A simple provider example

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

void main() {
  runApp(ChangeNotifierProvider(
    create: (_) => Foo(),
    child: MyApp(),
  ));
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    //Find the Provider above and get value from Foo object
    var value = Provider.of<Foo>(context).value;
    print(value);
    return MaterialApp(
      home: Scaffold(
          appBar: AppBar(
            title: Text('Home'),
          ),
          body: Column(
            children: [
              Center(child: Text(value)),
              Center(
                  child: RaisedButton(
                  onPressed: () {
                    //Call the method _goodBye from Foo using provider
                    Provider.of<Foo>(context, listen: false)._goodBye();
                    print(value);
                  },
                  child: Text('Click', style: TextStyle(fontSize: 20)),
              )
              ),
              Center(
                  child: RaisedButton(
                  onPressed: () {
                    //Call the method _goodBye from Foo using provider
                    Provider.of<Foo>(context, listen: false)._reset();
                    print(value);
                  },
                  child: Text('Reset', style: TextStyle(fontSize: 20)),
              )
              ),
            ],
          )),
    );
  }
}

class Foo extends ChangeNotifier {
//Initial private state of the value
  var _value = "Hello";

//Getter
  String get value {
    return _value;
  }

//Method to Update Value
  _goodBye() {
    _value = "Goodbye";
    notifyListeners();
  }

//Method to Reset
  _reset() {
    _value = "Rest";
    notifyListeners();
  }
}

Using the Consumer class.

You can rework the code above to use a Consumer. 

According to the documentation:

"The Consumer widget doesn't do any fancy work. It just calls Provider.of in a new widget, and delegates its build implementation to builder."

If you compare the code above to this example, you will see I was able to remove var value = Provider.of<Foo>(context).value; and let Consumer handle this work.

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

void main() {
  runApp(ChangeNotifierProvider(
    create: (_) => Foo(),
    child: MyApp(),
  ));
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    //This is no longer needed with Consumer
    //var value = Provider.of<Foo>(context).value;
    //print(value);
    return MaterialApp(
      home: Scaffold(
          appBar: AppBar(
            title: Text('Home'),
          ),
          body: Column(
            children: [
              Consumer<Foo>(
                builder: (_, foo, child) => Text(foo.value)
                ),
              Center(
                  child: RaisedButton(
                  onPressed: () {
                    //Call the method _goodBye from Foo using provider
                    Provider.of<Foo>(context, listen: false)._goodBye();
                  },
                  child: Text('Click', style: TextStyle(fontSize: 20)),
              )
              ),
              Center(
                  child: RaisedButton(
                  onPressed: () {
                    //Call the method _goodBye from Foo using provider
                    Provider.of<Foo>(context, listen: false)._reset();
                    //print(value);
                  },
                  child: Text('Reset', style: TextStyle(fontSize: 20)),
              )
              ),
            ],
          )),
    );
  }
}