Implementing State Persistence With Hydrated Bloc

Introduction

State persistence is essential for many Flutter mobile development apps: login tokens, user preferences, offline counters, and form drafts. Hydrated Bloc is a drop-in extension for bloc that persists state automatically to local storage with minimal boilerplate. This tutorial focuses on practical implementation: setup, creating a HydratedBloc, serializing complex state, testing, and migration strategies to keep persisted state reliable across releases.

Setup And Installation

Add hydrated_bloc (and its storage backend) to pubspec.yaml. For most apps, the default storage uses path_provider and builds on Hive. In main(), initialize HydratedBloc.storage before running the app. Ensure you call WidgetsFlutterBinding.ensureInitialized(); on mobile platforms.

Example main initialization:

import 'package:hydrated_bloc/hydrated_bloc.dart';
import 'package:path_provider/path_provider.dart';

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

This step wires persistent storage to the HydratedBloc base class so all hydrated blocs use the same storage instance.

Implementing A Hydrated Bloc

Convert an existing Bloc or Cubit to a hydrated variant by extending HydratedBloc or HydratedCubit and implementing toJson/fromJson. Keep state objects JSON-friendly (simple maps, lists, primitives). Use fromJson to restore and handle nulls or incompatible shapes gracefully.

Simple counter example:

class CounterCubit extends HydratedCubit<int> {
  CounterCubit() : super(0);
  void increment() => emit(state + 1);
  @override Map<String, dynamic>? toJson(int state) => {'value': state};
  @override int? fromJson(Map<String, dynamic> json) => json['value'] as int? ?? 0;
}

HydratedCubit/Bloc automatically persists state after each emit. Keep toJson/fromJson deterministic and fast; heavy serialization in the hot path impacts UI responsiveness.

Handling Complex State And Serialization

When your state contains nested objects, DateTime, enums, or binary data, convert them to and from map/primitive representations. Avoid storing raw objects. Strategies:

• Implement a toMap/fromMap on complex models and call jsonEncode/jsonDecode only when necessary.

• Use enum.name or index and map back in fromJson.

• For lists/maps of custom objects, serialize each item.

Example pattern for a model:

Model: toMap() returns Map, and fromMap(Map) reconstructs the instance. In Bloc.toJson, call model.toMap(); in fromJson, return Model.fromMap(map).

Also consider size: Hydrated storage is local and not meant for large blobs. For big binary assets, keep references (file paths) rather than embedding data. For mobile development, keep persisted state small to reduce I/O overhead and startup time.

Testing And Migration Strategies

Testing: HydratedBloc reads from HydratedBloc.storage. Provide a memory-backed storage or mock storage during tests by setting HydratedBloc.storage manually in setUp(). This lets you assert persisted values without hitting disk.

Migration: When state shape changes between app versions, implement defensive fromJson logic. Approaches:

• Version your persisted payloads: include a version key in toJson and branch in fromJson.

• Provide fallbacks for missing keys and map old shapes to new models.

• On breaking changes, consider running a one-time migration that reads old keys, transforms data, and writes new keys.

Example defensive fromJson pseudocode: check for null, handle legacy keys, and return a valid default state if parsing fails. Avoid throwing during fromJson; return stable defaults to let the app function.

Edge cases: clearing storage is easy via HydratedBloc.storage.clear(); use this for remote logout flows or when migrations fail irrecoverably.

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Conclusion

Hydrated Bloc brings robust, low-effort state persistence to Flutter mobile development. The pattern is straightforward: initialize storage early, extend HydratedBloc/HydratedCubit, and implement compact, deterministic toJson/fromJson methods. For complex models, serialize with toMap/fromMap and plan migration/versioning to protect users when state shapes evolve. Test with a memory storage and keep persisted payloads small and predictable to maintain startup performance and a smooth mobile experience.