FilePickerHelper.cs

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using System;
using System.Threading.Tasks;
using PhotoSharingApp.Portable.DataContracts;
using PhotoSharingApp.Portable.Extensions;
using Windows.Storage;
using Windows.Storage.Pickers;

namespace PhotoSharingApp.Universal.Storage
{
    /// <summary>
    /// Helper class which provides default settings for the file picker
    /// for opening photos.
    /// </summary>
    public class FilePickerHelper
    {
        /// <summary>
        /// Creates the file open picker with default parameters suitable
        /// for opening photos.
        /// </summary>
        public static FileOpenPicker CreateFileOpenPickerWithDefaultParams()
        {
            var openPicker = new FileOpenPicker
            {
                ViewMode = PickerViewMode.Thumbnail,
                SuggestedStartLocation = PickerLocationId.PicturesLibrary
            };

            openPicker.FileTypeFilter.Add(".jpg");
            openPicker.FileTypeFilter.Add(".jpeg");
            openPicker.FileTypeFilter.Add(".png");

            return openPicker;
        }

        /// <summary>
        /// Shows the file picker to let the user choose a single file.
        /// </summary>
        /// <exception cref="InvalidImageDimensionsException">Thrown when image is too small.</exception>
        /// <returns>The file that is chosen. Null, if user cancelled the dialog.</returns>
        public static async Task<StorageFile> PickSingleFile()
        {
            var openPicker = CreateFileOpenPickerWithDefaultParams();

            var file = await openPicker.PickSingleFileAsync();

            if (file != null)
            {
                var imageProperties = await file.Properties.GetImagePropertiesAsync();

                var sideLength = PhotoTypeContract.Standard.ToSideLength();

                if (imageProperties.Width < sideLength || imageProperties.Height < sideLength)
                {
                    throw new InvalidImageDimensionsException();
                }
            }

            return file;
        }
    }
}