My TransferState.onSerialize() callback has to get data from an EventEmitter.

However, to my understanding the first is synchronous, while the second is obviously asynchronous.

My, in this snippet minimized, implementation right now, with missing exactly this point, is:

import {
  OnInit
} from '@angular/core';
import {
  makeStateKey,
  StateKey,
  TransferState
} from '@angular/platform-browser';
import SerializerJsonld from '@rdfjs/serializer-jsonld';
import type {
  Store
} from '@rdfjs/types'
import { EventEmitter } from 'stream';

export class DatastoreService implements OnInit {

  private readonly _store: Store;

  private readonly RESULT_KEY: StateKey<string> = makeStateKey < string > ('result');

  async get_store(): Promise < Store > {
    await this._store.open();
    return this._store;
  }

  constructor(
    private transferState: TransferState,
  ) {}

  ngOnInit(): void {
    this._store.open().then(store => {
      this.transferState.onSerialize(this.RESULT_KEY, () => {
        const output = new SerializerJsonld().import(this._store.match());
        return ???;
      });
    });
  }
}

How can I solve this or in general such situations?