@Directive({
  selector: '[validatorUsername]',
  providers: [{ provide: NG_ASYNC_VALIDATORS, useExisting: ValidatorUsernameDirective, multi: true }]
})
export class ValidatorUsernameDirective implements Validator {
  validate(c: AbstractControl) {
    let username = c.value;

    return new Promise(resolve => {
      setTimeout(() => {
        if( username === "nemo" ) {
          resolve({
            'taken': true
          })
        } else {
          resolve(null);
        }
      }, 1000);
    });
  }
}

<input type="text" [(ngModel)]="username" name="username" required validatorUsername>

export class App implements OnInit {
  @ViewChild('myForm') myForm: NgForm;

  name: string;
  username: string;

  ngOnInit() {
    this.myForm.valueChanges.subscribe(_ => this.onValueChanged());
  }

  onValueChanged() {
    // fill 'formErrors' object
  }

  formErrors = {
    'name': '',
    'username': ''
  };
}