built_viewmodel.dart

The built_viewmodel.dart package provides a way to create ViewModel classes. It is a little bit inspired by the Android ViewModel but just a little bit ;-)

Usually, you call setState() whenever the state of the Widget should change. But in my opinion, it is not a good idea to call setState() when just a small part of the whole state has changed (read more: How fast is Flutter? I built a stopwatch app to find out., Avoiding Empty State Callbacks).

This package creates lots of Streams (as much as you need) to feed StreamBuilders.

Let's start

Create an abstract class, e.g. MyHomePageViewModel:

library main;

import 'dart:async';

import 'package:built_viewmodel/built_viewmodel.dart';

part 'main.g.dart';

abstract class MyHomePageViewModel implements ViewModel<MyHomePageViewModelController> {


  factory MyHomePageViewModel() = _$MyHomePageViewModel;
  MyHomePageViewModel._();
}

And add a reference of it to the Widget:

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

  final MyHomePageViewModel model = new MyHomePageViewModel();
  final String title;

  @override
  _MyHomePageState createState() {
    model.setCounter(0);
    return new _MyHomePageState();
  }
}

For now, the ViewModel makes nothing. You have to add your needed stream(s):

abstract class MyHomePageViewModel implements ViewModel<MyHomePageViewModelController> {

  Stream<int> get counter;   // new

  factory MyHomePageViewModel() = _$MyHomePageViewModel;
  MyHomePageViewModel._();

You also need to add a code generator. Create a tool/build.dart file with the following content:

import 'dart:async';

import 'package:build_config/build_config.dart';
import 'package:build_runner/build_runner.dart';
import 'package:built_viewmodel_generator/built_viewmodel_generator.dart';
import 'package:source_gen/source_gen.dart';

final builders = [
  applyToRoot(
    new PartBuilder([
      new BuiltViewModelGenerator(),
    ]),
    generateFor: const InputSet(
      include: const [
        'lib/*.dart',
      ],
    ),
  ),
];

Future main(List<String> args) async {
  await build(
    builders,
    deleteFilesByDefault: true,
    verbose: false,
  );
}

Execute the tool/build.dart file and it will generate a main.g.dart file which has all the logic for your stream(s).

Use the stream

To use the stream(s) you need a StreamBuilder where you have to set the stream to the one from the ViewModel:

new StreamBuilder(
  stream: widget.model.counter,
  builder: (BuildContext context, AsyncSnapshot<int> snapshot) {
    if (snapshot.hasError) return new Text('Error: ${snapshot.error}');
    switch (snapshot.connectionState) {
      case ConnectionState.none:
        return new Text('not connected');
      case ConnectionState.waiting:
        return new Text('awaiting interaction...');
      case ConnectionState.active:
        return new Text('counter: ${snapshot.data}');
      case ConnectionState.done:
        return new Text('counter: ${snapshot.data} (closed)');
      default:
        throw "Unknown: ${snapshot.connectionState}";
    }
  },
),

Whenever you send a new value to the stream the content of the StreamBuilder will change but only this small part and not the whole state.

To do that add a void setCounter(int value) method to the ViewModel ...

abstract class MyHomePageViewModel implements ViewModel<MyHomePageViewModelController> {
  Stream<int> get counter;

  void setCounter(int value) => controller.counter.add(value);   // new

  factory MyHomePageViewModel() = _$MyHomePageViewModel;
  MyHomePageViewModel._();
}

... and just call it.

Don't forget ...

... to call dispose()

Add this to your State class

@override
void dispose() {
  widget.model.dispose();
  super.dispose();
}

Advanced feature

Sometimes you need to know the state of the stream, for example, to start a download.

You can create methods and annotate them with one of this annotations: @OnListenHandler, @OnPauseHandler, @OnResumeHandler, @OnCancelHandler. These annotations have a name parameter which must match with the name of the stream.

For example, if you have a counter stream Stream<int> get counter, and you want to know when something is listening for it the annotation should look like this: @OnListenHandler('counter').

abstract class MyHomePageViewModel implements ViewModel<MyHomePageViewModelController> {
  Stream<int> get counter;

  @OnListenHandler('counter')                           // new
  onListen() => print('start listening for updates');   // new

  void setCounter(int value) => controller.counter.add(value);

  factory MyHomePageViewModel() = _$MyHomePageViewModel;

  MyHomePageViewModel._();
}

You can find the full example here.

Libraries

built_viewmodel_flutter_redux