Flutter / JavaScript bridge

nfc-service.ts

import {Inject, Injectable} from '@angular/core';
import {Observable} from 'rxjs/Observable';
import {Subscriber} from 'rxjs/Subscriber';

@Injectable()
export class NfcServiceProvider {
  private nfcAvailable: boolean = false;
  private tagDataSubscriber: Subscriber<any>;
  private tagRead: Observable<number> = new Observable<number>(
    observer => this.tagDataSubscriber = observer
  );

  public TagDataSubscriber(nfcTag: number) {
    this.tagDataSubscriber.next(nfcTag);
  }

  public get TagRead(): Observable<number> {
    return this.tagRead;
  }

  public get NfcAvailable(): boolean {
    return this.nfcAvailable;
  }

  constructor(@Inject('Window') private window: any) {
    /* function called by Flutter if a NFC tag was detected */
    window["nfcTagReceived"] = (nfcTag) => {
      // when a NFC tag was read, this point is reached without problems.
      
      // here the this-pointer is not available.
      // I need to inform the subscriber about the newly read NFC tag
      // that's unfortunately not working
      this.tagDataSubscriber.next(nfcTag);
    }
  }
}

webview-bridge.dart

class WebViewBridge {
  final BuildContext context;
  late WebViewBridgeChannelController webViewBridgeChannelController;

  WebViewBridge(this.context) {
    webViewBridgeChannelController = WebViewBridgeChannelController(webViewBridge: this);
  }

  WebViewController? webViewController;

  Set<JavascriptChannel> get channels => {
        webViewBridgeChannelController.onNewLoginToken(),
        webViewBridgeChannelController.onPlaySoundCheckInSucceeded(),
        webViewBridgeChannelController.onPlaySoundCheckInFailed(),
        webViewBridgeChannelController.onRequestInternalNfcAvailability(),
        webViewBridgeChannelController.onReadNfcOnce(),
        webViewBridgeChannelController.onReadNfcStream(),
        webViewBridgeChannelController.onStopNfcReading(),
        webViewBridgeChannelController.onChangeUrl(),
        webViewBridgeChannelController.onShowMessage(context),
      };

  Future<void> login(String token) async {
    await webViewController?.runJavascript('login("$token")');
  }

  // this function is called, when a new NFC tag was read
  Future<void> sendNfcTag(String? nfcId) async {
    await webViewController?.runJavascript('nfcTagReceived("$nfcId")');
  }

  Future<void> isInternalNfcAvailable(NfcReaderStatus status) async {
    await webViewController?.runJavascript('isInternalNfcAvailable("${status.name}")');
  }

  Future<void> changeUrl(String url) async {
    await webViewController?.loadUrl(url);
  }
}