如何在 Flutter 中开始加载主题

我希望用户可以在我的应用程序中更改并保存主题颜色。但是，我不知道如何在应用程序开始运行时加载保存的主题颜色。比如我想在下面的评论处直接加载保存的主题颜色。我尝试了共享首选项。但是，SharedPreference 实例需要运行await。这里好像不能用。有什么办法可以直接在这里加载保存的主题而不是使用setState之类的东西吗？

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  
  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
      title: 'Flutter Demo',
      theme: // how to load saved theme here?
      ),
      home: new MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

这个答案更进一步。它展示了如何加载和保存主题首选项，如何构建ThemeData，以及如何从应用程序的页面更改主题。

• 使用以下命令保存用户首选项（选择哪个主题）shared_preferences plugin.
• 使用整个 Flutter 框架中使用的“控制器模式”来为您的应用程序提供当前选择的主题（及其更改）。
• Use an InheritedWidget在应用程序的任何部分使用控制器。

控制器如下所示：

import 'package:flutter/material.dart';
import 'package:shared_preferences/shared_preferences.dart';

/// provides the currently selected theme, saves changed theme preferences to disk
class ThemeController extends ChangeNotifier {
  static const themePrefKey = 'theme';

  ThemeController(this._prefs) {
    // load theme from preferences on initialization
    _currentTheme = _prefs.getString(themePrefKey) ?? 'light';
  }

  final SharedPreferences _prefs;
  String _currentTheme;

  /// get the current theme
  String get currentTheme => _currentTheme;

  void setTheme(String theme) {
    _currentTheme = theme;

    // notify the app that the theme was changed
    notifyListeners();

    // store updated theme on disk
    _prefs.setString(themePrefKey, theme);
  }

  /// get the controller from any page of your app
  static ThemeController of(BuildContext context) {
    final provider = context.inheritFromWidgetOfExactType(ThemeControllerProvider) as ThemeControllerProvider;
    return provider.controller;
  }
}

/// provides the theme controller to any page of your app
class ThemeControllerProvider extends InheritedWidget {
  const ThemeControllerProvider({Key key, this.controller, Widget child}) : super(key: key, child: child);

  final ThemeController controller;

  @override
  bool updateShouldNotify(ThemeControllerProvider old) => controller != old.controller;
}

以下是您将如何使用控制器和InheritedWidget在你的应用程序中：

void main() async {
  // load the shared preferences from disk before the app is started
  final prefs = await SharedPreferences.getInstance();

  // create new theme controller, which will get the currently selected from shared preferences
  final themeController = ThemeController(prefs);

  runApp(MyApp(themeController: themeController));
}

class MyApp extends StatelessWidget {
  final ThemeController themeController;

  const MyApp({Key key, this.themeController}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    // use AnimatedBuilder to listen to theme changes (listen to ChangeNotifier)
    // the app will be rebuilt when the theme changes
    return AnimatedBuilder(
      animation: themeController,
      builder: (context, _) {
        // wrap app in inherited widget to provide the ThemeController to all pages
        return ThemeControllerProvider(
          controller: themeController,
          child: MaterialApp(
            title: 'Flutter Demo',
            theme: _buildCurrentTheme(),
            home: MyHomePage(),
          ),
        );
      },
    );
  }

  // build the flutter theme from the saved theme string
  ThemeData _buildCurrentTheme() {
    switch (themeController.currentTheme) {
      case "dark":
        return ThemeData(
          brightness: Brightness.dark,
          primarySwatch: Colors.orange,
        );
      case "light":
      default:
        return ThemeData(
          brightness: Brightness.light,
          primarySwatch: Colors.blue,
        );
    }
  }
}

class MyHomePage extends StatefulWidget {
  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: new AppBar(),
      body: Center(
        child: Column(
          children: <Widget>[
            RaisedButton(
              onPressed: () {
                // thanks to the inherited widget, we can access the theme controller from any page
                ThemeController.of(context).setTheme('light');
              },
              child: Text('Light Theme'),
            ),
            RaisedButton(
              onPressed: () {
                ThemeController.of(context).setTheme('dark');
              },
              child: Text('Dark Theme'),
            )
          ],
        ),
      ),
    );
  }
}