hydrated_bloc 0.6.0

An extension to the bloc state management library which automatically persists and restores bloc states.

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An extension to the bloc state management library which automatically persists and restores bloc states.

Usage

1. Use HydratedBlocDelegate

void main() async {
  // https://github.com/flutter/flutter/pull/38464
  // Changes in Flutter v1.9.4 require you to call WidgetsFlutterBinding.ensureInitialized()
  // before using any plugins if the code is executed before runApp.
  // As a result, you may have to uncomment the following line if you're using Flutter >=1.9.4.
  //
  // WidgetsFlutterBinding.ensureInitialized();
  BlocSupervisor.delegate = await HydratedBlocDelegate.build();
  runApp(App());
}

2. Extend HydratedBloc and override initialState, fromJson and toJson methods

class CounterBloc extends HydratedBloc<CounterEvent, CounterState> {
  // Use previously cached initialState if it's available
  @override
  CounterState get initialState {
    return super.initialState ?? CounterState(0);
  }

  // Called when trying to read cached state from storage.
  // Be sure to handle any exceptions that can occur and return null
  // to indicate that there is no cached data.
  @override
  CounterState fromJson(Map<String, dynamic> source) {
    try {
      return CounterState(source['value'] as int);
    } catch (_) {
      return null;
    }
  }

  // Called on each state change (transition)
  // If it returns null, then no cache updates will occur.
  // Otherwise, the returned value will be cached.
  @override
  Map<String, int> toJson(CounterState state) {
    try {
      return { 'value': state.value };
    } catch (_) {
      return null;
    }
  }

  @override
  Stream<CounterState> mapEventToState(CounterEvent event) async* {
    switch (event) {
      case CounterEvent.decrement:
        yield CounterState(currentState.value - 1);
        break;
      case CounterEvent.increment:
        yield CounterState(currentState.value + 1);
        break;
    }
  }
}

enum CounterEvent { increment, decrement }

class CounterState {
  int value;

  CounterState(this.value);
}

Now our CounterBloc is a HydratedBloc and will automatically persist its state. We can increment the counter value, hot restart, kill the app, etc... and our CounterBloc will always retain its state.

How it works

Overview

hydrated_bloc exports a HydratedStorage interface which means it can work with any storage provider. Out of the box, it comes with its own implementation: HydratedBlocStorage.

HydratedBlocStorage is built on top of path_provider for a platform-agnostic storage layer. The out-of-the-box storage implementation reads/writes to file using the toJson/fromJson methods on HydratedBloc and should perform very well for most use-cases (performance reports coming soon). HydratedBlocStorage is supported for desktop (example).

Considerations

On mobile, all data is written to temporary storage which means it can be wiped by the operating system at any point in time. As a result, hydrated_bloc is not intended to be used as a persistent database and should be viewed as a cache instead.

In addition, while the HydratedBlocStorage client doesn't automatically encrypt/decrypt the data, it is fairly straightforward to implement a custom HydratedStorage client which does support encryption.