angular behaviorsubject subscribe once code example

Example: behaviorsubject in angular 10

@Injectable()
export class TodoService {
  private _todos = new BehaviorSubject<Todo[]>([]);
  private baseUrl = 'https://56e05c3213da80110013eba3.mockapi.io/api';
  private dataStore: { todos: Todo[] } = { todos: [] };
  readonly todos = this._todos.asObservable();

  constructor(private http: HttpClient) {}

  get todos() {
    return this._todos.asObservable();
  }

  loadAll() {
    this.http.get(`${this.baseUrl}/todos`).subscribe(
      data => {
        this.dataStore.todos = data;
        this._todos.next(Object.assign({}, this.dataStore).todos);
      },
      error => console.log('Could not load todos.')
    );
  }
}