flutter camera パッケージの Example を読んでいく

flutter pub add camera
flutter pub add video_player

class _CameraExampleHomeState extends State<CameraExampleHome>
    with WidgetsBindingObserver, TickerProviderStateMixin {...}

  @override
  void didChangeAppLifecycleState(AppLifecycleState state) {
    final CameraController? cameraController = controller;

    // App state changed before we got the chance to initialize.
    if (cameraController == null || !cameraController.value.isInitialized) {
      return;
    }

    // 非アクティブになったらコントローラを破棄
    if (state == AppLifecycleState.inactive) {
      cameraController.dispose();
    } else if (state == AppLifecycleState.resumed) {
      // 再開したらコントローラを初期化（disposeされたらdescriptionにアクセスできないのでは...?）
      _initializeCameraController(cameraController.description);
    }
  }

  Future<void> _initializeCameraController(
      CameraDescription cameraDescription) async {
    final CameraController cameraController = CameraController(
      cameraDescription,
      kIsWeb ? ResolutionPreset.max : ResolutionPreset.medium,
      enableAudio: enableAudio,
      imageFormatGroup: ImageFormatGroup.jpeg, // 指定できるんか、知らんかった。
    );

    controller = cameraController;

    // カメラコントローラが変更されたら発火する
    // If the controller is updated then update the UI.
    cameraController.addListener(() {
      // mounted == false の時に setStateを呼び出すとエラーになる為
     // https://api.flutter.dev/flutter/widgets/State/mounted.html
      if (mounted) {
        // UI 更新
        setState(() {});
      }
     // エラーならスナックバーを表示
      if (cameraController.value.hasError) { 
        showInSnackBar(
            'Camera error ${cameraController.value.errorDescription}');
      }
    });

    try {
      await cameraController.initialize();
      // 非同期実行。 [] の処理が全部終わるまで待機。
      await Future.wait(<Future<Object?>>[
        // The exposure mode is currently not supported on the web.
        ...!kIsWeb
            ? <Future<Object?>>[
                // 露出オフセットのmin-maxを更新
                cameraController.getMinExposureOffset().then(
                    (double value) => _minAvailableExposureOffset = value),
                cameraController
                    .getMaxExposureOffset()
                    .then((double value) => _maxAvailableExposureOffset = value)
              ]
            : <Future<Object?>>[],
        // zoomのmin-maxを更新
        cameraController
            .getMaxZoomLevel()
            .then((double value) => _maxAvailableZoom = value),
        // zoomのmin-maxを更新
            .getMinZoomLevel()
            .then((double value) => _minAvailableZoom = value),
      ]);
    } on CameraException catch (e) {
      switch (e.code) {
        case 'CameraAccessDenied':
          showInSnackBar('You have denied camera access.');
        case 'CameraAccessDeniedWithoutPrompt':
          // iOS only
          showInSnackBar('Please go to Settings app to enable camera access.');
        case 'CameraAccessRestricted':
          // iOS only
          showInSnackBar('Camera access is restricted.');
        case 'AudioAccessDenied':
          showInSnackBar('You have denied audio access.');
        case 'AudioAccessDeniedWithoutPrompt':
          // iOS only
          showInSnackBar('Please go to Settings app to enable audio access.');
        case 'AudioAccessRestricted':
          // iOS only
          showInSnackBar('Audio access is restricted.');
        default:
          _showCameraException(e);
          break;
      }
    }

    if (mounted) {
      setState(() {}); // 初期化が終わったらUI更新する
    }
  }

  /// Releases the resources of this camera.
  @override
  Future<void> dispose() async {
    if (_isDisposed) {
      return;
    }
    _unawaited(_deviceOrientationSubscription?.cancel());
    _isDisposed = true;
    super.dispose();
    if (_initializeFuture != null) {
      await _initializeFuture;
      await CameraPlatform.instance.dispose(_cameraId);
    }
  }

This app has crashed because it attempted to access privacy-sensitive data without a usage description.  The app's Info.plist must contain an NSCameraUsageDescription key with a string value explaining to the user how the app uses this data.

<key>NSCameraUsageDescription</key>
<string>your usage description here</string>
<key>NSMicrophoneUsageDescription</key>
<string>your usage description here</string>