mvvm_plus 0.4.0

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+ uses 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+ organizes responsibilities into a UI group (called the View), a group for business logic associated with the View (called the View Model), and a group for 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 and Model are Dart models.

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" getter
  }
}

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.

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:

late final counter = ValueNotifier<int>(0)..addListener(buildView);

initState and dispose

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:

class MyWidgetViewModel extends ViewModel {
  late final StreamSubscription<bool> _streamSubscription;

  @override
  initState() {
    super.initState();
    _streamSubscription = Services.someStream.listen(myListener);
  }

  @override
  void dispose() {
    _streamSubscription.cancel();
    super.dispose();
  }
}

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;

Models

The Model class is a super class of ViewModel with much of the functionality of ViewModel. MVVM+ uses the Registrar package under the hood which has a widget named "Registrar" that adds Models 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 get method of View and ViewModel 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 are your MVVM Properties!

The MVVM pattern uses the term "Properties" to describe public values of View Models that are bound to Views and other objects. I.e., when the Property is changed, listeners are notified:

class MyViewModel {
  final counter = Property<int>(0);
}

In Flutter, this is how ValueNotifiers work. So, MVVM+ added a typedef that equates Property with ValueNotifier. As you use MVVM+, feel free to call your public members of ViewModels "Properties" or "ValueNotifiers", whichever is more comfortable to you. (In the MVVM+ documentation, I use "ValueNotifier" to be more transparent with the Flutter underpinnings, but in practice, I prefer to use "Property" because it clarifies its purpose and because "Property" has fewer characters! :)

So, for more granularity than listening to an entire registered Model, you can listen to one of its ValueNotifiers. So, if you have a Model that notifies in more than one place:

class CloudService extends Model {
  CloudService({super.register, super.name});
  late final currentUser = ValueNotifier<User>(null)..addListener(buildView);
  void doSomething() {
    // do something
    notifyListeners();
  }
}

you can listen to just one of its ValueNotifiers:

final cloud = get<CloudService>();
final currentUser = listenTo<ValueNotifier<User>>(notifier: cloud.currentUser).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.