Flutter error: the getter 'AudioPlayerState' isn't defined for the class '_HomePageState'

I have imported audioplayer.dart but it is still showing me the error: the getter 'AudioPlayerState' isn't defined for the class '_HomePageState'

import 'package:audioplayers/audioplayers.dart';
_audioPlayer.onPlayerStateChanged.listen((event) {
  if (event == AudioPlayerState.PLAYING) {
    _isPlaying = true;
  } else {
    _isPlaying = false;
  }
  setState(() {});
});