Flutter 如何更改無狀態小部件的 state

Question

我正在 flutter 中創建一個 web 面板，並且我有一個無狀態小部件。 我只需要更改另一個文件上的 int 代表我的 fonts 的顏色正在改變。

main.dart

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

  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primaryColor: Color(0xfff00B074),
        textTheme: const TextTheme(
          bodyText1: TextStyle(
              fontSize: 18.0,
              fontFamily: 'Barlow-Medium',
              color: Color(0xff464255)),
        ),
      ),
      home: MainScreen(),
    );
  }
}

main_screen.dart

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

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: SafeArea(
        child: Row(
          children: [
            if (Responsive.isDesktop(context))
              Expanded(
                // default flex = 1
                child: SideMenu(),
              ),
            Expanded(
              flex: 5,
              child: Container(),
            ),
          ],
        ),
      ),
    );
  }
}

還有我的側面菜單

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

  @override
  Widget build(BuildContext context) {
    return Drawer(
      child: ListView(
        children: [
          Padding(
            padding: const EdgeInsets.only(top: 30, bottom: 15),
            child: SvgPicture.asset(
              "assets/icon/logo.svg",
            ),
          ),
          DrawerListTile(
            intKey: 1,
            title: "Dashboard",
            svgSrc: "assets/icon/home_deactive.svg",
            press: () {
              sideMenuValue = 1;
              print('asd');
            },
          ),
          DrawerListTile(
            intKey: 2,
            title: "Transaction",
            svgSrc: "assets/icon/home_deactive.svg",
            press: () {
              sideMenuValue = 2;
            },
          ),
          DrawerListTile(
            intKey: 3,
            title: "Task",
            svgSrc: "assets/icon/home_deactive.svg",
            press: () {
              sideMenuValue = 3;
            },
          ),
          DrawerListTile(
            intKey: 4,
            title: "Documents",
            svgSrc: "assets/icon/home_deactive.svg",
            press: () {
              sideMenuValue = 4;
            },
          ),
          DrawerListTile(
            intKey: 5,
            title: "Store",
            svgSrc: "assets/icon/home_deactive.svg",
            press: () {
              sideMenuValue = 5;
            },
          ),
          DrawerListTile(
            intKey: 6,
            title: "Notification",
            svgSrc: "assets/icon/home_deactive.svg",
            press: () {
              sideMenuValue = 6;
            },
          ),
          DrawerListTile(
            intKey: 7,
            title: "Profile",
            svgSrc: "assets/icon/home_deactive.svg",
            press: () {},
          ),
          DrawerListTile(
            intKey: 8,
            title: "Settings",
            svgSrc: "assets/icon/home_deactive.svg",
            press: () {},
          ),
        ],
      ),
    );
  }
}

class DrawerListTile extends StatelessWidget {
  const DrawerListTile({
    Key? key,
    // For selecting those three line once press "Command+D"
    required this.intKey,
    required this.title,
    required this.svgSrc,
    required this.press,
  }) : super(key: key);

  final String title, svgSrc;
  final VoidCallback press;
  final int intKey;

  @override
  Widget build(BuildContext context) {
    return GestureDetector(
      onTap: press,
      child: Padding(
        padding: const EdgeInsets.only(left: 30, bottom: 30),
        child: Container(
            child: Row(
          children: [
            SvgPicture.asset(
              svgSrc,
              height: 18,
            ),
            SizedBox(
              width: 13,
            ),
            Text(
              title,
              style: sideMenuValue == intKey
                  ? TextStyle(
                      fontSize: 18.0,
                      fontFamily: 'Barlow-Medium',
                      color: Color(0xfff00B074))
                  : Theme.of(context).textTheme.bodyText1,
            ),
          ],
        )),
      ),
    );
  }
}

您可以在新聞上看到我正在更改 sideMenuValue 的值，該值沒有更改，因為我正在使用無狀態小部件並且不想使用有狀態小部件。

Answer 1

將需要更改模式的class更改如下

class HomePage extends StatefullWidget {
  const HomePage({
    Key? key,
  }) : super(key: key);

 @override
  State<HomePage> createState() => _HomePageState();
}

class _HomePageState extends State<_HomePage> {
 @override
  Widget build(BuildContext context) {
    return Scaffold(
      ..........
    );
}

Answer 2

請參考以下代碼

ValueNotifer 和 ValueListenableBuilder 可用於保存值並通過通知其偵聽器並減少重新構建小部件樹的次數來更新小部件。

main.dart文件

import 'package:elastic_search/SideMenu.dart';
import 'package:flutter/material.dart';

void main() => runApp(MyApp());


/* 
Since sideMenuValue is declared in main.dart file its available globally everywhere in the app
In order to update any value from other files we can make use of ValueNotifer 
and to update widget we can use ValueListenableBuilder which updates the particular widget when the value changes (ValueNotifier value)
*/
final ValueNotifier sideMenuValue = ValueNotifier(0);


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

  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primaryColor: Color(0xfff00B074),
        textTheme: const TextTheme(
          bodyText1: TextStyle(
              fontSize: 18.0,
              fontFamily: 'Barlow-Medium',
              color: Color(0xff464255)),
        ),
      ),
      home: MainScreen(),
    );
  }
}

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

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: SafeArea(
        child: Row(
          children: [
            Expanded(
              child: SideMenu(),
            ),
            // Expanded(
            //   flex: 5,
            //   child: Container(),
            // ),
          ],
        ),
      ),
    );
  }
}

SideMenu.dart 文件

import 'package:flutter/material.dart';
import 'package:elastic_search/main.dart';

class SideMenu extends StatelessWidget {
  const SideMenu({
    Key key,
  }) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return Drawer(
      child: ListView(
        children: [
          // Padding(
          //   padding: const EdgeInsets.only(top: 30, bottom: 15),
          //   child: SvgPicture.asset(
          //     "assets/icon/logo.svg",
          //   ),
          // ),
          DrawerListTile(
            intKey: 1,
            title: "Dashboard",
            svgSrc: "assets/icon/home_deactive.svg",
            press: () {
              sideMenuValue.value = 1;
              print('asd');
            },
          ),
          DrawerListTile(
            intKey: 2,
            title: "Transaction",
            svgSrc: "assets/icon/home_deactive.svg",
            press: () {
              sideMenuValue.value = 2;
            },
          ),
          DrawerListTile(
            intKey: 3,
            title: "Task",
            svgSrc: "assets/icon/home_deactive.svg",
            press: () {
              sideMenuValue.value = 3;
            },
          ),
          DrawerListTile(
            intKey: 4,
            title: "Documents",
            svgSrc: "assets/icon/home_deactive.svg",
            press: () {
              sideMenuValue.value = 4;
            },
          ),
          DrawerListTile(
            intKey: 5,
            title: "Store",
            svgSrc: "assets/icon/home_deactive.svg",
            press: () {
              sideMenuValue.value = 5;
            },
          ),
          DrawerListTile(
            intKey: 6,
            title: "Notification",
            svgSrc: "assets/icon/home_deactive.svg",
            press: () {
              sideMenuValue.value = 6;
            },
          ),
          DrawerListTile(
            intKey: 7,
            title: "Profile",
            svgSrc: "assets/icon/home_deactive.svg",
            press: () {
              sideMenuValue.value = 7;
            },
          ),
          DrawerListTile(
            intKey: 8,
            title: "Settings",
            svgSrc: "assets/icon/home_deactive.svg",
            press: () {
              sideMenuValue.value = 8;
            },
          ),
        ],
      ),
    );
  }
}

class DrawerListTile extends StatelessWidget {
  const DrawerListTile({
    Key key,
    // For selecting those three line once press "Command+D"
    @required this.intKey,
    @required this.title,
    @required this.svgSrc,
    @required this.press,
  }) : super(key: key);

  final String title, svgSrc;
  final VoidCallback press;
  final int intKey;

  @override
  Widget build(BuildContext context) {
    return GestureDetector(
      onTap: press,
      child: Padding(
        padding: const EdgeInsets.only(left: 30, bottom: 30),
        child: Container(
            child: Row(
          children: [
            // SvgPicture.asset(
            //   svgSrc,
            //   height: 18,
            // ),
            SizedBox(
              width: 13,
            ),
            /* 
              In order update widget we can use ValueListenableBuilder which updates the particular widget when the value changes (ValueNotifier value)
            */
            ValueListenableBuilder(
                valueListenable: sideMenuValue,
                builder: (context, snapshot, child) {
                  return Text(
                    title,
                    style: sideMenuValue.value == intKey
                        ? TextStyle(
                            fontSize: 18.0,
                            fontFamily: 'Barlow-Medium',
                            color: Color(0xfff00B074))
                        : Theme.of(context).textTheme.bodyText1,
                  );
                }),
          ],
        )),
      ),
    );
  }
}