alt_bloc

Description:

alt bloc

alt_bloc #
Library for BLoC design pattern implementation.
Authors #

Yegor Logachev [email protected] (DashDevs LLC)
Olivier Brand [email protected] (Ingenio LLC)

Intro #
Why we decided to create this solution? #
We are very excited to present our version of the BLoC pattern. Having implemented Flutter apps since its inception, we have built a strong expertise in the framework and kept up to date on all state management related architecture and packages. Our primary goal is to have a one stop shop for a state management system that is simple enough so anybody can understand and quickly adopt it to build powerful applications. One of our goal was also to avoid relying on complex and 'magical' frameworks such as Rx, or having to use multiple franeworks together to achieve the same outcome alt_bloc has to offer.
Existing solutions such as bloc and flutter_bloc have limitations in their design. Some of them are:


bloc library is designed in a way that it can only handle a single state and event class. As a result, developers have to create large hierarchies of classes that inherit, for each bloc, from a base state or event class. As a consequence, the number of events/states classes grows nonlinearly with the number of blocks that leads to an event/state hell. alt_bloc does not solve this problem completely, but provides the possibility to reduce the number of states per Bloc and simplify the overall hierarchy of events/states classes.


bloc library makes it very hard to create reusable state in other Bloc classes. For example a simple UX element such as showing a progress indicator would result in lots of Bloc classes containing a state class. Developers would then have to create their own progress state class for each Bloc or build a hierarchy of states in the state class. The first case leads to lots of code duplication, second leads to a very complicated state hierarchy.


Finally bloc and flutter_bloc libraries contains dependencies on complex libraries and packages such as rxdart and provider. As result this dependencies affects the build time , project size and more importantly increases the complexity and debugging when things do not go as planned.


P.S. We highly respect the authors and developers of bloc and flutter_bloc libraries and do not try to somehow descredit them. Our description above is an attempt to objectively describe the current problems of bloc and flutter_bloc libraries, and explain the reasons for creating this solution and provide to newcomers to Flutter an easy way to implement BLoC pattern without adding too much complexity.
Main Features #

Public interfaces of alt_bloc are similar to interfaces of popular libraries like Provider and BLoC. So you do not need to spend lots of time to learn how set up this library.
Support multi states. You can create few states per Bloc and you don't need to create hierarchy of states that inherited from one parent state, especially if such states are on different levels of abstraction.
Support Stream and Future objects as states provider.
Provides separate pipes to handle navigation actions.
Contains solution to receive result of navigation in the Bloc object.
Lightweight solution. The overall package is very small and it doesn't contains any third party libraries.

Components #
Bloc
Business Logic Component.
The class that implements the core business logic, and that should be placed between UI and data source. BLoC accepts UI actions from the widget and in return notifies the widget's about changes in UI state or initiate navigation actions.
This class implements methods that improve and simplify process of delivering business-logic states changes and navigation actions to widgets.
Current solution contains methods to register states that you want to provide to widgets (registerState()), send this states (addState(), addStateSource(), addStatesSource()) and navigation actions (addNavigation(), addNavigationSource()).
class CounterBloc extends Bloc {
int value = 0;

CounterBloc() {
registerState<int>(initialState: value);
}

void increment() => addState(++value);
}
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Widgets should use getStateStream() and navigationStream property to subscribe on Bloc events.
abstract class Bloc
void registerState<S>({bool isBroadcast = false, S initialState}) - registers state of S type
that can be processed by this Bloc.Creates stream and all necessary resources that need to process state of S type. isBroadcast define type of stream that will be created. You can pass object that will define initialState. Throws StateError if this method was called twice for the same type or if this Bloc was closed.
S initialState<S>() - returns initial value for state of S type. Returns null if this Bloc was closed. Throws ArgumentError if state of such type was not registered.
bool containsState<S>() - checks whether a state of S type was registered before. Returns false if this Bloc was closed.
isClosed - Defines whether that Bloc was closed.
bool addState<S>(S uiState) - adds state of S type to the stream that corresponding to state type. Returns false if this Bloc was closed and state was not added to the stream. Throws ArgumentError if state of such type was not registered.
StreamSubscription<S> addStateSource<S>(Future<S> source, {void Function(S data) onData, void Function() onDone, void Function(dynamic error) onError}) - adds Future that should be returned by state of S type as source of state. Callbacks onData, onDone, onError provide possibility to handle source. Throws ArgumentError if state of such type was not registered. Returns StreamSubscription that provide possibility to pause, resume or cancel source.
StreamSubscription<S> addStatesSource<S>(Stream<S> source, {void Function(S data) onData, void Function() onDone, void Function(dynamic error) onError}) - adds Stream that should be returned by state of S type as source of state. Callbacks onData, onDone, onError provide possibility to handle source. Throws ArgumentError if state of such type was not registered. Returns StreamSubscription that provide possibility to pause, resume or cancel source.
Future<Result> addNavigation<Result>({String routeName, dynamic arguments}) - adds navigation data to navigationStream. Method arguments wrap with RouteData object and pass to navigationStream. Returns a Future that completes to the result value when RouteData.resultConsumer function will be called. RouteData.resultConsumer can be called once and only once, otherwise StateError will be thrown. The 'Result' type argument is the type of the return value.
StreamSubscription<RouteData> addNavigationSource(Stream<RouteData> source, {void Function(RouteData data) onData, void Function() onDone, void Function(dynamic error) onError}) - adds Stream of RouteData as navigation events source. Callbacks onData, onDone, onError help to handle source. Returns StreamSubscription that provide possibility to pause, resume or cancel source. Preferable to use this method for aggregation of blocs.
class ConnectionBloc extends Bloc {

void startCall(Contact contact) async { ... }

void startChat(Contact contact) async { ... }
}

class ContactsBloc extends Bloc implements ConnectionBloc {
ContactsBloc(this._connectionBloc) {
addNavigationSource(_connectionBloc.navigationStream);
}

final ConnectionBloc _connectionBloc;

@override
void startCall(Contact contact) => _connectionBloc.startCall(contact);

@override
void startChat(Contact contact) => _connectionBloc.startChat(contact);

...
}
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Stream<S> getStateStream<S>() - returns states stream according to type S. Returns null if this Bloc was closed. Throws ArgumentError if state of such type was not registered.
navigationStream - returns navigation stream. Returns null if this Bloc was closed.
void close() - Releases resources and closes streams.
RouteData
The class that contains all information about navigation.
ResultConsumer
Signature of callback that use to return navigation result to the Bloc.
BlocProvider
Widget that responsible to create the Bloc with help of create function. Accepts any Widget as child and provides the ability for child to obtain the Bloc.
Function shouldNotify define whether that widgets that inherit from this widget should be rebuilt if Bloc was changed.
BlocProvider could subscribe on Bloc.navigationStream and receives navigation events if router function will be defined, similar as RouteListener.
class CounterScreen extends StatelessWidget {

@override
Widget build(BuildContext context) {
return BlocProvider<CounterBloc>(
routerPrecondition: (prevRouteData, routeData) => (routeData.settings.arguments as int) % 5 == 0,
create: () => CounterBloc(),
child: CounterLayout(title: 'Bloc Demo Home Page'),
router: (context, name, args) {
return showDialog(
context: context,
builder: (_) {
return WillPopScope(
child: AlertDialog(
title: Text('Congratulations! You clicked $args times'),
),
onWillPop: () async {
Navigator.of(context).pop('Dialog with $args clicks has been closed');
return false;
}
);
}
);
},
);
}
}
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Provider
InheritedWidget that encapsulated by BlocProvider and allow BlocProvider.child widgets tree to obtain the Bloc object.
static B of<B extends Bloc>(BuildContext context, {bool listen = false}) - static function that returns Bloc of type B. If listen defines as true, each time when Bloc object changes, this context is rebuilt. Custom Bloc comparison rules could be defined in BlocProvider.shouldNotify function.
BlocBuilder
Widget that accept Bloc of type B and subscribes on states stream of type S. If Bloc was not provided, so Provider.of uses by default.
Function builder calls each time when new state was added to stream and returns Widget depending on state. precondition allow to filter stats that will be delivered to builder.
BlocBuilder<CounterBloc, int>(
bloc: CounterBloc(),
precondition: (prevCount, count) => count % 2 == 0,
builder: (_, count) => Text('$count');
)
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BlocConsumer
Widget that accept Bloc of type B and subscribes on states stream of type S. If Bloc was not provided, so Provider.of uses by default.
consumer function will be triggered each time on new state.
RouteListener
Widget that subscribes on Bloc.navigationStream, listen for navigation actions and handle them. If bloc was not provided Provider.of will be used by default.
router function will be triggered each time on new state and returns navigation result to the
Bloc.
WARNING!!! Potentially few RouteListener widgets could be subscribed on the same stream, so same navigation event could be handled several times. We recommend to use precondition to avoid such situation.
RouteListener<CounterBloc>(
bloc: CounterBloc(),
child: CounterLayout(title: 'Bloc Demo Home Page'),
precondition: (prevData, data) => (data.settings.arguments as int) % 5 == 0,
router: (context, name, args) {
return showDialog(
context: context,
builder: (_) {
return WillPopScope(
child: AlertDialog(
title: Text('Congratulations! You clicked $args times'),
),
onWillPop: () async {
// Argument that Navigator.pop() function will be returned to the Bloc
Navigator.of(context).pop('Dialog with $args clicks has been closed');
return false;
});
});
},
)

class CounterBloc extends Bloc {

...
void increment() {
addState<bool>(true);
addStateSource<int>(repo.increment(),
onData: (count) async {
// print String that was passed to Navigator.pop()
print(await addNavigation(arguments: count));
},
onDone: () => addState<bool>(false));
}
...
}
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Example #
void main() => runApp(CounterApp());

class CounterApp extends StatelessWidget {

@override
Widget build(BuildContext context) {
return MaterialApp(
title: 'Flutter Demo',
theme: ThemeData(
primarySwatch: Colors.blue,
),
home: CounterScreen()
);
}
}
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Simple Bloc implementation.
You should register all states that will be used by Bloc with the help of the registerState<T>() method. To notify Widget about changes you should call addState<T>(T_object);.
WARNING!!! If you try to call addState<T>() method before registerState<T>() an error will be thrown.
If you want to send some navigation events, you need to call addNavigation({String routeName, dynamic arguments}) method.
class CounterBloc extends Bloc {

var _counter = 0;

CounterBloc() {
registerState<int>(initialState: 10);
registerState<bool>(initialState: false);
}

void increment() {
addState<bool>(true);
Future.delayed(const Duration(milliseconds: 500), () {
addState<bool>(false);
addState<int>(++_counter);
if ((_counter % 10) == 0) {
addNavigation(arguments: _counter);
}
});
}
}
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If you have some Future or Stream that contains state objects you can pass it as state source
with help addStatesSource() and addStateSource()
functions.
For example we have some repository that return Future with incrementation result.
class IncrementRepo {
int _counter = 0;

Future<int> increment() => Future.delayed(const Duration(seconds: 1), () => ++_counter);

Future<int> decrement() => Future.delayed(const Duration(seconds: 5), () => --_counter);
}

class CounterBloc extends Bloc {
final repo = IncrementRepo();

CounterBloc() {
registerState<int>(initialState: 0);
registerState<bool>(initialState: false);
}

void increment() {
addState<bool>(true);
addStateSource<int>(repo.increment(),
onData: (count) async {
print('Dialog result: ${await addNavigation(arguments: count)}');
},
onDone: () => addState<bool>(false));
}
}
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BlocProvider.
create function responsible for the Bloc creation.
class CounterScreen extends StatelessWidget {

@override
Widget build(BuildContext context) {
return BlocProvider<CounterBloc>(
create: () => CounterBloc(),
routerPrecondition: (_, routeData) => (routeData.settings.arguments as int) % 5 == 0,
child: CounterLayout(title: 'Bloc Demo Home Page'),
router: (context, name, args) {
return showDialog(
context: context,
builder: (_) => WillPopScope(
child: AlertDialog(
title: Text('Congratulations! You clicked $args times'),
),
onWillPop: () async {
Navigator.of(context).pop(args);
return false;
})
);
},
);
}
}
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Navigation handling.
You could handle navigation with the help of router as shown in the example above. Or you can use RoutreListener.
class CounterScreen extends StatelessWidget {

@override
Widget build(BuildContext context) {
return BlocProvider<CounterBloc>(
create: () => CounterBloc(),
child: RouteListener<CounterBloc>(
child: CounterLayout(title: 'Bloc Demo Home Page'),
router: (context, name, args) {
return showDialog(
context: context,
builder: (_) => AlertDialog(
title: Text('Congratulations! You clicked $args times'),
),
);
},
),
);
}
}
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Providing and observing Bloc on UI.
class CounterLayout extends StatelessWidget {

CounterLayout({Key key, this.title}) : super(key: key);

final String title;

@override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(
title: Text(title),
),
body: Stack(
children: <Widget>[
Center(
child: Column(
mainAxisAlignment: MainAxisAlignment.center,
children: <Widget>[
Text(
'You have pushed the button this many times:',
),
BlocBuilder<CounterBloc, int>(
builder: (_, count) {
return Text(
'$count',
style: Theme.of(context).textTheme.display1,
);
}),
],
),
),
Center(
child: BlocBuilder<CounterBloc, bool>(
builder: (_, inProgress) {
return inProgress ? CircularProgressIndicator() : Container();
}
),
)
],
),
floatingActionButton: FloatingActionButton(
onPressed: Provider.of<CounterBloc>(context).increment,
tooltip: 'Increment',
child: Icon(Icons.add),
),
);
}
}
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