figure-4.ts

class FormControl<Value = any, Data = any>
  implements AbstractControl<Value, Data> {
  id: ControlId;

  constructor() {
    this.source
      .pipe(
        filter(
          // make sure we don't process an event we already processed
          event => !event.processed.includes(this.id),
        ),
      )
      .subscribe(event => {
        event.processed.push(this.id);
        if (!event.meta) event.meta = {};
        if (!event.id) event.id = AbstractControl.eventId();

        const newEvent = this.processEvent(event as ControlEvent);

        if (newEvent) {
          this._events.next(newEvent);
        }
      });
  }

  // ...
}