我使用Angular 4.0.0进行自定义组验证时遇到问题此表单组始终显示null {{modelform.get（'passwords'） . json}}

组件

import { Component, OnInit } from '@angular/core';
import { AbstractControl, FormBuilder, FormControl, FormGroup, Validators } from '@angular/forms';

@Component({
    templateUrl: './mdf-costomvalidation.component.html'
})
export class MDFCustomValidationComponent implements OnInit {

    modelform: FormGroup;
    constructor(private _formBuilder: FormBuilder) { }

    ngOnInit() {
        this.modelform = this._formBuilder.group({
            firstname: ['', Validators.required],
            lastname: ['', Validators.required],
            passwords: this._formBuilder.group({
                firstpass: ['', Validators.required],
                secondpass: ['', Validators.required]
            }, { Validator: this.passwordMatch })
        });
    }
    passwordMatch(c: AbstractControl) {
        return c.get('firstpass').value === c.get('secondpass').value ? null : {'nomatch': true};
    }
    modelformsubmit() {
            console.log(this.modelform.value);
    }
}

我试图在类之外声明passwordMatch（）函数，并根据Angular.io API FormGroup示例将第二个参数称为'passwordMatch'而不是'this'关键字，但对返回值没有影响 .

function passwordMatch(c: AbstractControl) {
    return c.get('firstpass').value === c.get('secondpass').value ? null : {'nomatch': true};
}

passwords: this._formBuilder.group({
    firstpass: ['', Validators.required],
    secondpass: ['', Validators.required]
}, passwordMatch)
angular typescript angular2-forms angular-validation angular4-forms