mvvm_plus 0.3.0

MVVM+ is a lightweight Flutter state management library that implements MVVM, plus adds support for sharing business logic across widgets.

mvvm_plus (mvvm+)

MVVM+ is a lightweight Flutter implementation of MVVM, plus support for sharing business logic across widgets.

MVVM+ employs ChangeNotifiers and cherry picks the best bits of Provider, GetIt, and MVVM (plus adds a few bits of its own), so will be familiar to most Flutter developers.

Model-View-View Model (MVVM)

As with all MVVM implementations, MVVM+ divides responsibilities into a UI object (called the View), business logic associated with the View (called the View Model), and business logic that spans two or more View Models (called the Model):

States are mutated in the View Model and the Model, but not the View. With MVVM+, the View is a Flutter widget and the View Model is a Dart model.

MVVM+ goals:

• Clearly separate business logic from UI.
• Optionally support access to View Models from anywhere in the widget tree.
• Work well alone or with other state management packages (BLoC, RxDart, Provider, GetIt, ...).
• Be scalable and performant, so suitable for both indy and production apps.
• Be simple.
• Be small.

Views and View Models

Your ViewModel class is a Dart class that inherits from ViewModel:

class MyWidgetViewModel extends ViewModel {
  String someText;
}

For your View class, you give the super constructor a builder for your ViewModel (via the "viewModelBuilder" parameter) and extend the View class the same way you extend StatelessWidget widget--you override the build function:

class MyWidget extends View<MyWidgetViewModel> {
  MyWidget({super.key}) : super(viewModelBuilder: () => MyWidgetViewModel());
  @override
  Widget build(BuildContext context) {
    return Text(viewModel.someText); // <- your "viewModel" instance
  }
}

Views are frequently nested and can be large, like an app page, feature, or even an entire app. Or small, like a password field or a button.

Like the Flutter State class associated with StatefulWidget, the ViewModel class has initState and dispose member functions which are handy for subscribing to and canceling listeners to streams, subjects, change notifiers, etc.:

class MyWidgetViewModel extends ViewModel {
  @override
  initState() {
    super.initState();
    _streamSubscription = Services.someStream.listen(myListener);
  }
  @override
  void dispose() {
    _streamSubscription.cancel();
    super.dispose();
  }
  late final StreamSubscription<bool> _streamSubscription;
}

Rebuilding a View

ViewModel includes a buildView method for rebuilding the View. You can call it explicitly:

class MyWidgetViewModel extends ViewModel {
  int counter;
  void incrementCounter() {
    counter++;
    buildView(); // <- queues View to build
  }
}

Or use buildView as a listener to bind the ViewModel to the View with a ValueNotifier:

class MyWidgetViewModel extends ViewModel {
  final counter = ValueNotifier<int>(0);
  void initState() {
    super.initState();
    counter.addListener(buildView); // <- binds ViewModel to View
  }
}

Retrieving ViewModels from anywhere

Occasionally you need to access another widget's ViewModel instance (e.g., if it's an ancestor or on another branch of the widget tree). This is accomplished by "registering" the ViewModel with the "register" parameter of the ViewModel constructor (similar to how GetIt works):

class MyOtherWidget extends View<MyOtherWidgetViewModel> {
  MyOtherWidget(super.key) : super(
    viewModelBuilder: () => MyOtherWidgetViewModel(
      register: true, // <- registers ViewModel instance
    ),
  );
}

ViewModels can then "get" the other registered ViewModel:

final otherViewModel = get<MyOtherWidgetViewModel>();

Like GetIt, registered ViewModels are not managed by InheritedWidget. So, widgets don't need to be children of a View widget to get its registered ViewModel. This is a big plus for use cases where the accessed ViewModel is not an ancestor.

Unlike GetIt the lifecycle of all ViewModel instances (including registered) are bound to the lifecycle of the View instances that instantiated them. So, when a View instance is removed from the widget tree, its ViewModel is disposed.

On rare occasions when you need to register multiple ViewModels of the same type, just give each ViewModel instance a unique name:

class MyOtherWidget extends View<MyOtherWidgetViewModel> {
  MyOtherWidget(super.key) : super(
    viewModelBuilder: () => MyOtherWidgetViewModel(
      register: true,
      name: 'Header', // <- unique name
    ),
  );
}

and then get the ViewModel by type and name:

final headerText = get<MyOtherWidgetViewModel>(name: 'Header').someText;
final footerText = get<MyOtherWidgetViewModel>(name: 'Footer').someText;

Adding additional ChangeNotifiers

Sometimes you want registered models that are not associated with Views. MVVM+ uses the Registrar package under the hood which has a widget named "Registrar" that you can add to the widget tree:

Registrar<MyModel>(
  builder: () => MyModel(),
  child: MyWidget(),
);

The Registrar widget registers the model when added to the widget tree and unregisters it when removed. To register multiple models with a single widget, check out MultiRegistrar.

Listening to other widget's ViewModels

The ViewModel get method retrieves registered ViewModels but does not listen for future changes. For that, use listenTo from within your ViewModel:

final text = listenTo<MyOtherWidgetViewModel>().someText;

listenTo performs a one-time add of the buildView method as a listener that is called every time the notifyListeners method of MyOtherWidgetViewModel is called. If you want to do more than just queue a build, you can give listenTo a listener function:

listenTo<MyWidgetViewModel>(listener: myListener);

If you want to rebuild your View after your custom listener finishes, just call buildView within your listener:

void myListener() {
  // do some stuff
  buildView(); 
}

Either way, listeners added by listenTo are automatically removed when your ViewModel instance is disposed.

notifyListeners vs buildView

When your View and ViewModel classes are instantiated, buildView is added as a listener to your ViewModel. So, calling buildView or notifyListeners from within your ViewModel will both rebuild your View. So, what's the difference between calling buildView and notifyListeners? Nothing, unless your ViewModel is registered--any listeners to your registered ViewModel will be called on notifyListeners but not on buildView. So, to eliminate unnecessary View builds, it is a best practice to use buildView unless your use case requires listeners to be notified of a change.

ValueNotifiers

If you want more granularity than registering an entire ViewModel or service, you can register a ValueNotifier instead:

Registrar.register<MyValueNotifier>(instance: myValueNotifier);

And get or listenTo to the ValueNotifier the same way as a registered ViewModel (because they are both subclasses of ChangeNotifier):

final myValue = listenTo<MyValueNotifier>().value;

Example

(The source code for the repo example is under the Pub.dev "Example" tab and in the GitHub example/lib/main.dart file.)

This example increments a number (0, 1, 2, ...) and a letter character (a, b, c, ...) using a single increment floating action button (FAB) that toggles between incrementing the number and the letter. When the FAB displays "+1" a press increments the number and when it displays "+a" the character will increment.

Two View widgets are used in this example. One for the increment button/FAB which maintains the state ("+1"/"+a") and one for the page which maintains current count and other states.

The page listens to two services: one that changes the number color and another that changes the letter color. The number color service has a stream that emits a new color every N seconds. The letter color service is a ChangeNotifier with a timer that changes the current letter color and then calls notifyListeners.

That's it!

The example app demos much of the above functionality and shows how small and organized MVVM+ classes typically are.

If you have questions or suggestions on anything MVVM+, please do not hesitate to contact me.