使用相机包授予权限会在出现异常时暂停调试器

Question

我使用相机包来实现简单的功能。我主要遵循包提供的示例。当我打开相机小组件页面时，程序包会自动提示提供相机和麦克风的权限。单击允许这两种权限后，调试器将暂停，但出现异常：

Exception has occurred.
FlutterError (A CameraController was used after being disposed.
Once you have called dispose() on a CameraController, it can no longer be used.).

这是所需的代码：

class CameraPage extends StatefulWidget {
  @override
  _CameraPageState createState() => _CameraPageState();
}

class _CameraPageState extends State<CameraPage>
    with WidgetsBindingObserver {
  CameraController _controller;
  List<CameraDescription> _availableCameras;
  ...

  @override
  void initState() {
    super.initState();
    WidgetsBinding.instance.addObserver(this);
    _initialize();
  }

  Future<void> _initialize() async {
    await _getCameras();
    _controller = CameraController(_availableCameras[0], ResolutionPreset.high);
    await _controller.initialize();
    if (!mounted) {
      return;
    }
    setState(() {});
  }

  @override
  void didChangeAppLifecycleState(AppLifecycleState state) {
    if (state == AppLifecycleState.inactive) {
      _controller?.dispose();
    } else if (state == AppLifecycleState.resumed) {
      if (_controller != null) {
        _setCurrentCamera(_controller.description);
      }
    }
  }

  @override
  void dispose() {
    WidgetsBinding.instance.removeObserver(this);
    _controller.dispose();
    super.dispose();
  }


  Future<List<CameraDescription>> _getCameras() async {
    List<CameraDescription> camDescriptions;
      camDescriptions = await availableCameras();

      _availableCameras = camDescriptions;
    return camDescriptions;
  }

  @override
  Widget build(BuildContext context) {
    ...
  }


  Future<void> _setCurrentCamera(CameraDescription cameraDescription) async {
    if (_controller != null) {
      await _controller.dispose();
    }
    _controller = CameraController(
      cameraDescription,
      ResolutionPreset.high,
      enableAudio: false,
    );

    // If the _controller is updated then update the UI.
    _controller.addListener(() {
      if (mounted) setState(() {});

      if (_controller.value.hasError) {
        print('Camera error ${_controller.value.errorDescription}');
      }
    });

    try {
      await _controller.initialize();
    } on CameraException catch (e) {
      _showCameraException(e);
    }

    if (mounted) {
      setState(() {});
    }
  }

  void _switchCamera() {
    if (_controller != null && !_controller.value.isRecordingVideo) {
      CameraLensDirection direction = _controller.description.lensDirection;
      CameraLensDirection required = direction == CameraLensDirection.front
          ? CameraLensDirection.back
          : CameraLensDirection.front;
      for (CameraDescription cameraDescription in _availableCameras) {
        if (cameraDescription.lensDirection == required) {
          _setCurrentCamera(cameraDescription);
          return;
        }
      }
    }
  }


  void _showCameraException(CameraException e) {
    String errorText = 'Error: ${e.code}\nError Message: ${e.description}';
    print(errorText);
  }

}

调试器在这里指出异常：

  Future<void> _initialize() async {
    await _getCameras();
    _controller = CameraController(_availableCameras[0], ResolutionPreset.high);
    //-------------HERE------------------
    await _controller.initialize();
    if (!mounted) {
      return;
    }
    setState(() {});
  }

一旦我恢复调试器并尝试再次打开此相机页面，就不再出现错误/异常。仅在第一次接受权限后才会发生。

Answer 1

也许真正的罪魁祸首是didChangeAppLifecycleState。

一旦您调用await _controller.initialize();并显示权限对话框，AppLifecycleState.inactive就会触发生命周期事件，并根据您在 中的代码处理当前控制器didChangeAppLifecycleState，因此当应用程序在授予权限后恢复并尝试继续时，它会抛出错误。

尝试删除

if (state == AppLifecycleState.inactive) {
  _controller?.dispose();
}

或者有一个局部变量来检查是否正在初始化并在初始化时忽略处理，例如

Future<void> _initialize() async {
  await _getCameras();
  _controller = CameraController(_availableCameras[0], ResolutionPreset.high);
  _initializing = true;
  await _controller.initialize();
  _initializing = false;
  if (!mounted) {
    return;
  }
  setState(() {});
}

并在didChangeAppLifecycleState

if (state == AppLifecycleState.inactive && !_initializing) {
  _controller?.dispose();
}

编辑：

可能是，我想我发现了这个问题，实际的问题正如didChangeAppLifecycleState预期的那样，如果结果属实，则正在处理中的if条款，如果不是，则只是处理任何活动控制器。因此，当您调用初始化并等待权限时，在权限 future 解析之前，正在由 处置。didChangeAppLifecycleState_controller_setCurrentCamera_controllerdidChangeAppLifecycleState

我的解决方案只需进行简单的更改即可工作。将您的更改initState为

@override
void initState() {
  super.initState();
  _initializing = true; // set to true
  WidgetsBinding.instance.addObserver(this);
  _initialize();
}

初始化后更改您的_initialize函数，_initializing = false例如，

Future<void> _initialize() async {
  await _getCameras();
  _controller = CameraController(_availableCameras[0],ResolutionPreset.high);
  await _controller.initialize();
  _initializing = false; // set to false
  if (!mounted) {
    return;
  }
  setState(() {});
}

和你didChangeAppLifecycleState的

@override
void didChangeAppLifecycleState(AppLifecycleState state) {
  if(_initializing){
    return;
  }
  if (state == AppLifecycleState.inactive) {
    _controller?.dispose();
  } else if (state == AppLifecycleState.resumed) {
    if (_controller != null) {
      _setCurrentCamera(_controller.description);
    }
  }
}

这样，如果_initializing == true您从未处置当前控制器。

希望有帮助！