Creating a Habit Tracker app using Flutter involves designing the user interface, managing state, and implementing functionality to track and visualize habits. Below is a simple example to get you started. This example uses the provider package for state management.

First, add the necessary dependencies to your pubspec.yaml file:

dependencies:
  flutter:
    sdk: flutter
  provider: ^5.0.2
  shared_preferences: ^2.0.10

Run flutter pub get to fetch the dependencies.

Create a Habit class to represent a habit:

class Habit {
  final String name;
  bool isCompleted;

  Habit(this.name, this.isCompleted);
}

Create a HabitProvider using the provider package for state management:

import 'package:flutter/material.dart';
import 'package:provider/provider.dart';
import 'package:shared_preferences/shared_preferences.dart';

class HabitProvider extends ChangeNotifier {
  List<Habit> habits = [];

  HabitProvider() {
    _loadHabits();
  }

  void _loadHabits() async {
    SharedPreferences prefs = await SharedPreferences.getInstance();
    List<String>? habitNames = prefs.getStringList('habits');

    if (habitNames != null) {
      habits = habitNames.map((name) {
        bool isCompleted = prefs.getBool(name) ?? false;
        return Habit(name, isCompleted);
      }).toList();

      notifyListeners();
    }
  }

  void addHabit(String habitName) async {
    habits.add(Habit(habitName, false));
    _saveHabits();
  }

  void toggleHabit(int index) {
    habits[index].isCompleted = !habits[index].isCompleted;
    _saveHabits();
  }

  void _saveHabits() async {
    SharedPreferences prefs = await SharedPreferences.getInstance();
    prefs.setStringList('habits', habits.map((habit) => habit.name).toList());

    habits.forEach((habit) {
      prefs.setBool(habit.name, habit.isCompleted);
    });

    notifyListeners();
  }
}

Create the main app:

import 'package:flutter/material.dart';
import 'package:provider/provider.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return ChangeNotifierProvider(
      create: (context) => HabitProvider(),
      child: MaterialApp(
        title: 'Habit Tracker',
        theme: ThemeData(
          primarySwatch: Colors.blue,
        ),
        home: HabitTrackerScreen(),
      ),
    );
  }
}

Implement the HabitTrackerScreen:

import 'package:flutter/material.dart';
import 'package:provider/provider.dart';

class HabitTrackerScreen extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Habit Tracker'),
      ),
      body: Consumer<HabitProvider>(
        builder: (context, habitProvider, child) {
          return ListView.builder(
            itemCount: habitProvider.habits.length,
            itemBuilder: (context, index) {
              final habit = habitProvider.habits[index];
              return ListTile(
                title: Text(habit.name),
                trailing: Checkbox(
                  value: habit.isCompleted,
                  onChanged: (value) {
                    habitProvider.toggleHabit(index);
                  },
                ),
              );
            },
          );
        },
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: () {
          _addHabitDialog(context);
        },
        child: Icon(Icons.add),
      ),
    );
  }

  void _addHabitDialog(BuildContext context) {
    TextEditingController habitController = TextEditingController();

    showDialog(
      context: context,
      builder: (BuildContext context) {
        return AlertDialog(
          title: Text('Add a Habit'),
          content: TextField(
            controller: habitController,
            decoration: InputDecoration(labelText: 'Habit name'),
          ),
          actions: <Widget>[
            TextButton(
              onPressed: () {
                Navigator.of(context).pop();
              },
              child: Text('Cancel'),
            ),
            TextButton(
              onPressed: () {
                Navigator.of(context).pop();
                Provider.of<HabitProvider>(context, listen: false)
                    .addHabit(habitController.text);
              },
              child: Text('Add'),
            ),
          ],
        );
      },
    );
  }
}

In this example, the HabitProvider class is responsible for managing the state of the habits, loading habits from and saving habits to SharedPreferences. The HabitTrackerScreen widget displays a list of habits and allows users to add new habits. The state management is handled using the provider package.

Feel free to customize and expand upon this example to fit the specific requirements of your Habit Tracker app.