The MVN Architecture for Flutter & Riverpod

🏗️ Core Architecture: Three Layers, One Flow

The MVN (Model-View-Notifier) architecture is a modern, scalable pattern for Flutter applications leveraging the Riverpod state management library. It emphasizes a clear separation of concerns and a unidirectional data flow.

The architecture is composed of three distinct layers:

Key Benefits

• Clear Separation of Concerns: Distinguishes UI, state management, and business logic effectively.
• Enhanced Scalability: Encourages the use of multiple, focused notifiers to prevent "Fat Notifiers."
• Improved Testability: Layers are easily testable in isolation via Riverpod's dependency injection.
• Predictable State Management: Achieved through immutable state classes (often using sealed classes for states like Loading, Success, Error).
• Reactive and Efficient UI: Riverpod ensures the View only rebuilds when the specific state it's ref.watch-ing changes.

🛠️ Implementation Guide: Building a Feature with MVN

A practical, three-step guide for implementing an MVN feature (e.g., a movie list).

// movie_model.dart & movie_repository.dart
class Movie { /* ... movie properties ... */ }

class MovieRepository {
  Future<List<Movie>> fetchMovies() async { /* ... API call ... */ }
}

// movie_providers.dart
final movieRepositoryProvider = Provider((ref) => MovieRepository());

// movies_state.dart
sealed class MovieState {}
class MovieLoading extends MovieState {}
class MovieSuccess extends MovieState { final List<Movie> movies; }
class MovieError extends MovieState { final String errorMessage; }

// movies_notifier.dart
class MoviesNotifier extends Notifier<MovieState> {
  @override
  MovieState build() { /* ... initial state and reactive dependencies ... */ }
  
  Future<void> fetchMovies() async {
    // Reads Model (Repository), updates state based on result.
    final repo = ref.read(movieRepositoryProvider);
    // ... logic to update state to MovieLoading, then MovieSuccess/MovieError
  }
}

// movie_providers.dart
final moviesNotifierProvider = NotifierProvider<MoviesNotifier, MovieState>(() {
  return MoviesNotifier();
});

// movies_screen.dart
class MoviesScreen extends ConsumerWidget {
  @override
  Widget build(BuildContext context, WidgetRef ref) {
    final movieState = ref.watch(moviesNotifierProvider);
    
    // UI logic to render based on MovieLoading(), MovieSuccess(), or MovieError()
    if (movieState is MovieLoading) {
        return const CircularProgressIndicator();
    }
    
    // User actions call: 
    // ref.read(moviesNotifierProvider.notifier).fetchMovies();
  }
}

💡 Advanced Usage: Addressing Potential Pitfalls

While powerful, the multi-notifier MVN architecture has potential challenges. Here's how to anticipate and avoid them.

Pitfall 1: Complex State Synchronization

The Challenge: When a feature uses multiple notifiers (e.g., FilterNotifier and MoviesNotifier), coordinating state changes can be difficult. A change in the filter should automatically trigger a movie refetch.

How to Avoid It: Use ref.watch inside your providers. Create a reactive dependency chain where the MoviesNotifier watches the FilterNotifier. Riverpod will automatically handle re-running the logic when the dependency changes.

// Provider for the filter
final movieFilterProvider = StateProvider<MovieFilter>((ref) => MovieFilter.all);

// The MoviesNotifier WATCHES the filter provider
class MoviesNotifier extends Notifier<MovieState> {
  @override
  MovieState build() {
    // This creates a reactive link. When movieFilterProvider changes,
    // this build method will re-run automatically.
    final currentFilter = ref.watch(movieFilterProvider);
    _fetchMovies(filter: currentFilter); // Trigger the initial fetch
    return MovieLoading();
  }
  
  Future<void> _fetchMovies({required MovieFilter filter}) async { /* ... */ }
}

Pitfall 2: Handling One-Time Events (Side Effects)

The Challenge: You need to trigger actions that should only happen once per event, like showing a SnackBar or navigating to a new screen. Placing a boolean like showErrorSnackbar in your state is a bad practice, as it can re-trigger the event on screen rotation.

How to Avoid It: Use ref.listen in the View. This function is specifically designed to observe a provider's state and execute actions (side effects) without causing the widget to rebuild.

// Inside a ConsumerWidget's build method
ref.listen<MovieState>(moviesNotifierProvider, (previousState, newState) {
  if (newState is MovieError) {
    ScaffoldMessenger.of(context).showSnackBar(
      SnackBar(content: Text(newState.errorMessage)),
    );
  }
  // Add navigation logic here as well.
});

Pitfall 3: Overly Complex View Layer

The Challenge: As the View listens to multiple notifiers, the build method can become cluttered with ref.watch calls, potentially leading to unnecessary rebuilds of the entire screen.

How to Avoid It:

• Be Granular with select: If your widget only depends on a small piece of a larger state object, use .select to listen only to changes in that specific value.
• Split Your UI: Break down your UI into smaller ConsumerWidgets, each listening to only the state it needs.

Pitfall 4: Increased Boilerplate

The Challenge: Creating multiple files (state, notifier, provider) for each feature can feel repetitive and slow down initial development.

How to Avoid It:

• Accept it as a Trade-off: This boilerplate is the price for long-term maintainability and scalability.
• Use Good Project Structure: Organize your files by feature (e.g., a feature/movies folder) to keep everything related in one place.
• Use IDE Snippets: Create file templates or code snippets in your IDE to quickly generate the basic structure for your MVN components.

🎯 MVN vs. MVVM: The Idiomatic Choice for Riverpod

It's a fair and important question. The Model-View-ViewModel (MVVM) pattern is a powerful and proven architecture. The MVN pattern I've proposed does not seek to invalidate MVVM; rather, it provides a specialized evolution that is a more natural and productive fit for the Riverpod ecosystem.

While you can certainly implement MVVM using Riverpod, the MVN pattern offers a more direct, intuitive, and idiomatic path. Here's a breakdown of the key advantages:

1. Semantic Clarity and Intuitiveness

The single biggest advantage is in the name itself. The term "ViewModel" is abstract—it describes a role, but not an implementation. In the MVN pattern, the "Notifier" directly maps to a concrete class in the Riverpod library: Notifier or StateNotifier.

• Traditional MVVM: A developer on your team asks, "What are we using for our ViewModels?" The answer is, "We use Riverpod's StateNotifier classes to act as our ViewModels." This requires a mental translation.
• MVN Pattern: The same question is answered by the architecture itself. The "N" in MVN is the Notifier. This reduces cognitive load and makes the architecture instantly understandable for anyone familiar with Riverpod.

2. An Idiomatic, Riverpod-First Implementation

MVN is not just a pattern that uses Riverpod; it's a pattern designed around Riverpod's core principles.

• It embraces the reactive nature of providers, encouraging the use of ref.watch inside notifiers to create declarative dependency chains.
• It aligns with Riverpod's philosophy of having many small, focused providers rather than a few large, monolithic state objects.
• A generic MVVM implementation might treat the state management library as a pluggable detail, whereas MVN embraces Riverpod as the central nervous system of the state layer.

3. A Clear and Prescriptive Convention

By giving this pattern a specific name, we create a powerful convention for teams. When you say, "We are using the MVN architecture," it immediately communicates a clear set of rules and file structures.

This shared language removes ambiguity during code reviews and makes onboarding new developers significantly faster. Instead of debating how to implement a ViewModel, the team has a clear, established pattern to follow.

Conclusion: In essence, while you can absolutely build a great app by applying MVVM principles with Riverpod, the MVN pattern offers a path of less resistance. It provides a refined vocabulary and a structure that is purpose-built for the tools you are already using, leading to code that is not only robust and scalable but also a genuine pleasure to write and maintain.