StreamBuilderFiltered - For filtered and cast streams.

example.dart

abstract class MyBlocState{}
class Step1State extends MyBlocState{
  String title;
}
class Step2State extends MyBlocState{
  int progress;
}
class Step3State extends MyBlocState{
}


/// Example of showing only step2 details
example() {
  var widget= StreamBuilderFilter<Step2State>(
    noData: LinearProgressIndicator(),
    stream: myBloc.state,
    builder: (context, data) {
      return Text("Progress: ${data.progress}");
    },
  );
}

stream_builder_filtered.dart

/// I found useful to have this little function and build the StreamBuilderFilter around with it.
/// When ever BLoC state is returned as base class some views only react to some state and this helps to create filtered stream builder.

/// Data Widget builder to build widget with provided data type.
typedef DataWidgetBuilder<T> = Widget Function(BuildContext context, T data);
/// Stream builder with filtering by T type, or noData
class StreamBuilderFilter<T> extends StreamBuilder {

  /// [stream] - main stream that sends any type
  /// [noData] - widget returned on build when no data of desired type is streamed 
  /// [dataBuilder] - builder with the expected data type
  StreamBuilderFilter(
      {Key key, Stream stream, Widget noData, DataWidgetBuilder<T> dataBuilder})
      : super(key: key, stream:stream, builder: (c,s)=>_internalBuilder(c,s, dataBuilder, noData));

  /// Super constructor requires static method
  static Widget _internalBuilder<T>(BuildContext context, AsyncSnapshot snapshot, DataWidgetBuilder<T> dataBuilder, Widget noData) {
    if (snapshot.hasData && snapshot.data is T) {
      return dataBuilder(context, snapshot.data);
    } else {
      /// show noData widget or empty container
      return noData??Container();
    }
  }
}