自定义验证器不会调用Angular 2中的每个按键

import { AbstractControl, ValidatorFn } from '@angular/forms';

export function minLengthValidator(minLength: number): ValidatorFn {
  return (control: AbstractControl): { [key: string]: any } | null => {
    const value = control.value;
    if (value && value.length < minLength) {
      return { 'minLength': { requiredLength: minLength, actualLength: value.length } };
    }
    return null;
  };
}

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

@Component({
  selector: 'app-my-form',
  template: `
    <form [formGroup]="myForm">
      <input type="text" formControlName="myField">
      <div *ngIf="myForm.get('myField').errors?.minLength">
        Minimum length is {{ myForm.get('myField').errors.minLength.requiredLength }},
        actual length is {{ myForm.get('myField').errors.minLength.actualLength }}
      </div>
    </form>
  `,
})
export class MyFormComponent {
  myForm: FormGroup;

  constructor() {
    this.myForm = new FormGroup({
      myField: new FormControl('', [Validators.required, minLengthValidator(5)])
    });
  }
}