feat: add base implementation for realtime communication

Signed-off-by: Philip Molares <philip.molares@udo.edu>
Signed-off-by: Erik Michelson <github@erik.michelson.eu>
Co-authored-by: Erik Michelson <github@erik.michelson.eu>
Co-authored-by: Philip Molares <philip.molares@udo.edu>
Co-authored-by: Tilman Vatteroth <git@tilmanvatteroth.de>
Signed-off-by: Tilman Vatteroth <git@tilmanvatteroth.de>

src/realtime/realtime-note/websocket-doc.ts

@@ -0,0 +1,71 @@
+/*
+ * SPDX-FileCopyrightText: 2022 The HedgeDoc developers (see AUTHORS file)
+ *
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+import { encodeDocumentUpdateMessage } from '@hedgedoc/realtime';
+import { Doc } from 'yjs';
+
+import { RealtimeNote } from './realtime-note';
+import { WebsocketConnection } from './websocket-connection';
+
+/**
+ * This is the implementation of {@link Doc YDoc} which includes additional handlers for message sending and receiving.
+ */
+export class WebsocketDoc extends Doc {
+  private static readonly channelName = 'markdownContent';
+
+  /**
+   * Creates a new WebsocketDoc instance.
+   *
+   * The new instance is filled with the given initial content and an event listener will be registered to handle
+   * updates to the doc.
+   *
+   * @param realtimeNote - the {@link RealtimeNote} handling this {@link Doc YDoc}
+   * @param initialContent - the initial content of the {@link Doc YDoc}
+   */
+  constructor(private realtimeNote: RealtimeNote, initialContent: string) {
+    super();
+    this.initializeContent(initialContent);
+    this.bindUpdateEvent();
+  }
+
+  /**
+   * Binds the event that distributes updates in the current {@link Doc y-doc} to all clients.
+   */
+  private bindUpdateEvent(): void {
+    this.on('update', (update: Uint8Array, origin: WebsocketConnection) => {
+      const clients = this.realtimeNote
+        .getConnections()
+        .filter((client) => client !== origin && client.isSynced());
+      if (clients.length > 0) {
+        clients.forEach((client) => {
+          client.send(encodeDocumentUpdateMessage(update));
+        });
+      }
+    });
+  }
+
+  /**
+   * Sets the {@link YDoc's Doc} content to include the initialContent.
+   *
+   * This message should only be called when a new {@link RealtimeNote } is created.
+   *
+   * @param initialContent - the initial content to set the {@link Doc YDoc's} content to.
+   * @private
+   */
+  private initializeContent(initialContent: string): void {
+    this.getText(WebsocketDoc.channelName).insert(0, initialContent);
+  }
+
+  /**
+   * Gets the current content of the note as it's currently edited in realtime.
+   *
+   * Please be aware that the return of this method may be very quickly outdated.
+   *
+   * @return The current note content.
+   */
+  public getCurrentContent(): string {
+    return this.getText(WebsocketDoc.channelName).toString();
+  }
+}