Angular form: Equivalent of constructor ngForm injection for Reactive Forms?

I am trying to build a component to show validation error messages like this:

<form>
  <input ngModel name="firstname" required>
  <validation-error for="firstname"></validation-error> 
</form>

The <validation-error> component is amazingly simple for Template Driven forms as we can inject the NgFrom in the constructor and use ngFrom.form.get(myControlName) to find a control an its state:

@Component({
  selector: 'validation-error',
  template: `{{errorText}}`,
  styles: [`:host { display:block; color:red; }`]
})
export class ValidationErrorComponent  {
  @Input() for: string;  // name of input element
  constructor(
    @Optional() private ngForm: NgForm  // This works for Template forms only.
  ) { }

  get errorText() : string {
    const formControl = <FormControl>this.ngForm.form.get(this.for);
    return (formControl.errors) ? JSON.stringify(formControl.errors) : "[no error]";
  }
}

But the NgForm only seems to be injected for Template Forms. Is there an equivalent provider for Reactive Forms that gives access to the enclosing FormGroup?

Instead of injecting ngForm,use the ControlContainer to access FormControls.

ControlContainer

A base class for directives that contain multiple registered instances of NgControl.

@Component({
  selector: 'validation-error',
  template: `{{errorText}}`,
  styles: [`:host { display:block; color:red; }`]
})
export class ValidationErrorComponent  {
  @Input() for: string;  // name of input element
  constructor(
     @Optional() private control: ControlContainer
  ) { }

  get errorText() : string {
    const formControl = <FormControl>(this.control as any).form.get(this.for);
    return (formControl.errors) ? JSON.stringify(formControl.errors) : "[no error]";
  }
}

Example