gistfile1.txt

import 'package:flutter/material.dart';

// This class is a simulacrum of an internally complex opaque data object,
// whose values can only be retrieved via path lookup.
class IntegerTree {
  int _a = 0;
  int _b = 1;
  int _c = 2;

  int valueOf(String path) => path == 'a' ? _a : (path == 'b' ? _b : _c);
}

// Dependents identify the path they depend on. The IntegerModel records
// both the path and the path's current value.  See IntegerModel.getDependencyFor().
class PathDependency {
  PathDependency(this.path, this.value);
  final String path;
  final int value;
}

// This model only rebuilds dependents when the value of the paths they
// depend on changes.
class IntegerModel extends InheritedModel<PathDependency> {
  const IntegerModel({
    Key key,
    this.integerTree,
    Widget child
  }) : super(key: key, child: child);

  final IntegerTree integerTree;

  @override
  PathDependency getDependencyFor(dynamic aspect) {
    final String path = aspect;
    return new PathDependency(path, integerTree.valueOf(path));
  }

  @override
  bool updateShouldNotifyDependent(IntegerModel old, Set<PathDependency> dependencies ) {
    for (PathDependency dependency in dependencies) {
      if (dependency.value != integerTree.valueOf(dependency.path))
        return true;
    }
     return false;
   }

  static IntegerTree of(BuildContext context, { String path }) {
    final IntegerModel integerModel = context.inheritFromWidgetOfExactType(IntegerModel, aspect: path);
    return integerModel.integerTree;
  }
}

class ShowIntegerPath extends StatefulWidget {
  ShowIntegerPath({ Key key, this.path }) : super(key: key);

  final String path;

  _ShowIntegerPathState createState() => new _ShowIntegerPathState();
}

class _ShowIntegerPathState extends State<ShowIntegerPath> {
  int _buildCount = 0;

  @override
  Widget build(BuildContext context) {
    final IntegerTree integerTree = IntegerModel.of(context, path: widget.path);
    final int value = integerTree.valueOf(widget.path);
    return new Padding(
      padding: const EdgeInsets.symmetric(vertical: 8.0),
      child: new Text('${widget.path} : $value  [${_buildCount++}]', style: Theme.of(context).textTheme.title),
    );
  }
}

class IntegerPage extends StatefulWidget {
  @override
  _IntegerPageState createState() => new _IntegerPageState();
}

class _IntegerPageState extends State<IntegerPage> {
  final IntegerTree _integerTree = new IntegerTree();

  @override
  Widget build(BuildContext context) {
    final Widget showA = new ShowIntegerPath(path: 'a');
    final Widget showB = new ShowIntegerPath(path: 'b');
    final Widget showC = new ShowIntegerPath(path: 'c');

    return new Scaffold(
      appBar: new AppBar(title: const Text('InheritedModel')),
      body: new StatefulBuilder(
        builder: (BuildContext context, StateSetter setState) {
          return new IntegerModel(
            integerTree: _integerTree,
            child: new Center(
              child: new Column(
                mainAxisSize: MainAxisSize.min,
                children: <Widget>[
                  showA,
                  showB,
                  showC,
                  const SizedBox(height: 24.0),
                  new RaisedButton(
                    child: const Text('Increment a'),
                    onPressed: () {
                      setState(() {
                        _integerTree._a += 1;
                      });
                    },
                  ),
                  new RaisedButton(
                    child: const Text('Increment b'),
                    onPressed: () {
                      setState(() {
                        _integerTree._b += 1;
                      });
                    },
                  ),
                  new RaisedButton(
                    child: const Text('Increment c'),
                    onPressed: () {
                      setState(() {
                        _integerTree._c += 1;
                      });
                    },
                  ),
                ],
              ),
            ),
          );
        },
      ),
    );
  }
}

void main() {
  runApp(
    new MaterialApp(
      home: new IntegerPage(),
    ),
  );
}