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Flutter Bloc – A Widgets that make it easy to implement the BLoC

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Flutter Bloc Package

Widgets that make it easy to integrate blocs and cubits into Flutter. Built to work with package:bloc.

Learn more at bloclibrary.dev!

*Note: All widgets exported by the flutter_bloc package integrate with both Cubit and Bloc instances.

Usage

Lets take a look at how to use BlocBuilder to hook up a CounterPage widget to a CounterCubit.

counter_cubit.dart

class CounterCubit extends Cubit<int> {
  CounterCubit() : super(0);

  void increment() => emit(state + 1);
  void decrement() => emit(state - 1);
}

counter_page.dart

class CounterPage extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: const Text('Counter')),
      body: BlocBuilder<CounterCubit, int>(
        builder: (context, count) => Center(child: Text('$count')),
      ),
      floatingActionButton: Column(
        crossAxisAlignment: CrossAxisAlignment.end,
        mainAxisAlignment: MainAxisAlignment.end,
        children: <Widget>[
          Padding(
            padding: const EdgeInsets.symmetric(vertical: 5.0),
            child: FloatingActionButton(
              child: const Icon(Icons.add),
              onPressed: () => context.bloc<CounterCubit>().increment(),
            ),
          ),
          Padding(
            padding: const EdgeInsets.symmetric(vertical: 5.0),
            child: FloatingActionButton(
              child: const Icon(Icons.remove),
              onPressed: () => context.bloc<CounterCubit>().decrement(),
            ),
          ),
        ],
      ),
    );
  }
}

At this point we have successfully separated our presentational layer from our business logic layer. Notice that the CounterPage widget knows nothing about what happens when a user taps the buttons. The widget simply tells the CounterCubit that the user has pressed either the increment or decrement button.

Bloc Widgets

BlocBuilder is a Flutter widget which requires a cubit and a builder function. BlocBuilder handles building the widget in response to new states. BlocBuilder is very similar to StreamBuilder but has a more simple API to reduce the amount of boilerplate code needed. The builder function will potentially be called many times and should be a pure function that returns a widget in response to the state.

See BlocListener if you want to “do” anything in response to state changes such as navigation, showing a dialog, etc…

If the cubit parameter is omitted, BlocBuilder will automatically perform a lookup using BlocProvider and the current BuildContext.

BlocBuilder<BlocA, BlocAState>(
  builder: (context, state) {
    // return widget here based on BlocA's state
  }
)

Only specify the cubit if you wish to provide a cubit that will be scoped to a single widget and isn’t accessible via a parent BlocProvider and the current BuildContext.

BlocBuilder<BlocA, BlocAState>(
  cubit: blocA, // provide the local bloc instance
  builder: (context, state) {
    // return widget here based on BlocA's state
  }
)

For fine-grained control over when the builder function is called an optional buildWhen can be provided. buildWhen takes the previous cubit state and current cubit state and returns a boolean. If buildWhen returns true, builder will be called with state and the widget will rebuild. If buildWhen returns false, builder will not be called with state and no rebuild will occur.

BlocBuilder<BlocA, BlocAState>(
  buildWhen: (previousState, state) {
    // return true/false to determine whether or not
    // to rebuild the widget with state
  },
  builder: (context, state) {
    // return widget here based on BlocA's state
  }
)

BlocProvider is a Flutter widget which provides a cubit to its children via BlocProvider.of<T>(context). It is used as a dependency injection (DI) widget so that a single instance of a cubit can be provided to multiple widgets within a subtree.

In most cases, BlocProvider should be used to create new cubits which will be made available to the rest of the subtree. In this case, since BlocProvider is responsible for creating the cubit, it will automatically handle closing it.

BlocProvider(
  create: (BuildContext context) => BlocA(),
  child: ChildA(),
);

By default, BlocProvider will create the cubit lazily, meaning create will get executed when the cubit is looked up via BlocProvider.of<BlocA>(context).

To override this behavior and force create to be run immediately, lazy can be set to false.

BlocProvider(
  lazy: false,
  create: (BuildContext context) => BlocA(),
  child: ChildA(),
);

In some cases, BlocProvider can be used to provide an existing cubit to a new portion of the widget tree. This will be most commonly used when an existing cubit needs to be made available to a new route. In this case, BlocProvider will not automatically close the cubit since it did not create it.

BlocProvider.value(
  value: BlocProvider.of<BlocA>(context),
  child: ScreenA(),
);

then from either ChildA, or ScreenA we can retrieve BlocA with:

// with extensions
context.bloc<BlocA>();

// without extensions
BlocProvider.of<BlocA>(context)

MultiBlocProvider is a Flutter widget that merges multiple BlocProvider widgets into one. MultiBlocProvider improves the readability and eliminates the need to nest multiple BlocProviders. By using MultiBlocProvider we can go from:

BlocProvider<BlocA>(
  create: (BuildContext context) => BlocA(),
  child: BlocProvider<BlocB>(
    create: (BuildContext context) => BlocB(),
    child: BlocProvider<BlocC>(
      create: (BuildContext context) => BlocC(),
      child: ChildA(),
    )
  )
)

to:

MultiBlocProvider(
  providers: [
    BlocProvider<BlocA>(
      create: (BuildContext context) => BlocA(),
    ),
    BlocProvider<BlocB>(
      create: (BuildContext context) => BlocB(),
    ),
    BlocProvider<BlocC>(
      create: (BuildContext context) => BlocC(),
    ),
  ],
  child: ChildA(),
)

BlocListener is a Flutter widget which takes a BlocWidgetListener and an optional cubit and invokes the listener in response to state changes in the cubit. It should be used for functionality that needs to occur once per state change such as navigation, showing a SnackBar, showing a Dialog, etc…

listener is only called once for each state change (NOT including the initial state) unlike builder in BlocBuilder and is a void function.

If the bloc parameter is omitted, BlocListener will automatically perform a lookup using BlocProvider and the current BuildContext.

BlocListener<BlocA, BlocAState>(
  listener: (context, state) {
    // do stuff here based on BlocA's state
  },
  child: Container(),
)

Only specify the cubit if you wish to provide a cubit that is otherwise not accessible via BlocProvider and the current BuildContext.

BlocListener<BlocA, BlocAState>(
  cubit: blocA,
  listener: (context, state) {
    // do stuff here based on BlocA's state
  }
)

For fine-grained control over when the listener function is called an optional listenWhen can be provided. listenWhen takes the previous cubit state and current cubit state and returns a boolean. If listenWhen returns true, listener will be called with state. If listenWhen returns false, listener will not be called with state.

BlocListener<BlocA, BlocAState>(
  listenWhen: (previousState, state) {
    // return true/false to determine whether or not
    // to call listener with state
  },
  listener: (context, state) {
    // do stuff here based on BlocA's state
  },
  child: Container(),
)

MultiBlocListener is a Flutter widget that merges multiple BlocListener widgets into one. MultiBlocListener improves the readability and eliminates the need to nest multiple BlocListeners. By using MultiBlocListener we can go from:

BlocListener<BlocA, BlocAState>(
  listener: (context, state) {},
  child: BlocListener<BlocB, BlocBState>(
    listener: (context, state) {},
    child: BlocListener<BlocC, BlocCState>(
      listener: (context, state) {},
      child: ChildA(),
    ),
  ),
)

to:

MultiBlocListener(
  listeners: [
    BlocListener<BlocA, BlocAState>(
      listener: (context, state) {},
    ),
    BlocListener<BlocB, BlocBState>(
      listener: (context, state) {},
    ),
    BlocListener<BlocC, BlocCState>(
      listener: (context, state) {},
    ),
  ],
  child: ChildA(),
)

BlocConsumer exposes a builder and listener in order react to new states. BlocConsumer is analogous to a nested BlocListener and BlocBuilder but reduces the amount of boilerplate needed. BlocConsumer should only be used when it is necessary to both rebuild UI and execute other reactions to state changes in the cubit. BlocConsumer takes a required BlocWidgetBuilder and BlocWidgetListener and an optional cubit, BlocBuilderCondition, and BlocListenerCondition.

If the cubit parameter is omitted, BlocConsumer will automatically perform a lookup using BlocProvider and the current BuildContext.

BlocConsumer<BlocA, BlocAState>(
  listener: (context, state) {
    // do stuff here based on BlocA's state
  },
  builder: (context, state) {
    // return widget here based on BlocA's state
  }
)

An optional listenWhen and buildWhen can be implemented for more granular control over when listener and builder are called. The listenWhen and buildWhen will be invoked on each cubit state change. They each take the previous state and current state and must return a bool which determines whether or not the builder and/or listener function will be invoked. The previous state will be initialized to the state of the cubit when the BlocConsumer is initialized. listenWhen and buildWhen are optional and if they aren’t implemented, they will default to true.

BlocConsumer<BlocA, BlocAState>(
  listenWhen: (previous, current) {
    // return true/false to determine whether or not
    // to invoke listener with state
  },
  listener: (context, state) {
    // do stuff here based on BlocA's state
  },
  buildWhen: (previous, current) {
    // return true/false to determine whether or not
    // to rebuild the widget with state
  },
  builder: (context, state) {
    // return widget here based on BlocA's state
  }
)

RepositoryProvider is a Flutter widget which provides a repository to its children via RepositoryProvider.of<T>(context). It is used as a dependency injection (DI) widget so that a single instance of a repository can be provided to multiple widgets within a subtree. BlocProvider should be used to provide blocs whereas RepositoryProvider should only be used for repositories.

RepositoryProvider(
  create: (context) => RepositoryA(),
  child: ChildA(),
);

then from ChildA we can retrieve the Repository instance with:

// with extensions
context.repository<RepositoryA>();

// without extensions
RepositoryProvider.of<RepositoryA>(context)

MultiRepositoryProvider is a Flutter widget that merges multiple RepositoryProvider widgets into one. MultiRepositoryProvider improves the readability and eliminates the need to nest multiple RepositoryProvider. By using MultiRepositoryProvider we can go from:

RepositoryProvider<RepositoryA>(
  create: (context) => RepositoryA(),
  child: RepositoryProvider<RepositoryB>(
    create: (context) => RepositoryB(),
    child: RepositoryProvider<RepositoryC>(
      create: (context) => RepositoryC(),
      child: ChildA(),
    )
  )
)

to:

MultiRepositoryProvider(
  providers: [
    RepositoryProvider<RepositoryA>(
      create: (context) => RepositoryA(),
    ),
    RepositoryProvider<RepositoryB>(
      create: (context) => RepositoryB(),
    ),
    RepositoryProvider<RepositoryC>(
      create: (context) => RepositoryC(),
    ),
  ],
  child: ChildA(),
)

Gallery

Examples

Download Flutter Bloc Widget source code on GitHub

https://github.com/felangel/bloc/tree/master/packages/flutter_bloc

Check out the Flutter Bloc Package implementation guide on Pub.dev

https://pub.dev/packages/flutter_bloc

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