contentchild.ts

@Component({
  selector: 'parent-component',
  template: '<child-component><content-component></content-component></child-component>'
})
export class ParentComponent implements AfterViewInit {
}

@Component({
  selector: 'child-component',
  template: '<button (click)="this.increaseContentCounter()"></button><ng-content></ng-content>'
})
export class ChildComponent {
  @ContentChild(ContentComponent) contentComponent: ContentComponent;

  increaseContentCounter() {
    this.contentComponent.increase();
  }
}

@Component({
  selector: 'content-component',
  template: '<div>{{ counter }}</div>'
})
export class ContentComponent {
  protected counter = 0;

  increase() {
    this.counter++;
  }
}