我的异步验证器未按角度调​​用

如何解决我的异步验证器未按角度调​​用

这是异步验证器，我知道它没有被调用，因为它没有控制台记录响应，并且函数isUsernameAvailable返回一个Observable

import {AbstractControl,AsyncValidatorFn,ValidationErrors} from '@angular/forms';
import {Observable} from 'rxjs';
import {map} from 'rxjs/operators';
import {CredentialAvailabilityService} from '../services/auth/credential-availability.service';


let credentialAvailabilityService: CredentialAvailabilityService;

export  const  usernameValidator = (control: AbstractControl): Observable<ValidationErrors | null> => {
    return credentialAvailabilityService.isUsernameAvailable(control.value).pipe(
      map(res => {
        console.log(res);
        // if res is true,username exists,return true
        return res ? {usernameExists: true} : null;
        // NB: Return null if there is no error
      })
    );
}

下面是我加载表格的地方

loadForm() {
    this.form = this.formBuilder.group({
      username: new FormControl(null,{
        validators: [Validators.required,Validators.minLength(5),Validators.maxLength(20),alphaNumericValidator]
      },[usernameValidator]),fullName: new FormControl(null,{validators: [Validators.required,Validators.maxLength(100),Validators.pattern('^[a-zA-Z ]*$')]}),email: new FormControl(null,Validators.email]}),password: new FormControl(null,Validators.minLength(8),Validators.maxLength(40)]}),confirmPassword: new FormControl(null,{validators: [Validators.required]}),telephoneNumber: new FormControl(null,{validators: [Validators.minLength(9),Validators.maxLength(14),Validators.pattern('^[0-9]*$')]}),roleCode: new FormControl(null,{validators: [Validators.minLength(6),Validators.maxLength(6),//image: new FormControl(null,{},mimeType)
    },{
      validator: MustMatch('password','confirmPassword')
    });
    //Todo : add an async validator to check if current username exists as user types on the field
  }