CategoriesPage.xaml.cs

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using Microsoft.Practices.ServiceLocation;
using PhotoSharingApp.Universal.Unity;
using PhotoSharingApp.Universal.ViewModels;
using Windows.Foundation;
using Windows.UI.Xaml;
using Windows.UI.Xaml.Navigation;

namespace PhotoSharingApp.Universal.Views
{
    /// <summary>
    /// The categories page that displays the most recent categories
    /// with photo thumbnails.
    /// </summary>
    public sealed partial class CategoriesPage : BasePage
    {
        private int _thumbnailImageSideLength;
        private CategoriesViewModel _viewModel;

        /// <summary>
        /// The constructor.
        /// </summary>
        public CategoriesPage()
        {
            InitializeComponent();

            UpdateThumbnailSize();

            SizeChanged += CategoriesPage_SizeChanged;

            // We want to scroll to the previously selected item,
            // so we need to watch the Loaded event as this is the
            // earliest time we can change the scroll position.
            categoriesList.Loaded += OnCategoriesListLoaded;
        }

        /// <summary>
        /// The side length of a thumbnail
        /// </summary>
        public int ThumbnailImageSideLength
        {
            get { return _thumbnailImageSideLength; }
            set
            {
                if (value != _thumbnailImageSideLength)
                {
                    _thumbnailImageSideLength = value;
                    NotifyPropertyChanged();
                }
            }
        }

        private void CategoriesPage_SizeChanged(object sender, SizeChangedEventArgs e)
        {
            UpdateThumbnailSize();
        }

        private void OnCategoriesListLoaded(object sender, RoutedEventArgs e)
        {
            // If there is a category selected already, we want the view to update
            // the scroll position to show it.
            if (_viewModel.SelectedCategoryPreview != null)
            {
                // MainScrollViewer control holds both the hero images and categories,
                // so we need to find the offset of the selected category in 
                // the parent ScrollViewer control.
                var container =
                    categoriesList.ContainerFromItem(_viewModel.SelectedCategoryPreview) as FrameworkElement;

                if (container != null)
                {
                    var focusedVisualTransform = container.TransformToVisual(mainScrollViewer);
                    var transformPoint = focusedVisualTransform.TransformPoint(new Point(0, 0));

                    // Now adjust the scroll position.
                    mainScrollViewer.ChangeView(null, transformPoint.Y, null, true);
                }
            }
        }

        protected override void OnNavigatedFrom(NavigationEventArgs e)
        {
            base.OnNavigatedFrom(e);

            _viewModel.StopHeroImageSlideShow();
        }

        protected override async void OnNavigatedTo(NavigationEventArgs e)
        {
            base.OnNavigatedTo(e);

            var loadData = e.NavigationMode != NavigationMode.Back;
            _viewModel = ServiceLocator.Current.GetInstance<CategoriesViewModel>(loadData);
            DataContext = _viewModel;

            if (loadData)
            {
                await _viewModel.LoadState();
            }

            _viewModel.StartHeroImageSlideShow();
        }

        private void UpdateThumbnailSize()
        {
            if (pageRoot.ActualWidth > 1300)
            {
                ThumbnailImageSideLength = 150;
            }
            else
            {
                ThumbnailImageSideLength = 100;
            }
        }
    }
}