Flutter Update String in Statefull Widget

Why the String Value didn't update even though I'm pressed the button, I don't know which part is wrong. can you guys help me?

The way it works is that let's say I have an initial value of 100. If I input a value in the text field, for example, 5, then it will become 105, and so on.

import 'dart:ffi';

import 'package:flutter/material.dart';

class InitialValue extends StatefulWidget{
  InitialValue({Key? key}):super(key: key);

  @override
  _InitialValue createState()=> _InitialValue();
}

class _InitialValue extends State<InitialValue>{
  String Value = "100";
  final TextEditingController ivalue = TextEditingController();
  int value = 0;

  void hitungtambah(){
    String Ivalue = ivalue.text;
    int Fvalue = int.tryParse(Ivalue)?? 0;
    int value = int.tryParse(Value)??0;


    setState(() {
        value+=Fvalue;
    });
    
  }

  @override
  Widget build(BuildContext context){
    return Scaffold(
      appBar: AppBar(
        title: Text("Initial Value"),
      ),
      body: Column(
        children: <Widget>[
          Container(
            alignment: Alignment.center,
            child: Text("$value",
            style: TextStyle(fontSize: 20),
            
            ),
          ),
          Container(
            padding: EdgeInsets.only(left: 20,right: 20,top: 20),
            child: 
            TextField(
              decoration: InputDecoration(
              hintText: 'Input Value',),
              controller: ivalue,),
          ),
          
          TextButton(onPressed: (){
            hitungtambah();
          }, child: Text("Plus"))
        ],
      ) ,
    );
  }
}

To set initial text on TextEditingController you can use

TextEditingController.fromValue(TextEditingValue(text: "100"));

Now use the controller to get text when button is pressed,

class InitialValue extends StatefulWidget {
  InitialValue({Key? key}) : super(key: key);

  @override
  _InitialValue createState() => _InitialValue();
}

class _InitialValue extends State<InitialValue> {
  final TextEditingController controller =
      TextEditingController.fromValue(TextEditingValue(text: "100"));
  int value = 0; ///you can change this value to 100

  void hitungtambah() {
    int inputValue = int.tryParse(controller.text) ?? 0;

    setState(() {
      value += inputValue;
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text("Initial Value"),
      ),
      body: Column(
        children: <Widget>[
          Container(
            alignment: Alignment.center,
            child: Text(
              "$value",
              style: TextStyle(fontSize: 20),
            ),
          ),
          Container(
            padding: EdgeInsets.only(left: 20, right: 20, top: 20),
            child: TextField(
              decoration: InputDecoration(
                hintText: 'Input Value',
              ),
              controller: controller,
            ),
          ),
          TextButton(
              onPressed: () {
                hitungtambah();
              },
              child: Text("Plus"))
        ],
      ),
    );
  }
}

You can check /effective-dart/style and get-started/codelab.