State management

Intro

By extending the AppStateModel interface it is possible to create a class to drive the AppStateProvider in order to provide the data to the widgets.

From the "theme changer" example (N.B. in this "light" version of the frideos package there is no SharedPreferences helper class):

1. Create a model for the app state:

class AppState extends AppStateModel {
  List<MyTheme> themes;
  StreamedValue<MyTheme> currentTheme;

  AppState() {
    print('-------APP STATE INIT--------');

    themes = List<MyTheme>();

    themes.addAll([
      MyTheme(
        name: 'Default',
        brightness: Brightness.light,
        backgroundColor: Colors.blue[50],
        scaffoldBackgroundColor: Colors.blue[50],
        primaryColor: Colors.blue,
        primaryColorBrightness: Brightness.dark,
        accentColor: Colors.blue[300],
      ),
      MyTheme(
        name: 'Teal',
        brightness: Brightness.light,
        backgroundColor: Colors.teal[50],
        scaffoldBackgroundColor: Colors.teal[50],
        primaryColor: Colors.teal[600],
        primaryColorBrightness: Brightness.dark,
        accentColor: Colors.teal[300],
      ),
      MyTheme(
        name: 'Orange',
        brightness: Brightness.light,
        backgroundColor: Colors.orange[50],
        scaffoldBackgroundColor: Colors.orange[50],
        primaryColor: Colors.orange[600],
        primaryColorBrightness: Brightness.dark,
        accentColor: Colors.orange[300],
      ),
    ]);

    currentTheme = StreamedValue();
  }

  void setTheme(MyTheme theme) {
    currentTheme.value = theme;
    Prefs.savePref<String>('theme', theme.name);
  }

  @override
  void init() async {
    String lastTheme = await Prefs.getPref('theme');
    if (lastTheme != null) {
      currentTheme.value = themes.firstWhere((theme) => theme.name == lastTheme,
          orElse: () => themes[0]);
    } else {
      currentTheme.value = themes[1];
    }
  }

  @override
  dispose() {
    print('---------APP STATE DISPOSE-----------');
    currentTheme.dispose();
  }
}

2. Wrap the MaterialApp in the AppStateProvider:

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

class App extends StatefulWidget {
  @override
  _AppState createState() => _AppState();
}

class _AppState extends State<App> {
  AppState appState;

  @override
  void initState() {
    super.initState();
    appState = AppState();
  }

  @override
  Widget build(BuildContext context) {
    return AppStateProvider<AppState>(
      appState: appState,
      child: MaterialPage(),
    );
  }
}

3. Consume the data:

class MaterialPage extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    var theme = AppStateProvider.of<AppState>(context).currentTheme;

    return ValueBuilder<MyTheme>(
        streamed: theme,
        builder: (context, snapshot) {
          return MaterialApp(
              title: "Theme and drawer starter app",
              theme: _buildThemeData(snapshot.data),
              home: HomePage());
        });
  }

  _buildThemeData(MyTheme appTheme) {
    return ThemeData(
      brightness: appTheme.brightness,
      backgroundColor: appTheme.backgroundColor,
      scaffoldBackgroundColor: appTheme.scaffoldBackgroundColor,
      primaryColor: appTheme.primaryColor,
      primaryColorBrightness: appTheme.primaryColorBrightness,
      accentColor: appTheme.accentColor,
    );
  }
}

4. Change the data (using a stream):

class SettingsPage extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    var appState = AppStateProvider.of<AppState>(context);

    _buildThemesList() {
      return appState.themes.map((MyTheme appTheme) {
        return DropdownMenuItem<MyTheme>(
          value: appTheme,
          child: Text(appTheme.name, style: TextStyle(fontSize: 14.0)),
        );
      }).toList();
    }

    return Scaffold(
      appBar: AppBar(
        centerTitle: true,
        title: Text(
          "Settings",
        ),
      ),
      body: Container(
        padding: EdgeInsets.all(8.0),
        child: Column(
          children: <Widget>[
            Column(
              mainAxisAlignment: MainAxisAlignment.start,
              children: <Widget>[
                Padding(
                  padding: const EdgeInsets.all(8.0),
                  child: Text(
                    'Choose a theme:',
                    style: TextStyle(fontWeight: FontWeight.w500),
                  ),
                ),
                ValueBuilder<MyTheme>(
                    streamed: appState.currentTheme,
                    builder: (context, snapshot) {
                      return DropdownButton<MyTheme>(
                        hint: Text("Status"),
                        value: snapshot.data,
                        items: _buildThemesList(),
                        onChanged: appState.setTheme,
                      );
                    }),
              ],
            ),
          ],
        ),
      ),
    );
  }
}