If you want to show alert dialog you should use ‘showDialog’, then in its builder use your Alert Dialog.

Like this

class ChooseCity extends StatefulWidget {
  @override
  State<StatefulWidget> createState() {
    return ChooseCityState();
  }
}

class ChooseCityState extends State<ChooseCity> {
  var cities = ["Ankara", "İzmir", "İstanbul", "Edirne", "Antalya"];
  String chosenCity = "Edirne";

  @override
  Widget build(BuildContext context) {
    return Column(
      mainAxisAlignment: MainAxisAlignment.center,
      children: <Widget>[
        DropdownButton<String>(
          items: cities.map((String city) {
            return DropdownMenuItem<String>(
              child: Text(city),
              value: city,
            );
          }).toList(),
          value: chosenCity,
          onChanged: (String choosen) {
            chosenCity = choosen;
            showAlertDialog();
            setState(() {});
          },
        ),
        SizedBox(
          height: 60,
        ),
        Text(
          "    You choosed  :" + chosenCity,
          textAlign: TextAlign.center,
        ),
      ],
    );
  }

  showAlertDialog() {
    showDialog(
      context: context,
      builder: (context) {
        return AlertDialog(
          title: Text("ATTENTION"),
          content: Text("You chose the best place!!"),
          actions: [
            FlatButton(
              child: Text("I see,I agree"),
              onPressed: () => {},
            )
          ],
        );
      },
    );
  }
}

I started to work with DropdownButton widget and I created List of cities.
I want to see AlertDialog when i choose my favorite city on the list but it can’t work.
Here is codes :

import 'package:flutter/material.dart';
import './func.dart';
class ChooseCity extends StatefulWidget {
  @override
  State<StatefulWidget> createState() {
    return ChooseCityState();
  }
}

class ChooseCityState extends State<ChooseCity> {
  var cities = ["Ankara", "İzmir", "İstanbul", "Edirne", "Antalya"];
  String choosenCity = "Edirne";

  @override
  Widget build(BuildContext context) {
    return Column(

      mainAxisAlignment: MainAxisAlignment.center,
      children: <Widget>[



        DropdownButton<String>(
          items: cities.map((String city) {
            return DropdownMenuItem<String>(
              child: Text(city),
              value: city,
            ); 
          }).toList(),
          value: choosenCity,
          onChanged: (String choosen) {

            setState(() {
          choosenCity = choosen;});
          choosen=="Antalya" ? 
            AlertDialog(
              title:Text("ATTENTION"),
              content: Text("You chose the best place!!"),
              actions: [
                 FlatButton(child: Text("I see,I agree"),onPressed: ()=>{},)
                   ],


                )

              : Text("go on");

          },


        ),
        SizedBox(
          height: 60,
        ),
        Text(
          "    You choosed  :" + choosenCity,
          textAlign: TextAlign.center,


        ),
      ],
    );

  }
}

View of my page :

It doesn’t look like as i want. I want to see AlertDialog when i chose “Antalya” . Where is the error? Where should i put the bool function? I tried to put this bool function out of setState function but it didn’t be as i want.