<div class="flash-signal">
  Signal visuel :
  <input type="radio" name="flashsignal" id="flashsignal1" checked="checked" (click)="flashSignal()">
  <label for="flashsignal1">Oui</label>
  <input type="radio" name="flashsignal" id="flashsignal2" (click)="notFlashSignal()">
  <label for="flashsignal2">Non</label>
</div>

export class ChatThreadsComponent implements OnInit {

flash = true;

constructor(public threadsService: ThreadsService,
          public messagesService: MessagesService,
          public boxService: BoxService) {
  this.threads = threadsService.orderedThreads;
}

ngOnInit(): void {
   this.flash = true;
}

notFlashSignal(): void {
  this.flash = false;
}

flashSignal(): void {
    this.flash = true;
}
}

<div [ngClass]="{'notification-message-unread' : !box.thread.lastMessage.isRead && flash}">