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/// A provide widget can rebuild on changes to any class that implements
/// the listenable interface.
///
/// Here, we mixin ChangeNotifier so we don't need to manage listeners
/// ourselves.
///
/// Extending ValueNotifier<int> would be another simple way to do this.
class Counter with ChangeNotifier {
  int _value;

  int get value => _value;

  Counter(this._value);

  void increment() {
    _value++;
    notifyListeners();
  }
}

/// CounterApp which obtains a counter from the widget tree and uses it.
class CounterApp extends StatelessWidget {
  // The widgets here get the value of Counter in three different
  // ways.
  //
  // - Provide<Counter> creates a widget that rebuilds on change
  // - Provide.value<Counter> obtains the value directly
  // - Provide.stream<Counter> returns a stream
  @override
  Widget build(BuildContext context) {
    // Gets the Counter from the nearest ProviderNode that contains a Counter.
    // This does not cause this widget to rebuild when the counter changes.
    final currentCounter = Provide.value<Counter>(context);

    return Column(children: [
      // Simplest way to retrieve the provided value.
      //
      // Each time the counter changes, this will get rebuilt. This widget
      // requires the value to be a Listenable or a Stream. Otherwise
      Provide<Counter>(
        builder: (context, child, counter) => Text('${counter.value}'),
      ),

      // This widget gets the counter as a stream of changes.
      // The stream is filtered so that this only rebuilds on even numbers.
      StreamBuilder<Counter>(
          initialData: currentCounter,
          stream: Provide.stream<Counter>(context)
              .where((counter) => counter.value % 2 == 0),
          builder: (context, snapshot) =>
              Text('Last even value: ${snapshot.data.value}')),

      // This button just needs to call a method on Counter. No need to rebuild
      // it as the value of Counter changes. Therefore, we can use the value of
      // `Provide.value<Counter>` from above.
      FlatButton(child: Text('increment'), onPressed: currentCounter.increment),

      Text('Another widget that does not depend on the Counter'),
    ]);
  }
}

void main() {
    // The class that contains all the providers. This shouldn't change after
    // being used.
    //
    // In this case, the Counter gets instantiated the first time someone uses
    // it, and lives as a singleton after that.
    final providers = Providers()
      ..provide(Provider.function((context) => Counter(0)));

    runApp(ProviderNode(
      providers: providers,
      child: CounterApp(),
    ));
}