Hydrated Bloc: Persisting State in Flutter Applications

5.3.1 Hydrated Bloc: Persisting State in Flutter Applications

In the realm of mobile app development, ensuring that an application maintains its state across sessions is crucial for providing a seamless user experience. This is where the Hydrated Bloc package comes into play, offering an elegant solution for state persistence in Flutter applications. In this section, we’ll explore the purpose of Hydrated Bloc, how to integrate it into your Flutter project, and best practices for its use.

Purpose of Hydrated Bloc

Hydrated Bloc is an extension of the Bloc library, designed to automatically persist and restore the state of your Blocs. This means that when a user closes and reopens your app, the state is retained, providing a consistent experience. This feature is particularly useful for applications where state persistence is critical, such as in e-commerce apps where a user’s cart should remain intact between sessions.

Key Benefits:

• Automatic State Persistence: Hydrated Bloc handles the serialization and deserialization of state, saving developers from implementing custom persistence logic.
• Seamless User Experience: By maintaining state across app launches, users enjoy a more fluid and consistent interaction with the app.
• Reduced Boilerplate: With Hydrated Bloc, much of the complexity of state management is abstracted away, allowing developers to focus on building features.

Adding the hydrated_bloc Package

To get started with Hydrated Bloc, you need to add the package to your Flutter project. Open your pubspec.yaml file and include the following dependency:

dependencies:
  flutter:
    sdk: flutter
  hydrated_bloc: ^8.0.0
  path_provider: ^2.0.0

The path_provider package is necessary to determine the appropriate storage directory for the persisted state.

Setting Up HydratedBloc

Once the package is added, the next step is to set up HydratedBlocStorage. This involves initializing the storage in the main function before running the app. Here’s how you can do it:

import 'package:flutter/material.dart';
import 'package:flutter_bloc/flutter_bloc.dart';
import 'package:hydrated_bloc/hydrated_bloc.dart';
import 'package:path_provider/path_provider.dart';

void main() async {
  WidgetsFlutterBinding.ensureInitialized();
  HydratedBloc.storage = await HydratedStorage.build(
    storageDirectory: await getApplicationDocumentsDirectory(),
  );
  runApp(MyApp());
}

Explanation:

• WidgetsFlutterBinding.ensureInitialized(): Ensures that the Flutter engine is fully initialized before any asynchronous operations.
• HydratedStorage.build(): Creates a storage instance using the directory provided by getApplicationDocumentsDirectory(), which is platform-specific.

Updating Bloc Classes

To leverage Hydrated Bloc, update your Bloc classes to extend HydratedBloc instead of the standard Bloc. This change allows your Bloc to automatically handle state persistence.

import 'package:hydrated_bloc/hydrated_bloc.dart';

class CounterBloc extends HydratedBloc<CounterEvent, int> {
  CounterBloc() : super(0);

  @override
  Stream<int> mapEventToState(CounterEvent event) async* {
    if (event is Increment) {
      yield state + 1;
    } else if (event is Decrement) {
      yield state - 1;
    }
  }

  @override
  int fromJson(Map<String, dynamic> json) => json['value'] as int;

  @override
  Map<String, dynamic> toJson(int state) => {'value': state};
}

Key Methods:

• fromJson: Converts the JSON map back into the Bloc’s state.
• toJson: Serializes the state into a JSON map for storage.

Implementing fromJson and toJson

The fromJson and toJson methods are crucial for defining how your state is serialized and deserialized. This allows Hydrated Bloc to store the state in a format that can be easily retrieved and reconstructed.

@override
int fromJson(Map<String, dynamic> json) => json['value'] as int;

@override
Map<String, dynamic> toJson(int state) => {'value': state};

Considerations:

• Ensure that the data types used in fromJson and toJson match the state type.
• Handle potential null values or missing keys gracefully to avoid runtime errors.

Testing Persistence

To verify that your state is being persisted correctly, follow these steps:

1. Run the App: Launch your Flutter application.
2. Interact with the App: Perform actions that change the state, such as incrementing a counter.
3. Close the App: Terminate the app completely.
4. Relaunch the App: Open the app again and observe that the state is restored to its previous value.

This simple test confirms that the state persistence mechanism is functioning as expected.

Best Practices

While Hydrated Bloc simplifies state persistence, there are several best practices to consider:

• Data Security and Privacy: Ensure that sensitive data is encrypted before being persisted. This is especially important for applications handling personal or financial information.
• Versioning and Data Migrations: As your application evolves, the structure of your state may change. Implement versioning in your fromJson and toJson methods to handle migrations gracefully.
• Efficient State Management: Only persist the necessary state to minimize storage usage and improve performance.
• Testing Across Platforms: Test your persistence logic on all target platforms (iOS, Android, etc.) to ensure consistent behavior.

Real-World Application

Consider an e-commerce app where users can add items to a shopping cart. Using Hydrated Bloc, the cart’s state can be persisted across app launches, ensuring that users don’t lose their selections if they accidentally close the app.

class CartBloc extends HydratedBloc<CartEvent, List<Item>> {
  CartBloc() : super([]);

  @override
  Stream<List<Item>> mapEventToState(CartEvent event) async* {
    if (event is AddItem) {
      yield List.from(state)..add(event.item);
    } else if (event is RemoveItem) {
      yield List.from(state)..remove(event.item);
    }
  }

  @override
  List<Item> fromJson(Map<String, dynamic> json) {
    return (json['items'] as List)
        .map((item) => Item.fromJson(item))
        .toList();
  }

  @override
  Map<String, dynamic> toJson(List<Item> state) {
    return {'items': state.map((item) => item.toJson()).toList()};
  }
}

Conclusion

Hydrated Bloc is a powerful tool for managing state persistence in Flutter applications. By following the steps outlined in this section, you can ensure that your app provides a consistent and reliable user experience across sessions. As you implement Hydrated Bloc, keep in mind the best practices discussed, particularly around data security and versioning, to build robust and future-proof applications.

Additional Resources

• Hydrated Bloc Documentation
• Bloc Library
• Path Provider Documentation

These resources provide further insights and examples to deepen your understanding of Hydrated Bloc and its capabilities.

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