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/realtime-note.service.ts

@@ -0,0 +1,57 @@
+/*
+ * SPDX-FileCopyrightText: 2022 The HedgeDoc developers (see AUTHORS file)
+ *
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+import { Injectable } from '@nestjs/common';
+
+import { Note } from '../../notes/note.entity';
+import { RevisionsService } from '../../revisions/revisions.service';
+import { RealtimeNote } from './realtime-note';
+
+@Injectable()
+export class RealtimeNoteService {
+  constructor(private revisionsService: RevisionsService) {}
+
+  private noteIdToRealtimeNote = new Map<string, RealtimeNote>();
+
+  /**
+   * Creates or reuses a {@link RealtimeNote} that is handling the real time editing of the {@link Note} which is identified by the given note id.
+   * @param note The for which a {@link RealtimeNote realtime note} should be retrieved.
+   * @throws NotInDBError if note doesn't exist or has no revisions.
+   * @return A {@link RealtimeNote} that is linked to the given note.
+   */
+  public async getOrCreateRealtimeNote(note: Note): Promise<RealtimeNote> {
+    return (
+      this.noteIdToRealtimeNote.get(note.id) ??
+      (await this.createNewRealtimeNote(note))
+    );
+  }
+
+  /**
+   * Creates a new {@link RealtimeNote} for the given {@link Note} and memorizes it.
+   *
+   * @param note The note for which the realtime note should be created
+   * @throws NotInDBError if note doesn't exist or has no revisions.
+   * @return The created realtime note
+   */
+  private async createNewRealtimeNote(note: Note): Promise<RealtimeNote> {
+    const initialContent = (await this.revisionsService.getLatestRevision(note))
+      .content;
+    const realtimeNote = new RealtimeNote(note.id, initialContent);
+    realtimeNote.on('destroy', () => {
+      this.noteIdToRealtimeNote.delete(note.id);
+    });
+    this.noteIdToRealtimeNote.set(note.id, realtimeNote);
+    return realtimeNote;
+  }
+
+  /**
+   * Retrieves a {@link RealtimeNote} that is linked to the given {@link Note} id.
+   * @param noteId The id of the {@link Note}
+   * @return A {@link RealtimeNote} or {@code undefined} if no instance is existing.
+   */
+  public getRealtimeNote(noteId: string): RealtimeNote | undefined {
+    return this.noteIdToRealtimeNote.get(noteId);
+  }
+}