如何为Angular中的空ng-content提供回退？

<ng-template #fallback>
  <p>这是空内容的回退文本。</p>
</ng-template>

<ng-content *ngIf="!content; else fallback"></ng-content>

import { Component, ViewChild, ElementRef } from '@angular/core';

@Component({
  selector: 'app-example',
  template: `
    <ng-content-example>
      <p>这是传入的内容。</p>
    </ng-content-example>
  `,
})
export class ExampleComponent {
  @ViewChild('fallback') fallback: ElementRef;
  content: boolean;

  ngAfterContentInit() {
    this.content = this.fallback.nativeElement.children.length === 0;
  }
}