setter-ex.comp.ts

@Component({
  selector: 'app-root',
  template: `
    <button (click)="show = !show">Toggle</button> 
    <input *ngIf="show" #input>
  `
})
export class AppComponent {
   @ViewChild('input', { static: false }) 
   set input(element: ElementRef<HTMLInputElement>) {
     if(element) {
       element.nativeElement.focus()
     }
  }
}