Flutter：嘗試根據 DropDownButton 輸入將變量傳遞到下一個屏幕

Question

我正在嘗試根據用戶下拉輸入降低產品（智能手機）價格並將價格傳遞到下一個屏幕。 我的代碼沒有錯誤並且運行完美，但問題是，它僅基於一個 switch 語句（為下拉值全局選擇的初始語句）降低了價格，因為setState方法僅適用於更改屏幕上顯示的下拉值，而不是在開關中包含 function 的語句。 有人可以幫我解決這個問題。 我嘗試了很多東西，但沒有任何幫助。 下面是我的代碼（它很長，所以我在這里跳過了小部件構建部件和設計部件）。

值已被定義為這樣

   double price = 18000;  
   String bdropDownValue = 'Less than 6 hours'; //initialvalue
    final List<String> bItems = [
    'Less than 6 hours',
    'Less than 12 hours',
    'Less than 24 hours'],

下拉按鈕表單字段

DropdownButtonFormField(value: bdropDownValue,
                        items: bItems.map((bItems) {
                          return DropdownMenuItem(
                            value: bItems,
                            child: Text(bItems),
                          );
                        }).toList(),
                        onChanged: (String? value) {
                          setState(() {
                            bdropDownValue = value!;
                          });
                        },
                        ),
                  /// button used to calculate next
                   TextButton(
                          onPressed: () {
                            calculator(bdropDownValue);
                          },
                          child: Text(
                            "confirm",
                            ),

我正在使用這種方法來計算

void calculator(String bdropDownValue) {
    switch (bdropDownValue) {
      case 'Less than 6 hours':
        price = price - 3000;
        break;
      case 'Less than 12 hours':
        price = price - 1500;
        break;
      case 'Less than 24 hours':
        price = price - 1000;
        break;
    }

並在計算器方法中使用這樣的Navigator

Navigator.push(
        context,
        MaterialPageRoute(
            builder: (context) => priceScreen(
                  price: price,
                )));
  }
}

Answer 1

請檢查我在下面添加的示例，它工作得很好。

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
        debugShowCheckedModeBanner: false,
        title: 'Flutter Demo',
        theme: ThemeData(
          primarySwatch: Colors.blue,
        ),
        home: MainHomePage());
  }
}

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

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

class _MainHomePageState extends State<MainHomePage> {
  double price = 18000;
  String bdropDownValue = 'Less than 6 hours'; //initialvalue
  final List<String> bItems = [
    'Less than 6 hours',
    'Less than 12 hours',
    'Less than 24 hours'
  ];

  void calculator(String bdropDownValue) {
    var actualPrice = price;
    switch (bdropDownValue) {
      case 'Less than 6 hours':
        actualPrice = actualPrice - 3000;
        break;
      case 'Less than 12 hours':
        actualPrice = actualPrice - 1500;
        break;
      case 'Less than 24 hours':
        actualPrice = actualPrice - 1000;
        break;
    }

    Navigator.push(
        context,
        MaterialPageRoute(
            builder: (context) => PriceScreen(
                  price: actualPrice,
                )));
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Center(
        child: Column(
          children: [
            DropdownButtonFormField(
              value: bdropDownValue,
              items: bItems.map((bItems) {
                return DropdownMenuItem(
                  value: bItems,
                  child: Text(bItems),
                );
              }).toList(),
              onChanged: (String value) {
                setState(() {
                  bdropDownValue = value;
                });
              },
            ),
            TextButton(
              onPressed: () {
                calculator(bdropDownValue);
              },
              child: Text(
                "confirm",
              ),
            ),
          ],
        ),
      ),
    );
  }
}

class PriceScreen extends StatelessWidget {
  final double price;
  const PriceScreen({Key key, this.price}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Container(
        child: Text(price.toString()),
      ),
    );
  }
}