angular4:使用valueChanges observable进行自定义“equals”验证

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

@Component({
  selector: 'app-my-form',
  template: `
    <form [formGroup]="myForm">
      <input formControlName="password1" type="password" placeholder="Password">
      <input formControlName="password2" type="password" placeholder="Confirm Password">
      <span *ngIf="myForm.hasError('equals', ['password1', 'password2'])">
        Passwords do not match!
      </span>
    </form>
  `,
})
export class MyFormComponent implements OnInit {
  myForm: FormGroup;

  ngOnInit(): void {
    this.myForm = new FormGroup({
      password1: new FormControl('', Validators.required),
      password2: new FormControl('', Validators.required),
    });

    this.myForm.setValidators(this.equalsValidator);
  }

  equalsValidator(form: FormGroup): { [key: string]: boolean } {
    const password1 = form.get('password1').value;
    const password2 = form.get('password2').value;

    if (password1 !== password2) {
      return { 'equals': true };
    }

    return null;
  }
}