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社区首页 >问答首页 >如果我在我的just_audio中显示超过10种声音，颤振ListView包将抛出空异常。

问如果我在我的just_audio中显示超过10种声音，颤振ListView包将抛出空异常。
EN

Stack Overflow用户
提问于 2022-02-15 23:02:35
回答 1查看 311关注 0票数 0

我正在尝试构建一个简单的应用程序，在资产文件夹中播放音频文件。

我正在使用一个流生成器返回一个卡片为每个声音，以显示暂停和播放图标。但奇怪的是，当我在声音模型中添加超过10种声音时，我的应用程序会抛出一个空错误。

这里是我的声音模型类:

代码语言：javascript
复制
class CoughViewModel extends ChangeNotifier{

  final List<SoundModel> _soundsList = [
    SoundModel(soundIcon: 'assets/vectors/death_bed.svg', soundName: 'DEATH BED', soundPath: 'assets/sounds/cough_sounds/death_bed.mp3'),
  ];

  List<SoundModel> get soundsList => _soundsList;
}

这里是我用来显示声卡的代码:

代码语言：javascript
复制
    Expanded(
      child: ListView.builder(
          physics: const BouncingScrollPhysics(),
          itemCount: soundModel.length,
          itemBuilder: (BuildContext context,int index){
          return SoundCard(soundTitle: soundModel[index].soundName,soundPath: soundModel[index].soundPath,cardPicture: soundModel[index].soundIcon,);
      }),
    ),

这里是我的流生成器:

代码语言：javascript
复制
return StreamBuilder<PlayerState>(
      stream: player.playerStateStream,
      builder: (context, snapshot) {
        print("STATEOFPLAYER : ${player.playerStateStream}");
        final playerState = snapshot.data;
        if(snapshot.hasData){
          return _cardReturner(playerState!);
        }
        else{
          return const CircularProgressIndicator(
            color: MyColors.color3,
          );
        }
      },
    );

这里是我的小部件返回函数:

代码语言：javascript
复制
  Widget _cardReturner(PlayerState playerState) {
    final processingState = playerState.processingState;
    if (player.playing != true) {
      return soundCard(isPlaying: false);
    } else if (processingState != ProcessingState.completed) {
      return soundCard(isPlaying: true);
    } else {
      return soundCard(isPlaying: false);
    }
  }

由于一些奇怪的原因，当模型中的声音数增加到11时，我的player.playerStateStream's playerState就会变为空。

如果我手动将ListView.Builder的itemCount设置为10，则应用程序运行时没有问题。但一旦人数超过，我就得到一个例外

​

​

​

​

，这里还有我的soundCard代码

代码语言：javascript
复制
class SoundCard extends StatefulWidget {
  final String? cardPicture;
  final String soundTitle;
  final String soundPath;

  const SoundCard({Key? key,this.cardPicture,required this.soundTitle,required this.soundPath}) : super(key: key);

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

class _SoundCardState extends State<SoundCard> {
  final player = AudioPlayer();
  final Icon playIcon = const Icon(
    Icons.play_arrow,
    color: MyColors.color3,
  );
  final Icon pauseIcon = const Icon(
    Icons.pause,
    color: MyColors.color3,
  );

  final String playString = 'TAP TO PLAY';
  final String pauseString = 'TAP TO PAUSE';

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

  @override
  void initState() {
    player.setAsset(widget.soundPath);
    super.initState();
  }

  @override
  Widget build(BuildContext context) {
    ScreenUtil.setContext(context);
    return StreamBuilder<PlayerState>(
      stream: player.playerStateStream,
      builder: (context, snapshot) {
        print("STATEOFPLAYER : ${player.playerStateStream}");
        final playerState = snapshot.data;
        if(snapshot.hasData){
          return _cardReturner(playerState!);
        }
        else{
          return const CircularProgressIndicator(
            color: MyColors.color3,
          );
        }
      },
    );
  }

  Widget _cardReturner(PlayerState playerState) {
    final processingState = playerState.processingState;
    if (player.playing != true) {
      return soundCard(isPlaying: false);
    } else if (processingState != ProcessingState.completed) {
      return soundCard(isPlaying: true);
    } else {
      return soundCard(isPlaying: false);
    }
  }

  Widget soundCard({required bool isPlaying}) {
    return Container(
      margin: EdgeInsets.only(left:25.w,right: 25.w,top: 10.h,bottom: 10.h),
      decoration: BoxDecoration(
          color: MyColors.color2,
          borderRadius: BorderRadius.all(Radius.circular(10.r))),
      child: GestureDetector(
        onTap: () async {
          if (isPlaying) {
            player.stop();
          } else {
            player.setAsset(widget.soundPath);
            player.play();
          }
        },
        child: ListTile(
          leading: CircleAvatar(
            child: SvgPicture.asset(
              widget.cardPicture??'assets/vectors/sound_icon_cough.svg',
              height: 30.h,
            ),
            backgroundColor: MyColors.color1,
          ),
          title: Text(
            widget.soundTitle,
            style: MyTextStyles.bold15.copyWith(color: MyColors.color3),
          ),
          subtitle: Text(
            isPlaying ? pauseString : playString,
            style: MyTextStyles.medium15.copyWith(color: MyColors.color1),
          ),
          trailing: isPlaying ? pauseIcon : playIcon,
        ),
      ),
    );
  }
}

STACKTRACE

代码语言：javascript
复制
flutter: STATEOFPLAYER : Instance of 'BehaviorSubject<PlayerState>'

======== Exception caught by widgets library =======================================================
The following _CastError was thrown building StreamBuilder<PlayerState>(dirty, state: _StreamBuilderBaseState<PlayerState, AsyncSnapshot<PlayerState>>#075ca):
Null check operator used on a null value

The relevant error-causing widget was: 
  StreamBuilder<PlayerState> StreamBuilder:file:///Users/junaidtariq/Documents/Documents/AndroidStudioProjects/the_hoax_flu/lib/global_widgets.dart:141:12
When the exception was thrown, this was the stack: 
#0      Element.widget (package:flutter/src/widgets/framework.dart:3203:31)
#1      debugCheckHasMediaQuery.<anonymous closure> (package:flutter/src/widgets/debug.dart:229:17)
#2      debugCheckHasMediaQuery (package:flutter/src/widgets/debug.dart:245:4)
#3      MediaQuery.of (package:flutter/src/widgets/media_query.dart:859:12)
#4      ScreenUtil.screenWidth (package:flutter_screenutil/screen_util.dart:69:51)
#5      ScreenUtil.scaleWidth (package:flutter_screenutil/screen_util.dart:88:28)
#6      ScreenUtil.setWidth (package:flutter_screenutil/screen_util.dart:101:41)
#7      SizeExtension.w (package:flutter_screenutil/size_extension.dart:5:32)
#8      _SoundCardState.soundCard (package:the_hoax_flu/global_widgets.dart:171:39)
#9      _SoundCardState._cardReturner (package:the_hoax_flu/global_widgets.dart:161:14)
#10     _SoundCardState.build.<anonymous closure> (package:the_hoax_flu/global_widgets.dart:147:18)
#11     StreamBuilder.build (package:flutter/src/widgets/async.dart:442:81)
#12     _StreamBuilderBaseState.build (package:flutter/src/widgets/async.dart:124:48)
#13     StatefulElement.build (package:flutter/src/widgets/framework.dart:4870:27)
#14     ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:4754:15)
#15     StatefulElement.performRebuild (package:flutter/src/widgets/framework.dart:4928:11)
#16     Element.rebuild (package:flutter/src/widgets/framework.dart:4477:5)
#17     BuildOwner.buildScope (package:flutter/src/widgets/framework.dart:2659:19)
#18     WidgetsBinding.drawFrame (package:flutter/src/widgets/binding.dart:882:21)
#19     RendererBinding._handlePersistentFrameCallback (package:flutter/src/rendering/binding.dart:363:5)
#20     SchedulerBinding._invokeFrameCallback (package:flutter/src/scheduler/binding.dart:1144:15)
#21     SchedulerBinding.handleDrawFrame (package:flutter/src/scheduler/binding.dart:1081:9)
#22     SchedulerBinding._handleDrawFrame (package:flutter/src/scheduler/binding.dart:995:5)
#26     _invoke (dart:ui/hooks.dart:151:10)
#27     PlatformDispatcher._drawFrame (dart:ui/platform_dispatcher.dart:308:5)
#28     _drawFrame (dart:ui/hooks.dart:115:31)
(elided 3 frames from dart:async)
====================================================================================================

我正处于最后期限，因为我必须把这个应用交付给客户。任何帮助都是非常感谢的，

flutter
dart
flutter-dependencies
just-audio
EN

回答 1

Stack Overflow用户

回答已采纳

发布于 2022-02-16 00:51:58

在调用.w时访问25.w扩展。只有当ScreenUtil中的上下文是最新的时，它才能使用过时的上下文(从上一次构建开始)。

build回调StreamBuilder独立于小部件build函数进行更新。

StreamBuilder构建函数被更频繁地调用。因此，您必须再次调用ScreenUtil.setContext(context);来更新BuildContext。

代码语言：javascript
复制
  @override
  Widget build(BuildContext context) {
    ScreenUtil.setContext(context);
    return StreamBuilder<PlayerState>(
      stream: player.playerStateStream,
      builder: (context, snapshot) {

        // Update context
        ScreenUtil.setContext(context);

        print("STATEOFPLAYER : ${player.playerStateStream}");
        final playerState = snapshot.data;
        if (snapshot.hasData){
          return _cardReturner(playerState!);
        } else {
          return const CircularProgressIndicator(
            color: MyColors.color3,
          );
        }
      },
    );
  }
票数 2
EN
页面原文内容由Stack Overflow提供。腾讯云小微IT领域专用引擎提供翻译支持
原文链接：

https://stackoverflow.com/questions/71134491

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