与此同时，我找到了解决方案.我的子组件现在引用了相应的FormGroup.

Meanwhile I found a solution. My child component now references the according FormGroup.

<email [(emailAddress)]="model.email" 
       [formGroup]="form.form" 
       [required]="emailRequired()">
</email>

通过FormGroup，您可以到达子组件实际的FormControl并在验证器标志的设置器内调用 updateValueAndValidity (在我的情况下为 set required ):

Via the FormGroup, you can then reach to the child components actual FormControl and call updateValueAndValidity inside the setter of the validator flag (set required in my case):

import {Component, EventEmitter, Input, Output} from '@angular/core';
import {FormGroup} from '@angular/forms';

@Component({
  selector: 'email',
  templateUrl: './email.component.html',
  styleUrls: ['./email.component.css']
})
export class EmailInputComponent {

  private _emailAddress: string;

  public _required = true;

  @Input()
  private formGroup: FormGroup;

  @Output()
  emailAddressChange = new EventEmitter();

  constructor() { }

  @Input()
  get emailAddress(): string {
    return this._emailAddress;
  }

  set emailAddress(value: string) {
    this._emailAddress = value;
    this.emailAddressChange.emit(this._emailAddress);
  }

  @Input()
  get required(): boolean {
    return this._required;
  }

  set required(value: boolean) {
    this._required = value;
    // this is where the magic happens
    const refEmailControl = this.formGroup.controls.email;
    if (refEmailControl) {
      refEmailControl.updateValueAndValidity(); // updates the validity state      
      refEmailControl.markAsTouched(); // necessary in my case to make UI render error message
    }
  }
}