LazyImageBehavior

LazyBitmapImage.cs

using System;
using System.IO;
using System.Net;
using System.Net.Cache;
using System.Threading.Tasks;
using System.Windows.Media.Imaging;

public static class LazyBitmapImage
{
    public static Task<BitmapImage> GetImage(Uri uri)
    {
        return Task.Run(() =>
        {
            var wc = new WebClient { CachePolicy = new RequestCachePolicy(RequestCacheLevel.CacheIfAvailable) };
            try
            {
                var image = new BitmapImage();
                image.BeginInit();
                image.StreamSource = new MemoryStream(wc.DownloadData(uri));
                image.EndInit();
                image.Freeze();
                return image;
            }
            catch (WebException) { }
            catch (IOException) { }
            catch (InvalidOperationException) { }
            finally
            {
                wc.Dispose();
            }
            return null;
        });
    }

    public static Task<BitmapImage> GetImage(string uri)
    {
        return GetImage(new Uri(uri));
    }
}

LazyImageBehavior.cs

using System;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Media;
using System.Windows.Media.Imaging;

public class LazyImageBehavior
{
    #region LazySource 添付プロパティ

    [AttachedPropertyBrowsableForType(typeof(Image))]
    public static Uri GetLazySource(Image element)
    {
        return (Uri)element.GetValue(LazySourceProperty);
    }

    [AttachedPropertyBrowsableForType(typeof(Image))]
    public static void SetLazySource(Image element, Uri value)
    {
        element.SetValue(LazySourceProperty, value);
    }

    public static readonly DependencyProperty LazySourceProperty =
        DependencyProperty.RegisterAttached("LazySource", typeof(Uri), typeof(LazyImageBehavior), new PropertyMetadata(null, LazySource_Changed));

    #endregion

    #region LazyImageSource 添付プロパティ

    [AttachedPropertyBrowsableForType(typeof(ImageBrush))]
    public static Uri GetLazyImageSource(ImageBrush element)
    {
        return (Uri)element.GetValue(LazyImageSourceProperty);
    }

    [AttachedPropertyBrowsableForType(typeof(ImageBrush))]
    public static void SetLazyImageSource(ImageBrush element, Uri value)
    {
        element.SetValue(LazyImageSourceProperty, value);
    }

    public static readonly DependencyProperty LazyImageSourceProperty =
        DependencyProperty.RegisterAttached("LazyImageSource", typeof(Uri), typeof(LazyImageBehavior), new PropertyMetadata(null, LazyImageSource_Changed));

    #endregion

    private static async void LazySource_Changed(DependencyObject sender, DependencyPropertyChangedEventArgs e)
    {
        var element = sender as Image;
        if (element == null)
        {
            return;
        }
        var image = await LazyBitmapImage.GetImage(e.NewValue as Uri);
        if (image != null)
        {
            element.Source = image;
        }
    }

    private static async void LazyImageSource_Changed(DependencyObject sender, DependencyPropertyChangedEventArgs e)
    {
        var element = sender as ImageBrush;
        if (element == null)
        {
            return;
        }
        var image = await LazyBitmapImage.GetImage(e.NewValue as Uri);
        if (image != null)
        {
            element.ImageSource = image;
        }
    }
}