Angular 2 Reactive Custom Form Control验证函数不止一次调用

Question

当我注意到，当我没有尝试将任何数据提交到html输入时，我正在尝试构建自定义反应式表单控件验证器函数。 我想知道为什么在检查chrome控制台中的代码时我会将log 2打印三次。

@Component({
    selector: 'fourth-app',
    templateUrl: './app/Fourth_Component/fourthcomponent.html'
})

@Injectable()
export class FourthComponent implements OnInit {


profileForm: FormGroup;

private ProductName: FormControl;
private Price: FormControl;
private InStock: FormControl;
private ProductCompany: FormControl;

constructor(private http: Http) { }

ngOnInit() {
    console.log("1");
    this.ProductName = new FormControl('', [Validators.required, Validators.minLength(2)]);
    this.Price = new FormControl('',Validators.required);
    this.InStock = new FormControl('', [Validators.required, this.inStock]);
    this.ProductCompany = new FormControl('', [Validators.required, Validators.minLength(5)]);

    this.profileForm = new FormGroup({
        ProductName: this.ProductName,
        Price: this.Price,
        InStock: this.InStock,
        ProductCompany: this.ProductCompany
    });
}

inStock(control: FormControl) {
    console.log("2");
    return control.value == "true" ? null : { valid: false };
}

addProduct(formValues: any): void {

    if (this.profileForm.valid) {
        let headers = new Headers({ 'Content-Type': 'application/json' });
        let options = new RequestOptions({ headers: headers });

        this.http.post('api/Combiners', JSON.stringify(formValues), options)
            .map((res: Response) => res.json())
            .subscribe(
            data => console.log(data),
            err => console.log(err),
            () => console.log('yay post')
            );
    }
    else {
        console.log("User input was invalid!");
    }
}

}

Chrome console Screenshot