mvp

Flutter package to build applications using Model View Presenter (MVP) architecture.

What's MVP?

MVP stands for Model View Presenter.

It is a well known architecture, widely used in app development. The way it works is as follows:

• We have objects that represents our data (the model).
• We have views which renders the UI components and gathers user input (the view).
• And finally we have components that receives user input and decides what to do with it, updates the model and then passes it to the view to redraw everything with the new data (the presenter).

MVP advantages

• UI operations separated from the application logic.
• Easier to test (in the case of Flutter, unit tests for the presenter, widget tests for the view).

Usage of this library

Let's say we have our loved counter sample:

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);
  final String title;

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

class _MyHomePageState extends State<MyHomePage> {
  int _counter = 0;

  void _incrementCounter() {
    setState(() {
      _counter++;
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            Text(
              'You have pushed the button this many times:',
            ),
            Text(
              '$_counter',
              style: Theme.of(context).textTheme.display1,
            ),
          ],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: _incrementCounter,
        tooltip: 'Increment',
        child: Icon(Icons.add),
      ), 
    );
  }
}

In this simple app our model is the actual counter value, let's extract the counter var into a dedicated model class:

class MainModel {
  MainModel({this.counter = 0});
  
  int counter;
} 

We need a presenter that will receive the user input (click on the increment counter button), compute the new counter value, and then apply this to the view:

class MainPresenter extends MvpPresenter<MainModel, MainScreenState> {

  @override
  void initializeViewModel() {
    viewModel = MainModel();
  }

  void incrementButtonClick() {
    viewModel.counter++;
    callback(viewModel);
  }

}

Finally, we release the view from the task of computing the new counter value, only receive user input and render the current view model:

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'MVP Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: MainScreen(title: 'MVP Demo'),
    );
  }
}

class MainScreen extends StatefulWidget {
  final presenter = MainPresenter();

  MainScreen({Key key, this.title}) : super(key: key);
  final String title;

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

class MainScreenState extends MvpScreen<MainScreen, MainModel> {

  @override
  void initializeViewModel() {
    viewModel = MainModel();
  }

  @override
  void initState() {
    super.initState();
    // Always remember to bind the presenter in initState().
    widget.presenter.bind(applyState, this);
  }

  @override
  void dispose() {
    super.dispose();
    // And remember to release presenter binding in dispose().
    widget.presenter.unbind();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            Text(
              'You have pushed the button this many times:',
            ),
            Text(
              "${viewModel.counter}",
              style: Theme.of(context).textTheme.display1,
            ),
          ],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: () {
          widget.presenter.incrementButtonClick();
        },
        tooltip: 'Increment',
        child: Icon(Icons.add),
      ),
    );
  }
}