combine_latest_bloc.dart

import 'dart:async';

import 'package:bloc/bloc.dart';
import 'package:flutter/material.dart';

typedef CombineLatestBlocWidgetBuilder<S1, S2> = Widget Function(BuildContext context, S1 state1, S2 state2);

class CombineLatestBlocBuilder<E1, S1, E2, S2> extends StatefulWidget {

  final Bloc<E1, S1> bloc1;
  final Bloc<E2, S2> bloc2;
  final CombineLatestBlocWidgetBuilder<S1, S2> builder;

  const CombineLatestBlocBuilder({Key key, this.bloc1, this.bloc2, this.builder}) : super(key: key);

  Widget build(BuildContext context, S1 state1, S2 state2) => builder(context, state1, state2);

  @override
  _CombineLatestBlocBuilderState<E1, S1, E2, S2> createState() => _CombineLatestBlocBuilderState<E1, S1, E2, S2>();
}

class _CombineLatestBlocBuilderState<E1, S1, E2, S2> extends State<CombineLatestBlocBuilder<E1, S1, E2, S2>> {

  StreamSubscription<S1> _subscription1;
  StreamSubscription<S2> _subscription2;
  S1 _state1;
  S2 _state2;

  @override
  void initState() {
    super.initState();
    _state1 = widget.bloc1.currentState;
    _subscribe1();
    _state2 = widget.bloc2.currentState;
    _subscribe2();
  }

  @override
  void didUpdateWidget(CombineLatestBlocBuilder<E1, S1, E2, S2> oldWidget) {
    super.didUpdateWidget(oldWidget);
    if (oldWidget.bloc1.state != widget.bloc1.state) {
      if (_subscription1 != null) {
        _subscription1.cancel();
        _state1 = widget.bloc1.currentState;
      }
      _subscribe1();
    }
    if (oldWidget.bloc2.state != widget.bloc2.state) {
      if (_subscription2 != null) {
        _subscription2.cancel();
        _state2 = widget.bloc2.currentState;
      }
      _subscribe2();
    }
  }

  @override
  Widget build(BuildContext context) => widget.build(context, _state1, _state2);

  @override
  void dispose() {
    _unsubscribe1();
    _unsubscribe2();
    super.dispose();
  }

  void _subscribe1() {
    if (widget.bloc1.state != null) {
      _subscription1 = widget.bloc1.state.skip(1).listen((S1 state) {
        setState(() {
          _state1 = state;
        });
      });
    }
  }

  void _subscribe2() {
    if (widget.bloc2.state != null) {
      _subscription2 = widget.bloc2.state.skip(1).listen((S2 state) {
        setState(() {
          _state2 = state;
        });
      });
    }
  }

  void _unsubscribe1() {
    if (_subscription1 != null) {
      _subscription1.cancel();
      _subscription1 = null;
    }
  }

  void _unsubscribe2() {
    if (_subscription2 != null) {
      _subscription2.cancel();
      _subscription2 = null;
    }
  }
}