# BlocListener

### BlocListener with automatically retrieved Bloc

```kotlin
@Composable
inline fun <reified BlocA : BlocBase<BlocAState>, BlocAState : Any>
BlocListener(
    blocTag: String? = null,
    noinline listenWhen: (@DisallowComposableCalls (previousState: BlocAState ?, currentState: BlocAState ) -> Boolean) = null,
    crossinline listener: @DisallowComposableCalls suspend (BlocAState) -> Unit
)
```

`BlocListener` handles retrieving a [`Bloc` ](https://beyondeye.gitbook.io/compose-bloc/bloc-documentation/bloc-and-cubit-overview/bloc)of the specified type from the registered Blocs in the current composable subtree (see [BlocProvider](https://beyondeye.gitbook.io/compose-bloc/bloc-documentation/blocs-and-compose-overview/blocprovider)) and start listening to the associated stream of Bloc state updates.&#x20;

the  `listener` function is invoked in response to `state` changes in the `Bloc`.

&#x20;Note that `listener` is not a Composable function: It is a (potentially) suspendable method, that is invoked with [LaunchedEffect](https://developer.android.com/jetpack/compose/side-effects#launchedeffect).&#x20;

The `listener` is guaranteed to be called only  once for each `state` change,  unlike the `content` method in [BlocBuilder](https://beyondeye.gitbook.io/compose-bloc/bloc-documentation/blocs-and-compose-overview/blocbuilder), that can be triggered because of recomposition.

An optional `blocTag` parameter can be specified in order to identify a specific bloc instance in the case where there is more than one instance of a bloc of same type registered for the current composable subtree (see [BlocProvider](https://beyondeye.gitbook.io/compose-bloc/bloc-documentation/blocs-and-compose-overview/blocprovider)).

&#x20;The `blocTag` parameter is not present in the original `flutter_bloc` implementation&#x20;

&#x20;An optional `listenWhen` parameter can be provided for more granular control over when `listener` is called.

&#x20;`listenWhen` takes the previous `state` and current `state` and must return a `Boolean` which determines whether or not the `listener` function will be invoked.

For the first call to `listenWhen` the previous `state` will be initialized to the `state` of the `Bloc` when the `BlocListener` was initialized.

### BlocListener for externally provided Bloc

```kotlin
@Composable
inline fun <reified BlocA : BlocBase<BlocAState>, BlocAState : Any>
BlocListener(
    externallyProvidedBlock: BlocA,
    noinline listenWhen: (@DisallowComposableCalls (previousState: BlocAState ?, currentState: BlocAState ) -> Boolean) = null,
    crossinline listener: @DisallowComposableCalls suspend (BlocAState) -> Unit
)
```

This is the same as the previous method but with an explicitly specified `Bloc` instance `externallyProvidedBlock` , not retrieved implicitly from current registered blocs in the current composable subtree.

&#x20;Use this method if for example you have retrieved the Bloc already with [`rememberProvidedBlocOf` ](https://beyondeye.gitbook.io/compose-bloc/bloc-documentation/blocprovider#rememberprovidedblocof)