您没有使用StateFull Widget和setState()。
当你想动态改变屏幕上的值时，调用setState()
检查以下示例：

import 'package:flutter/material.dart';

void main() {
  runApp(const MyApp());
}

int currentValue = 0;

class MyApp extends StatelessWidget {
  const MyApp({Key? key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primaryColor: Colors.indigo,
        useMaterial3: true,
      ),
      home: const MyHomePage(),
    );
  }
}

//use stateful widget when you want to change state
class MyHomePage extends StatefulWidget {
  const MyHomePage({super.key});

  @override
  State<MyHomePage> createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Column(
        crossAxisAlignment: CrossAxisAlignment.stretch,
        mainAxisAlignment: MainAxisAlignment.center,
        children: [
          //show the current value on screen
          Text(
            '$currentValue',
            textAlign: TextAlign.center,
          ),

          //button to show dialog
          ElevatedButton(
            child: Text("show Dialog "),
            onPressed: () {
              showDialog(
                  context: context,
                  builder: (BuildContext context) {
                    return Dialog();
                  });
            },
          ),
        ],
      ),
    );
  }
}

class Dialog extends StatefulWidget {
  const Dialog({super.key});

  @override
  State<Dialog> createState() => _DialogState();
}

class _DialogState extends State<Dialog> {
  @override
  Widget build(BuildContext context) {
    return AlertDialog(
      title: Text('$currentValue'),
      actions: [
        //button to increase the value of currentValue
        ElevatedButton(
            onPressed: () {
              //call setState, it will Update the state
              setState(() {
                //increment the value
                currentValue++;
              });
            },
            child: Text('Increase')),
        ElevatedButton(
            onPressed: () {
              setState(() {
                //reduce the value
                currentValue--;
              });
            },
            child: Text('Decrease')),
        ElevatedButton(
            onPressed: () {
              setState(() {
                //reduce the value
                currentValue = 0;
              });
            },
            child: Text('Reset')),
        ElevatedButton(
            onPressed: () {
              Navigator.pop(context);
            },
            child: Text('Remove Dialog')),
      ],
    );
  }
}

请将类StatelessWidget设置为StatefullWidget，并在调用onPressed()时使用setState

onPressed: () { 
     setState(() { 
             //logic
     }); 
}

希望这有帮助！