BehaviorStreamBuilder, no more flicker because of initialData ( https://twitter.com/filiphracek/status/1050798900968181761 )

BehaviorStreamBuilder.dart

class BehaviorStreamBuilder<T> extends StreamBuilder<T>
{ 
  BehaviorStreamBuilder({
    Key key,
    BehaviorSubject<T> stream,
    @required AsyncWidgetBuilder<T> builder
  }) : assert(builder != null),
    super
    (
      key: key, 
      stream: stream.stream
        .distinct() // next values won't redraw until different
        .skip(1) // skips the first value from behavior, it's the same as initialData
        ,
      initialData: stream.value,
      builder: builder, 
    );
}

usage.dart

int _counter = 0;
var _bloc = MyBloc();

//
  BehaviorStreamBuilder<String>(
    stream: _bloc.letterSubject,
    builder: (c, s) => Text("${s.data} #${(++_counter).toString()}"),
  )
//
  
class MyBloc
{
  var letterSubject = BehaviorSubject<String>.seeded("A");

  MyBloc(){
    // wont redraw cause value is the same
    Observable.timer("A", Duration(seconds: 2)).listen(letterSubject.add);
    // will redraw because new value
    Observable.timer("B", Duration(seconds: 5)).listen(letterSubject.add);
  }
}