main.dart

  void main() => runApp(
    ChangeNotifierProvider<VolumeManager>(
      create: (_) => VolumeManager(),
      child: DemoApp(),
    )
  );

  class DemoApp extends StatelessWidget {
    const DemoApp();

    @override
    Widget build(BuildContext context) {
      return MaterialApp(
        home: Scaffold(
          appBar: AppBar(
            title: const Text("Play a video"),
          ),
          body: const Center(
            child: VideoWidget(),
          )
        ),
      );
    }
  }
  

volume_manager.dart

  class VolumeManager with ChangeNotifier {
    double _volume = 50.0;
    double get volume => _volume;

    void setVolume({
      required double volumeValue, 
      required VideoPlayerController controller
    }) {
      _volume = volumeValue;
      controller.setVolume(_volume);

      notifyListeners();
    }
  }
  

video_widget.dart

  class VideoWidget extends StatefulWidget {
    const VideoWidget();

    @override
    _VideoWidgetState createState() => _VideoWidgetState();
  }

  class _VideoWidgetState extends State<VideoWidget> {
    late final VideoPlayerController controller;
    late final Future<void> initVideo;

    @override
    void initState() {
      super.initState();

      controller = VideoPlayerController.asset(
        "assets/butterfly.mp4"
      );

      controller.setLooping(true);
      initVideo = controller.initialize();
    }

    @override
    void dispose() {
      controller.dispose();
      super.dispose();
    }

    @override
    Widget build(BuildContext context) {
      return FutureBuilder(
        future: initVideo,
        builder: (context, snapshot) {
          if (snapshot.connectionState == ConnectionState.done) {
            return PlayWidget(controller);
          }

          return const Center(
            child: CircularProgressIndicator(),
          );
        },
      );
    }
  }
  

video_play_widget.dart

  class PlayWidget extends StatelessWidget {
    final VideoPlayerController controller;
    const PlayWidget(this.controller);

    @override
    Widget build(BuildContext context) {
      return Column(
        mainAxisAlignment: MainAxisAlignment.spaceAround,
        children: <Widget>[
          AspectRatio(
            aspectRatio: controller.value.aspectRatio,
            child: VideoPlayer(controller),
          ),

          Row(
            mainAxisAlignment: MainAxisAlignment.spaceAround,
            children: <Widget>[
              RaisedButton(
                child: const Text("Play!"),
                onPressed: _play,
              ),

              RaisedButton(
                child: const Text("Pause"),
                onPressed: _pause,
              )
            ],
          ),

          Consumer<VolumeManager>(
            builder: (context, manager, _) {
              return Slider(
                value: manager.volume,
                onChanged: (value) {
                  manager.setVolume(
                    volumeValue: value,
                    controller: controller,
                  );
                },
                min: 0,
                max: 1,
                divisions: 10,
                label: "${manager.volume * 100}%",
              );
            },
          )
        ]
      );
    }

    void _play() {
      if (!controller.value.isPlaying)
        controller.play();
    }

    void _pause() {
      if (controller.value.isPlaying)
        controller.pause();
    }
  }
  

error_widget.dart

  class ErrorWidget extends StatelessWidget {
    const ErrorWidget();

    @override
    Widget build(BuildContext context) {
      return const Icon(
        Icons.error,
        size: 25,
        color: Colors.red,
      );
    }

  }