MVVM pattern in Flutter

Introduction

The MVVM pattern helps organize your Flutter app by separating the user interface from the business logic. This makes your app easier to build, test, and maintain.

When you want to keep your UI code simple and focused only on displaying data.

When your app has complex logic that you want to keep separate from the UI.

When you want to write tests for your app's logic without involving the UI.

When working in a team where designers and developers work on UI and logic separately.

When you want to make your app easier to update and add new features.

Syntax

Flutter

class ViewModel {
  // Holds data and logic
}

class View extends StatelessWidget {
  final ViewModel viewModel;

  View(this.viewModel);

  @override
  Widget build(BuildContext context) {
    // Use viewModel to build UI
    return Container();
  }
}

The ViewModel holds the app data and logic.

The View listens to the ViewModel and updates the UI.

Examples

This ViewModel holds a counter and a method to increase it.

Flutter

class CounterViewModel {
  int count = 0;

  void increment() {
    count++;
  }
}

This View shows the current count from the ViewModel.

Flutter

class CounterView extends StatelessWidget {
  final CounterViewModel viewModel;

  CounterView(this.viewModel);

  @override
  Widget build(BuildContext context) {
    return Text('Count: ${viewModel.count}');
  }
}

Sample App

This app uses MVVM to separate the counter logic from the UI. The ViewModel holds the count and notifies the View when it changes. The View listens and updates the displayed count.

Flutter

import 'package:flutter/foundation.dart';
import 'package:flutter/widgets.dart';
import 'package:flutter/material.dart';

class CounterViewModel extends ChangeNotifier {
  int _count = 0;

  int get count => _count;

  void increment() {
    _count++;
    notifyListeners();
  }
}

class CounterView extends StatelessWidget {
  final CounterViewModel viewModel;

  CounterView({required this.viewModel});

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: Text('MVVM Counter')),
      body: Center(
        child: AnimatedBuilder(
          animation: viewModel,
          builder: (context, _) {
            return Text('Count: ${viewModel.count}', style: TextStyle(fontSize: 30));
          },
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: viewModel.increment,
        child: Icon(Icons.add),
        tooltip: 'Increment',
      ),
    );
  }
}

void main() {
  final viewModel = CounterViewModel();
  runApp(MaterialApp(home: CounterView(viewModel: viewModel)));
}

Important Notes

Use ChangeNotifier in ViewModel to notify the View about data changes.

Use AnimatedBuilder or ValueListenableBuilder in the View to rebuild UI on changes.

Keep UI code simple and put all logic in the ViewModel.

Summary

MVVM separates UI (View) from logic (ViewModel) for cleaner code.

ViewModel holds data and notifies View when data changes.

View listens to ViewModel and updates UI automatically.

Practice

1. What is the main role of the ViewModel in the MVVM pattern in Flutter?

easy

A. To handle user input events directly from the UI

B. To hold data and business logic, notifying the View of changes

C. To display the user interface elements on the screen

D. To manage navigation between different screens

Solution

1. Step 1: Understand MVVM roles

   The ViewModel acts as a middleman holding data and logic separate from UI.

2. Step 2: Identify ViewModel responsibility

   It notifies the View when data changes so UI updates automatically.

3. Final Answer:

   To hold data and business logic, notifying the View of changes -> Option B

4. Quick Check:

   ViewModel = Data + Logic holder [OK]

Hint: ViewModel holds data and logic, not UI display [OK]

Common Mistakes:

• Confusing ViewModel with View
• Thinking ViewModel handles UI rendering
• Assuming ViewModel manages navigation

2. Which of the following is the correct way to notify the View about data changes in a ViewModel using Flutter's ChangeNotifier?

easy

A. call notifyListeners() inside the ViewModel after data update

B. call setState() inside the ViewModel after data update

C. call build() method directly from the ViewModel

D. call dispose() method after data update

Solution

1. Step 1: Recall ChangeNotifier usage

   ChangeNotifier requires calling notifyListeners() to update listeners.

2. Step 2: Identify correct notification method

   notifyListeners() triggers UI rebuilds in listening Views.

3. Final Answer:

   call notifyListeners() inside the ViewModel after data update -> Option A

4. Quick Check:

   notifyListeners() updates View [OK]

Hint: Use notifyListeners() in ViewModel to update UI [OK]

Common Mistakes:

• Using setState() inside ViewModel (wrong place)
• Trying to call build() manually
• Calling dispose() instead of notifyListeners()

3. Given this ViewModel code snippet in Flutter:

class CounterViewModel extends ChangeNotifier {
  int _count = 0;
  int get count => _count;

  void increment() {
    _count++;
    notifyListeners();
  }
}

What will be the value of count after calling increment() twice starting from 0?

medium

A. 0

B. 1

C. 2

D. Error: notifyListeners() not called

Solution

1. Step 1: Analyze initial value and increment

   _count starts at 0, increment() adds 1 each call.

2. Step 2: Calculate after two increments

   After two calls, _count = 0 + 1 + 1 = 2.

3. Final Answer:

   2 -> Option C

4. Quick Check:

   Two increments = count 2 [OK]

Hint: Each increment adds 1; two calls add 2 total [OK]

Common Mistakes:

• Forgetting notifyListeners() updates UI but doesn't change count
• Assuming count resets after notifyListeners()
• Confusing initial value with updated value

4. Identify the error in this ViewModel code snippet:

class UserViewModel extends ChangeNotifier {
  String _name = '';
  String get name => _name;

  void updateName(String newName) {
    _name = newName;
  }
}

Why won't the UI update when updateName is called?

medium

A. notifyListeners() is missing after updating _name

B. Getter method name is incorrect

C. ChangeNotifier should not be extended

D. updateName should be async

Solution

1. Step 1: Check data update method

   updateName changes _name but does not notify listeners.

2. Step 2: Understand UI update trigger

   Without notifyListeners(), UI won't rebuild on data change.

3. Final Answer:

   notifyListeners() is missing after updating _name -> Option A

4. Quick Check:

   Missing notifyListeners() blocks UI update [OK]

Hint: Always call notifyListeners() after data changes [OK]

Common Mistakes:

• Forgetting notifyListeners() after state change
• Thinking getter name causes UI update
• Assuming async needed without async code

5. You want to build a Flutter app using MVVM where the View updates automatically when the ViewModel's list of items changes. Which approach correctly implements this behavior?

hard

A. Store the list in View and pass it to ViewModel without notifications

B. Directly modify the list in the View and call setState() in ViewModel

C. Use a StatefulWidget for ViewModel and call build() manually after list changes

D. Use a ChangeNotifier in ViewModel, call notifyListeners() after modifying the list, and use a Consumer widget in View

Solution

1. Step 1: Understand MVVM data flow

   ViewModel holds data and notifies View on changes using ChangeNotifier.

2. Step 2: Identify correct Flutter pattern

   Calling notifyListeners() after list changes triggers Consumer widget rebuild in View.

3. Step 3: Eliminate incorrect options

   setState() belongs in View, not ViewModel; manual build() calls are incorrect; storing list only in View breaks MVVM.

4. Final Answer:

   Use a ChangeNotifier in ViewModel, call notifyListeners() after modifying the list, and use a Consumer widget in View -> Option D

5. Quick Check:

   ChangeNotifier + notifyListeners() + Consumer = MVVM update [OK]

Hint: Use ChangeNotifier + notifyListeners() + Consumer for auto UI update [OK]

Common Mistakes:

• Calling setState() in ViewModel instead of View
• Modifying data only in View without notifications
• Manually calling build() method