bloc-provider-bloc
Use when providing, accessing, and automatically disposing BLoC (Business Logic Component) instances across Flutter widget subtrees using bloc_provider.
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- Use when providing, accessing, and automatically disposing BLoC (Business Logic Component) instances across Flutter widget subtrees using bloc_provider.
# bloc_provider BLoC Management Guide
`bloc_provider` manages the lifecycle of BLoC (Business Logic Component) classes, providing $O(1)$ subtree lookups and automatic resource disposal when the enclosing widget tree unmounts.
## Guidelines
- **Implementing the `Bloc` Interface**:
- Always implement the `Bloc` interface on your business logic classes.
- Implement `void dispose()` to close all active `StreamController`s, sinks, and subscriptions.
- **Providing BLoCs**:
- Wrap the target subtree with `BlocProvider<MyBloc>(creator: (context, bag) => MyBloc(), child: ...)`.
- The BLoC is lazily created upon first access and automatically disposed when the provider's state unmounts.
- **Accessing BLoCs**:
- Call `BlocProvider.of<MyBloc>(context)` to retrieve the nearest ancestor BLoC.
- Unlike standard InheritedWidget lookups that cause rebuilding, `BlocProvider.of()` does not trigger widget rebuilds itself; connect outputs to `StreamBuilder` widgets to rebuild only the necessary parts of the UI.
## Examples
### 1. Defining a BLoC
```dart
import 'dart:async';
import 'package:bloc_provider/bloc_provider.dart';
import 'package:rxdart/rxdart.dart';
class CounterBloc implements Bloc {
CounterBloc() {
_incrementController.listen((_) {
_countSubject.add(_countSubject.value + 1);
});
}
final _countSubject = BehaviorSubject<int>.seeded(0);
final _incrementController = StreamController<void>();
ValueStream<int> get count => _countSubject;
Sink<void> get increment => _incrementController.sink;
@override
void dispose() async {
await _incrementController.close();
await _countSubject.close();
}
}
```
### 2. Providing and Consuming in the Widget Tree
```dart
import 'package:bloc_provider/bloc_provider.dart';
import 'package:flutter/material.dart';
class CounterPage extends StatelessWidget {
const CounterPage({super.key});
@override
Widget build(BuildContext context) {
return BlocProvider<CounterBloc>(
creator: (context, bag) => CounterBloc(),
child: const _CounterView(),
);
}
}
class _CounterView extends StatelessWidget {
const _CounterView();
@override
Widget build(BuildContext context) {
final bloc = BlocProvider.of<CounterBloc>(context);
return Scaffold(
appBar: AppBar(title: const Text('BLoC Counter')),
body: Center(
child: StreamBuilder<int>(
stream: bloc.count,
initialData: bloc.count.value,
builder: (context, snapshot) {
return Text(
'Count: ${snapshot.data}',
style: Theme.of(context).textTheme.headlineMedium,
);
},
),
),
floatingActionButton: FloatingActionButton(
onPressed: () => bloc.increment.add(null),
child: const Icon(Icons.add),
),
);
}
}
```
## Common Pitfalls & Anti-Patterns
- ❌ **Anti-pattern**: Forgetting to implement `void dispose()` in BLoC classes, causing stream controllers to leak in memory.
- ✔️ **Correct**: Always close subjects and controllers inside `dispose()`.
- ❌ **Anti-pattern**: Instantiating BLoC directly inside a `StatelessWidget.build()` method without `BlocProvider`.
- ✔️ **Correct**: Use `BlocProvider(creator: ...)` to tie the BLoC's lifespan to the widget tree lifecycle.
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