如果另一个字段中包含Angular 6中的值，则输入为必填

<form [formGroup]="myForm">
  <input type="text" formControlName="inputField">
  <input type="checkbox" formControlName="triggerField">
</form>

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

@Component({
  selector: 'app-my-component',
  templateUrl: './my-component.component.html',
  styleUrls: ['./my-component.component.css']
})
export class MyComponentComponent implements OnInit {
  myForm: FormGroup;

  constructor(private formBuilder: FormBuilder) { }

  ngOnInit() {
    this.myForm = this.formBuilder.group({
      inputField: ['', Validators.required],
      triggerField: ['']
    });

    this.myForm.get('triggerField').valueChanges.subscribe(value => {
      const inputField = this.myForm.get('inputField');
      if (value === 'specificValue') {
        inputField.setValidators(Validators.required);
      } else {
        inputField.clearValidators();
      }
      inputField.updateValueAndValidity();
    });
  }
}