如何解决如何将数据发送到从 UI
好吧,我被这个问题困住了。我有一个 audioservice
(audioplayer.dart
) 的代码,它需要一个 queue
才能播放。我正在使用 playlist.dart
从 audioplayer.dart
中的 ModalRoute
获取队列并保存在全局变量 queue
中。然后,我初始化 AudioPlayerService。现在一切都很好,但是在扩展 AudioPlayerTask
的 BackgroundAudioTask
类中,当我尝试访问变量(在 onStart
中)时,它出现的是一个空列表。我不知道问题出在哪里,而且我对 BackgroundAudioTask
类不太熟悉。这是它的样子:
import .....
List<MediaItem> queue = [];
class TempScreen extends StatefulWidget {
@override
_TempScreenState createState() => _TempScreenState();
}
class _TempScreenState extends State<TempScreen> {
@override
Widget build(BuildContext context) {
queue = ModalRoute.of(context).settings.arguments;
// Now HERE THE QUEUE IS FINE
return Container(.....all ui code);
}
// I'm using this button to start the service
audioPlayerButton() {
AudioService.start(
backgroundTaskEntrypoint: _audioPlayerTaskEntrypoint,androidNotificationChannelName: 'Audio Service Demo',androidNotificationColor: 0xFF2196f3,androidNotificationIcon: 'mipmap/ic_launcher',androidEnableQueue: true,);
AudioService.updateQueue(queue);
print('updated queue at the start');
print('queue Now is $queue');
AudioService.setRepeatMode(AudioServiceRepeatMode.none);
AudioService.setShuffleMode(AudioServiceShuffleMode.none);
AudioService.play();
}
}
void _audioPlayerTaskEntrypoint() async {
AudioServiceBackground.run(() => AudioPlayerTask());
}
class AudioPlayerTask extends BackgroundAudioTask {
AudioPlayer _player = AudioPlayer();
Seeker _seeker;
StreamSubscription<PlaybackEvent> _eventSubscription;
String kUrl = '';
String key = "38346591";
String decrypt = "";
String preferredQuality = '320';
int get index => _player.currentIndex == null ? 0 : _player.currentIndex;
MediaItem get mediaItem => index == null ? queue[0] : queue[index];
// This is just a function i'm using to get song URLs
fetchSongUrl(songId) async {
print('starting fetching url');
String songUrl =
"https://www.jiosaavn.com/api.PHP?app_version=5.18.3&api_version=4&readable_version=5.18.3&v=79&_format=json&__call=song.getDetails&pids=" +
songId;
var res = await get(songUrl,headers: {"Accept": "application/json"});
var resEdited = (res.body).split("-->");
var getMain = jsonDecode(resEdited[1]);
kUrl = await DesPlugin.decrypt(
key,getMain[songId]["more_info"]["encrypted_media_url"]);
kUrl = kUrl.replaceAll('96','$preferredQuality');
print('fetched url');
return kUrl;
}
@override
Future<void> onStart(Map<String,dynamic> params) async {
print('inside onStart of audioPlayertask');
print('queue Now is $queue');
// Now HERE QUEUE COMES OUT TO BE AN EMPTY LIST
final session = await AudioSession.instance;
await session.configure(AudioSessionConfiguration.speech());
if (queue.length == 0) {
print('queue is found to be null.........');
}
_player.currentIndexStream.listen((index) {
if (index != null) AudioServiceBackground.setMediaItem(queue[index]);
});
// Propagate all events from the audio player to AudioService clients.
_eventSubscription = _player.playbackEventStream.listen((event) {
_broadcastState();
});
// Special processing for state transitions.
_player.processingStateStream.listen((state) {
switch (state) {
case ProcessingState.completed:
AudioService.currentMediaItem != queue.last
? AudioService.skipToNext()
: AudioService.stop();
break;
case ProcessingState.ready:
break;
default:
break;
}
});
// Load and broadcast the queue
print('queue is');
print(queue);
print('Index is $index');
print('MediaItem is');
print(queue[index]);
try {
if (queue[index].extras == null) {
queue[index] = queue[index].copyWith(extras: {
'URL': await fetchSongUrl(queue[index].id),});
}
await AudioServiceBackground.setQueue(queue);
await _player.setUrl(queue[index].extras['URL']);
onPlay();
} catch (e) {
print("Error: $e");
onStop();
}
}
@override
Future<void> onSkipToQueueItem(String mediaId) async {
// Then default implementations of onSkipToNext and onSkipToPrevIoUs will
// delegate to this method.
final newIndex = queue.indexWhere((item) => item.id == mediaId);
if (newIndex == -1) return;
_player.pause();
if (queue[newIndex].extras == null) {
queue[newIndex] = queue[newIndex].copyWith(extras: {
'URL': await fetchSongUrl(queue[newIndex].id),});
await AudioServiceBackground.setQueue(queue);
// AudioService.updateQueue(queue);
}
await _player.setUrl(queue[newIndex].extras['URL']);
_player.play();
await AudioServiceBackground.setMediaItem(queue[newIndex]);
}
@override
Future<void> onUpdateQueue(List<MediaItem> queue) {
AudioServiceBackground.setQueue(queue = queue);
return super.onUpdateQueue(queue);
}
@override
Future<void> onPlay() => _player.play();
@override
Future<void> onPause() => _player.pause();
@override
Future<void> onSeekTo(Duration position) => _player.seek(position);
@override
Future<void> onFastForward() => _seekRelative(fastForwardInterval);
@override
Future<void> onRewind() => _seekRelative(-rewindInterval);
@override
Future<void> onSeekForward(bool begin) async => _seekContinuously(begin,1);
@override
Future<void> onSeekBackward(bool begin) async => _seekContinuously(begin,-1);
@override
Future<void> onStop() async {
await _player.dispose();
_eventSubscription.cancel();
await _broadcastState();
// Shut down this task
await super.onStop();
}
Future<void> _seekRelative(Duration offset) async {
var newPosition = _player.position + offset;
// Make sure we don't jump out of bounds.
if (newPosition < Duration.zero) newPosition = Duration.zero;
if (newPosition > mediaItem.duration) newPosition = mediaItem.duration;
// Perform the jump via a seek.
await _player.seek(newPosition);
}
void _seekContinuously(bool begin,int direction) {
_seeker?.stop();
if (begin) {
_seeker = Seeker(_player,Duration(seconds: 10 * direction),Duration(seconds: 1),mediaItem)
..start();
}
}
/// broadcasts the current state to all clients.
Future<void> _broadcastState() async {
await AudioServiceBackground.setState(
controls: [
MediaControl.skipToPrevIoUs,if (_player.playing) MediaControl.pause else MediaControl.play,MediaControl.stop,MediaControl.skipToNext,],systemActions: [
MediaAction.seekTo,MediaAction.seekForward,MediaAction.seekBackward,androidCompactActions: [0,1,3],processingState: _getProcessingState(),playing: _player.playing,position: _player.position,bufferedPosition: _player.bufferedPosition,speed: _player.speed,);
}
AudioProcessingState _getProcessingState() {
switch (_player.processingState) {
case ProcessingState.idle:
return AudioProcessingState.stopped;
case ProcessingState.loading:
return AudioProcessingState.connecting;
case ProcessingState.buffering:
return AudioProcessingState.buffering;
case ProcessingState.ready:
return AudioProcessingState.ready;
case ProcessingState.completed:
return AudioProcessingState.completed;
default:
throw Exception("Invalid state: ${_player.processingState}");
}
}
}
This 是 AudioService 的完整代码,以防万一。
解决方法
audio_service 在单独的隔离中运行您的 BackgroundAudioTask
。在README中是这样写的:
请注意,您的 UI 和后台任务在单独的隔离区中运行,并且不共享内存。他们交流的唯一方式是通过消息传递。您的 Flutter UI 将仅使用 AudioService
API 与后台任务进行通信,而您的后台任务将仅使用 AudioServiceBackground
API 与 UI 和其他客户端进行交互。
这里的关键是隔离不共享内存。如果在 UI 隔离中设置“全局”变量,则不会在后台隔离中设置它,因为后台隔离有自己单独的内存块。这就是为什么您的全局 queue
变量为 null。它实际上不是同一个变量,因为现在你实际上有两个变量副本:一个在 UI 隔离中已经设置了一个值,另一个在后台隔离中还没有设置一个值.
现在,您的后台隔离会“稍后”将其自己的队列变量副本设置为某些内容,这是通过消息传递 API 实现的,您可以在该 API 中将队列从 UI 隔离传递到 updateQueue
和后台隔离接收该消息并将其存储到 onUpdateQueue
中变量的自己的副本中。如果您在此之后打印出队列,它将不再为空。
在您的 onStart
中还有一行是您尝试设置队列的地方,尽管您可能应该删除该代码并让队列仅在 onUpdateQueue
中设置。您不应尝试访问 onStart
中的队列,因为您的队列直到 onUpdateQueue
才会收到其值。如果您想在设置之前避免任何空指针异常,您可以将后台隔离中的队列初始化为一个空列表,它最终将被 onUpdateQueue
中的非空列表替换而不会为空。
我还建议您避免将 queue
设为全局变量。全局变量通常很糟糕,但在这种情况下,实际上可能会让您误以为队列变量在 UI 和后台隔离中是相同的,而实际上每个隔离都有自己的变量副本,可能具有不同的值。因此,如果您创建两个单独的“本地”变量,您的代码会更清晰。一个在 UI 中,一个在后台任务中。
另外一个建议是,您应该注意消息传递 API 中的方法是异步方法。您应该等待音频服务启动,然后再向其发送消息,例如设置队列。并且您应该等待队列设置,然后再尝试从队列中播放:
await AudioService.start(....);
// Now the service has started,it is safe to send messages.
await AudioService.updateQueue(...);
// Now the queue has been updated,it is safe to play from it.
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