使用http的Angular自定义验证器不起作用

import { HttpClient } from '@angular/common/http';
import { AsyncValidatorFn, AbstractControl, ValidationErrors } from '@angular/forms';
import { Observable, of } from 'rxjs';
import { map, catchError } from 'rxjs/operators';

// 异步验证器工厂函数
export function httpValidator(http: HttpClient): AsyncValidatorFn {
  return (control: AbstractControl): Observable<ValidationErrors | null> => {
    const value = control.value;
    // 发起HTTP请求
    return http.get('http://example.com/validate', { params: { value } }).pipe(
      map(response => {
        // 根据响应结果判断验证结果
        if (response.valid) {
          return null; // 通过验证
        } else {
          return { httpError: true }; // 验证失败
        }
      }),
      catchError(() => {
        return of({ httpError: true }); // 处理HTTP请求错误
      })
    );
  };
}

import { Component } from '@angular/core';
import { FormControl, FormGroup, Validators } from '@angular/forms';
import { HttpClient } from '@angular/common/http';
import { httpValidator } from './http-validator';

@Component({
  selector: 'app-my-form',
  template: `
    <form [formGroup]="myForm">
      <input formControlName="myControl">
      <div *ngIf="myControl.errors?.httpError">验证失败</div>
    </form>
  `,
})
export class MyFormComponent {
  myForm: FormGroup;

  constructor(private http: HttpClient) {
    this.myForm = new FormGroup({
      myControl: new FormControl('', Validators.required, httpValidator(http)),
    });
  }
}