当我在ListBox中有图像时,为什么会出现OutOfMemoryException？

Question

我想在我的自定义库中显示存储在Windows Phone 8照片文件夹中的所有图像,该图库ListBox用于显示图像.

的ListBox代码如下:

    <phone:PhoneApplicationPage.Resources>
        <MyApp:PreviewPictureConverter x:Key="PreviewPictureConverter" />
    </phone:PhoneApplicationPage.Resources>

    <ListBox Name="previewImageListbox" VirtualizingStackPanel.VirtualizationMode="Recycling">
        <ListBox.ItemsPanel>
            <ItemsPanelTemplate>
                <VirtualizingStackPanel CleanUpVirtualizedItemEvent="VirtualizingStackPanel_CleanUpVirtualizedItemEvent_1">
                </VirtualizingStackPanel>
            </ItemsPanelTemplate>
        </ListBox.ItemsPanel>
        <ListBox.ItemTemplate>
            <DataTemplate>
                <Grid>
                    <Image Source="{Binding Converter={StaticResource PreviewPictureConverter}}" HorizontalAlignment="Center" VerticalAlignment="Center" />
                </Grid>
            </DataTemplate>
        </ListBox.ItemTemplate>
     </ListBox>

使用以下转换器:

public class PreviewPictureConverter : System.Windows.Data.IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        PreviewImageItem c = value as PreviewImageItem;
        if (c == null)
            return null;
        return c.ImageData;
    }

    public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        throw new NotImplementedException();
    }
}

图像存储在自定义类中:

class PreviewImageItem
{
    public Picture _picture = null;
    public BitmapImage _bitmap = null;

    public PreviewImageItem(Picture pic)
    {
        _picture = pic;
    }

    public BitmapImage ImageData 
    {
        get
        {
            System.Diagnostics.Debug.WriteLine("Get picture " + _picture.ToString());
            _bitmap = new BitmapImage();
            Stream data = _picture.GetImage();
            try
            {
                _bitmap.SetSource(data); // Out-of memory exception (see text)
            }
            catch (Exception ex)
            {
                System.Diagnostics.Debug.WriteLine("Exception : " + ex.ToString());
            }
            finally
            {
                data.Close();
                data.Dispose();
                data = null;
            }

            return _bitmap;
        }
    }
}

以下代码用于设置ListBox数据源:

private List<PreviewImageItem> _galleryImages = new List<PreviewImageItem>();

using (MediaLibrary library = new MediaLibrary())
{
    PictureCollection galleryPics = library.Pictures;
    foreach (Picture pic in galleryPics)
    {
        _galleryImages.Add(new PreviewImageItem(pic));
    }

    previewImageListbox.ItemsSource = _galleryImages;
};

最后这里是"清理"代码:

private void VirtualizingStackPanel_CleanUpVirtualizedItemEvent_1(object sender, CleanUpVirtualizedItemEventArgs e)
{
    PreviewImageItem item = e.Value as PreviewImageItem;

    if (item != null)
    {
        System.Diagnostics.Debug.WriteLine("Cleanup");
        item._bitmap = null;
    }
}

所有这一切工作正常,但代码崩溃OutOfMemoryException后几个图像(特别是当滚动快速).滚动VirtualizingStackPanel_CleanUpVirtualizedItemEvent_1时,该方法称为regulary(例如,每2或3个列表框条目)ListBox.

这个示例代码有什么问题？

为什么内存没有被释放(足够快)？

Answer 1

哦,我最近杀了一整天才使这个工作!

所以解决方案是:

使您的Image控制免费资源.所以设置

BitmapImage bitmapImage = image.Source as BitmapImage;
bitmapImage.UriSource = null;
image.Source = null;

正如之前提到的那样.

确保在列表的每个项目上虚拟化_bitmap.你应该按需加载它(LongListSelector.Realized方法),你必须销毁它!它不会自动收集,GC.Collect也不起作用.空引用也不起作用:(但是这里的方法是:制作1x1像素文件.将其复制到程序集中并从中创建资源流以使用1x1像素空白处理图像.将自定义配置方法绑定到LongListSelector.UnRealized事件(例如,容器处理您的列表项).

public static void DisposeImage(BitmapImage image)
{
    Uri uri= new Uri("oneXone.png", UriKind.Relative);
    StreamResourceInfo sr=Application.GetResourceStream(uri);
    try
    {
        using (Stream stream=sr.Stream)
        {
            image.DecodePixelWidth=1; //This is essential!
            image.SetSource(stream);
        }
    }
    catch { }
}

在LongListSelector中为我工作,每张图像宽度为1000张.

如果您错过了数据收集的第2步,您可以看到良好的结果,但滚动了100-200个项目后内存溢出.

Answer 2

您刚刚使用Windows Phone在屏幕上显示用户媒体库"pictures"文件夹中的所有图片.这是令人难以置信的内存密集型,考虑到WP8应用程序的150MB限制,难怪你得到了OOM异常.

您应该考虑添加的一些事项:

1)将listboxitem滚动到视图外时,将Source和SourceUri属性设置为null.请参阅Stefan的文章中的"缓存图像"@ http://blogs.msdn.com/b/swick/archive/2011/04/07/image-tips-for-windows-phone-7.aspx

  BitmapImage bitmapImage = image.Source as BitmapImage;
  bitmapImage.UriSource = null;
  image.Source = null;

2)如果你在WP8上,请确保设置DecodePixelWidth和/或DecodePixelHeight.这样,图像将被加载到内存中,永久调整大小,只有调整大小的副本存储在内存中.加载到内存中的图像可能比手机本身的屏幕尺寸大得多.因此,将那些缩小到合适尺寸并且仅存储调整大小的图像是至关重要的.设置BitmapImage.DecodePixelWidth = 480(最多)以帮助解决这个问题.

var bmp = new BitmapImage();

// no matter the actual size, 
// this bitmap is decoded to 480 pixels width (aspect ratio preserved)
// and only takes up the memory needed for this size
bmp.DecodePixelWidth = 480;

bmp.UriSource = new Uri(@"Assets\Demo.png", UriKind.Relative);
ImageControl.Source = bmp;

(来自这里的代码示例)

3)为什么使用Picture.GetImage()而不是Picture.GetThumbnail()？你真的需要图像占据整个屏幕吗？

4)如果这是一个WP8独家应用程序,请考虑从ListBox移动到LongListSelector.LLS具有比ListBox更好的虚拟化.查看代码示例,只需将XAML ListBox元素更改为LongListSelector元素就足够了.