即使setValidators设置为required，如果原始，角度反应式输入也有效

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

@Component({
  selector: 'app-example',
  template: `
    <input [(ngModel)]="value" (ngModelChange)="onValueChange($event)">
    <div *ngIf="formControl.hasError('required')">Value is required</div>
  `,
})
export class ExampleComponent implements OnInit {
  value: string;
  formControl: FormControl;

  ngOnInit() {
    this.formControl = new FormControl('', Validators.required);
    this.formControl.valueChanges.subscribe(() => {
      this.formControl.updateValueAndValidity();
    });
  }

  onValueChange(value: string) {
    this.value = value;
  }
}