Having trouble updating passed variable in flutter

flutterprob.dart

class NavigationState extends State<MyNavigationHolder> {
  var goToPass = "/page1";
  
  
  assignPass(page){
    print("inside assignPass");
    print("value of page: " + page);
    setState(() {goToPass = page;});
  }
 
  //button gets pressed and assignPass updates goToPass with new page val
  //then I call HeaderPage
  
   new Flexible(
                    child: new HeaderPage(goTo: goToPass)
                  ),
  
  //Inside header page: 
  
  
  
import 'package:flutter/material.dart';
import 'main.dart';


class HeaderPage extends StatefulWidget {
  const HeaderPage({
    Key key,
    this.goTo
  }) : super(key: key);

  final String goTo;

  @override
  HeaderPageState createState() => new HeaderPageState(goTo);
}


class HeaderPageState extends State<HeaderPage> {
  HeaderPageState(this.goTo);
  final String goTo;
  String goToPass;

  
  sunPressed(){
    
  }
  
  moonPressed(){
    
  }

  @override
  void initState() {
    // TODO: implement initState
    super.initState();
    setState(() {goToPass = goTo;});
  }

  @override
  Widget build(BuildContext context) {
    print("value of goTo in Widget build: " + goToPass);

    Widget ButtonGrad(){
      print("value of goTo: " + goToPass);
      if (goToPass=="/page3"){
        //magic happens
        //i only want to fire buttongrad if goToPass="/page3")
        
        
//PROBLEM: goToPass is always == "/page1" and I don't know why;
        
//Output from console: 
        
Performing full restart...
Restarted app in 2,570ms.
I/flutter ( 5550): value of goTo in Widget build: /page1
I/flutter ( 5550): value of goTo: /page1
I/flutter ( 5550): inside assignPass
I/flutter ( 5550): value of page: /page2
I/flutter ( 5550): value of goTo in Widget build: /page1
I/flutter ( 5550): value of goTo: /page1
I/flutter ( 5550): inside assignPass
I/flutter ( 5550): value of page: /page4
I/flutter ( 5550): value of goTo in Widget build: /page1
I/flutter ( 5550): value of goTo: /page1
I/flutter ( 5550): inside assignPass
I/flutter ( 5550): value of page: /page1
I/flutter ( 5550): value of goTo in Widget build: /page1
I/flutter ( 5550): value of goTo: /page1
I/flutter ( 5550): inside assignPass
I/flutter ( 5550): value of page: /page3
I/flutter ( 5550): value of goTo in Widget build: /page1
I/flutter ( 5550): value of goTo: /page1
I/flutter ( 5550): inside getActivityList
  
  
 //What am I doing wrong?