just_audio/example/lib/main.dart

import 'dart:math';

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

import 'package:flutter/services.dart';
import 'package:just_audio/just_audio.dart';

void main() => runApp(MyApp());

class MyApp extends StatefulWidget {
  @override
  _MyAppState createState() => _MyAppState();
}

class _MyAppState extends State<MyApp> {
  final AudioPlayer _player = AudioPlayer();

  @override
  void initState() {
    super.initState();
    _player.setUrl(
        "https://s3.amazonaws.com/scifri-episodes/scifri20181123-episode.mp3");
  }

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

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(
          title: const Text('Audio Player Demo'),
        ),
        body: Center(
          child: Column(
            crossAxisAlignment: CrossAxisAlignment.center,
            mainAxisAlignment: MainAxisAlignment.center,
            children: [
              Text("Science Friday"),
              Text("Science Friday and WNYC Studios"),
              StreamBuilder<AudioPlaybackState>(
                stream: _player.playbackStateStream,
                builder: (context, snapshot) {
                  final state = snapshot.data;
                  return Row(
                    mainAxisSize: MainAxisSize.min,
                    children: [
                      if (state == AudioPlaybackState.playing)
                        IconButton(
                          icon: Icon(Icons.pause),
                          iconSize: 64.0,
                          onPressed: _player.pause,
                        )
                      else if (state == AudioPlaybackState.buffering ||
                          state == AudioPlaybackState.connecting)
                        Container(
                          margin: EdgeInsets.all(8.0),
                          width: 64.0,
                          height: 64.0,
                          child: CircularProgressIndicator(),
                        )
                      else
                        IconButton(
                          icon: Icon(Icons.play_arrow),
                          iconSize: 64.0,
                          onPressed: _player.play,
                        ),
                      IconButton(
                        icon: Icon(Icons.stop),
                        iconSize: 64.0,
                        onPressed: state == AudioPlaybackState.stopped ||
                                state == AudioPlaybackState.none
                            ? null
                            : _player.stop,
                      ),
                    ],
                  );
                },
              ),
              StreamBuilder<Duration>(
                stream: _player.durationStream,
                builder: (context, snapshot) {
                  final duration = snapshot.data ?? Duration.zero;
                  return StreamBuilder<Duration>(
                    stream: _player.getPositionStream(),
                    builder: (context, snapshot) {
                      final position = snapshot.data ?? Duration.zero;
                      return SeekBar(
                        duration: duration,
                        position: position,
                        onChangeEnd: (newPosition) {
                          _player.seek(newPosition);
                        },
                      );
                    },
                  );
                },
              ),
            ],
          ),
        ),
      ),
    );
  }
}

class SeekBar extends StatefulWidget {
  final Duration duration;
  final Duration position;
  final ValueChanged<Duration> onChanged;
  final ValueChanged<Duration> onChangeEnd;

  SeekBar({
    @required this.duration,
    @required this.position,
    this.onChanged,
    this.onChangeEnd,
  });

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

class _SeekBarState extends State<SeekBar> {
  double _dragValue;

  @override
  Widget build(BuildContext context) {
    return Slider(
      min: 0.0,
      max: widget.duration.inMilliseconds.toDouble(),
      value: _dragValue ?? widget.position.inMilliseconds.toDouble(),
      onChanged: (value) {
        setState(() {
          _dragValue = value;
        });
        if (widget.onChanged != null) {
          widget.onChanged(Duration(milliseconds: value.round()));
        }
      },
      onChangeEnd: (value) {
        _dragValue = null;
        if (widget.onChangeEnd != null) {
          widget.onChangeEnd(Duration(milliseconds: value.round()));
        }
      },
    );
  }
}