example-6.ts

const usernameControl = new FormControl('');

usernameControl.validationEvents
  .pipe(
    filter(event => event.label === 'End'),
    tap(() => this.control.markPending(true, { source: 'usernameValidator' })),
    switchMap(username =>
      interval(500).pipe(
        take(1),
        switchMap(() => this.userService.findUserByHandle(username)),
        map(user => !!user),
      ),
    ),
    tap(() => this.control.markPending(false, { source: 'usernameValidator' })),
  )
  .subscribe(usernameExists => {
    const errors = usernameExists ? { usernameExists: true } : null;

    this.usernameControl.setErrors(errors, {
      source: 'usernameValidator',
    });
  });