Constructor for QR scanner example

MainPage.cs

        private readonly DispatcherTimer _timer;
        private readonly ObservableCollection<string> _matches;

        private PhotoCameraLuminanceSource _luminance;
        private QRCodeReader _reader;
        private PhotoCamera _photoCamera;
        
        public MainPage()
        {            
            InitializeComponent();

            _matches = new ObservableCollection<string>();
            _matchesList.ItemsSource = _matches;

            _timer = new DispatcherTimer();
            _timer.Interval = TimeSpan.FromMilliseconds(250);
            _timer.Tick += (o, arg) => ScanPreviewBuffer();
        }

        protected override void OnNavigatedTo(NavigationEventArgs e)
        {
            _photoCamera = new PhotoCamera();
            _photoCamera.Initialized += OnPhotoCameraInitialized;            
            _previewVideo.SetSource(_photoCamera);

            CameraButtons.ShutterKeyHalfPressed += (o, arg) => _photoCamera.Focus();

            base.OnNavigatedTo(e);
        }