestjs/common';
+
+import { User } from '../../users/user.entity';
+import { generateRandomName } from './random-word-lists/name-randomizer';
+import { RealtimeNote } from './realtime-note';
+import { RealtimeUserStatusAdapter } from './realtime-user-status-adapter';
+
+/**
+ * Manages the connection to a specific client.
+ */
+export class RealtimeConnection {
+  protected readonly logger = new Logger(RealtimeConnection.name);
+  private readonly transporter: MessageTransporter;
+  private readonly yDocSyncAdapter: YDocSyncServerAdapter;
+  private readonly realtimeUserStateAdapter: RealtimeUserStatusAdapter;
+
+  private displayName: string;
+
+  /**
+   * Instantiates the connection wrapper.
+   *
+   * @param messageTransporter The message transporter that handles the communication with the client.
+   * @param user The user of the client
+   * @param realtimeNote The {@link RealtimeNote} that the client connected to.
+   * @throws Error if the socket is not open
+   */
+  constructor(
+    messageTransporter: MessageTransporter,
+    private user: User | null,
+    private realtimeNote: RealtimeNote,
+  ) {
+    this.displayName = user?.displayName ?? generateRandomName();
+    this.transporter = messageTransporter;
+
+    this.transporter.on('disconnected', () => {
+      realtimeNote.removeClient(this);
+    });
+    this.yDocSyncAdapter = new YDocSyncServerAdapter(this.transporter);
+    this.yDocSyncAdapter.setYDoc(realtimeNote.getRealtimeDoc());
+    this.realtimeUserStateAdapter = new RealtimeUserStatusAdapter(
+      this.user?.username ?? null,
+      this.getDisplayName(),
+      this,
+    );
+  }
+
+  public getRealtimeUserStateAdapter(): RealtimeUserStatusAdapter {
+    return this.realtimeUserStateAdapter;
+  }
+
+  public getTransporter(): MessageTransporter {
+    return this.transporter;
+  }
+
+  public getUser(): User | null {
+    return this.user;
+  }
+
+  public getSyncAdapter(): YDocSyncServerAdapter {
+    return this.yDocSyncAdapter;
+  }
+
+  public getDisplayName(): string {
+    return this.displayName;
+  }
+
+  public getRealtimeNote(): RealtimeNote {
+    return this.realtimeNote;
+  }
+}