How to get focused form control name in angular reactive forms with the help of ReactiveForms

One way i tried is adding OnFocus event on the form control.

Is there any possibility to get it with the help of reactiveFoms itself.

Based on your comment I don't see the relevance of determining focus on a control to validate such that 5 input fields don't exceed the sum of 100. So I propose we could use a common validator that does the work for us (checking all inputs in the group).

@Component({
  selector: 'my-app',
  templateUrl: './app.component.html',
  styleUrls: ['./app.component.css'],
})
export class AppComponent {
  name = 'Angular 5';

  myForm: FormGroup;

  constructor(private fb: FormBuilder) {
    this.myForm = fb.group(
      {
        field1: [null],
        field2: [null],
        field3: [null],
        field4: [null],
        field5: [null],
      },
      { validator: this.myValidator }
    );
  }

  myValidator(group: FormGroup) {
    let sum = 0;
    for (let a in group.controls) {
      sum += group.get([a]).value;
    }
    console.log(sum > 100);
    return sum > 100 ? { notValid: true } : null;
  }
}

Working example: https://stackblitz.com/edit/angular-qxxmke?file=app%2Fapp.component.ts