app_model_v0.dart

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

// Enables sharing keyword value data with an entire app.
//
// AppModel.set(context, keyword, value) adds an entry to the shared data table.
//
// AppModel.get(context, keyword) returns the value for keyword or null.
//
// A widget whose build method uses AppModel.get(context, keyword)
// creates a dependency on the AppModel: when the value of keyword
// changes with AppModel.set(), the widget will be rebuilt.
//
// This Widget would be created above the WidgetApp, so that it's always available.

class AppModel extends StatefulWidget {
  AppModel({ Key? key, required this.child }) : super(key: key);

  final Widget child;

  @override
  _AppModelState createState() => _AppModelState();

  static Object? get(BuildContext context, Object key) {
    final _AppModelData model = InheritedModel.inheritFrom<_AppModelData>(context, aspect: key)!;
    return model.appModelState.get(key);
  }

  static void set(BuildContext context, Object key, Object value) {
    final _AppModelData model = context.findAncestorWidgetOfExactType<_AppModelData>()!;
    model.appModelState.set(key, value);
  }
}

class _AppModelState extends State<AppModel> {
  late Map<Object, Object> data;

  @override
  void initState() {
    super.initState();
    data = Map<Object, Object>();
  }

  @override
  Widget build(BuildContext context) {
    return _AppModelData(appModelState: this, child: widget.child);
  }

  Object? get(Object key) => data[key];

  void set(Object key, Object value) {
    if (data[key] != value) {
      setState(() {
        data = Map.from(data);
        data[key] = value;
      });
    }
  }
}

class _AppModelData extends InheritedModel<Object> {
  _AppModelData({
    Key? key,
    required this.appModelState,
    required Widget child
  }) : data = appModelState.data, super(key: key, child: child);

  final _AppModelState appModelState;
  final Map<Object, Object> data;

  @override
  bool updateShouldNotify(_AppModelData old) {
    return data != old.data;
  }

  @override
  bool updateShouldNotifyDependent(_AppModelData old, Set<Object> keys) {
    for (Object key in keys) {
      if (data[key] != old.data[key]) {
        return true;
      }
    }
    return false;
  }
}

// Displays the value of AppModel.get(context, appModelKey) and the number of times
// that this widget has been built.

class ShowAppModelValue extends StatefulWidget {
  const ShowAppModelValue(this.appModelKey);

  final Object appModelKey;

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

class _ShowAppModelValueState extends State<ShowAppModelValue> {
  int _buildCount = 0;

  @override
  Widget build(BuildContext context) {
    _buildCount += 1;
    return Padding(
      padding: EdgeInsets.only(bottom: 16),
      child: Text('${widget.appModelKey}: ${AppModel.get(context, widget.appModelKey)} [$_buildCount]'),
    );
  }
}


// Demonstrates that changes to the AppModel _only_ cause the dependent widgets
// to be rebuilt.

class Home extends StatefulWidget {
  @override
  _HomeState createState() => _HomeState();
}

class _HomeState extends State<Home> {
  int _fooCount = 0;
  int _barCount = 0;

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Center(
        child: Column(
          mainAxisSize: MainAxisSize.min,
          children: <Widget>[
            ShowAppModelValue('Home'), // Only built when the app is launched
            ShowAppModelValue('foo'),  // Built each time the foo button is pressed
            ShowAppModelValue('bar'),  // Built each time the bar button is pressed
            ElevatedButton(
              child: Text('foo = FOO'),
              onPressed: () {
                _fooCount += 1;
                AppModel.set(context, 'foo', 'FOO $_fooCount'); // note: no setState()
              },
            ),
            SizedBox(height: 16),
            ElevatedButton(
              child: Text('bar = BAR'),
              onPressed: () {
                _barCount += 1;
                AppModel.set(context, 'bar', 'BAR $_barCount');  // note: no setState()
              },
            ),
          ],
        ),
      ),
    );
  }
}

void main() {
  runApp(
    AppModel(child: MaterialApp(home: Home())),
  );
}