具有自动内存清理的图像下载机

Question

我有一个列表(简单的ListBox)的项目与图像的主细节基础(如果用户点击列表项目，详细信息页打开)。我面临着著名的图像内存泄漏问题，描述了here、here、here和here。

一种可能的方法是当run through all images NavigatingFrom and clean them时。

在one of the threads中，我发现了更有趣的解决方案:它自动清除图像，但这不适用于虚拟化(如果要添加用于存储ImageSource的私有字段，则图像丢失或混合)。建议的修复方法是添加依赖项属性。

但是我仍然面临着同样的问题:图片在向下滚动和返回后被混淆了。看起来依赖项属性是随机变化的，但我无法捕捉到它们变化的时刻。

public class SafePicture : ContentControl
{
    public static readonly DependencyProperty SafePathProperty =
        DependencyProperty.RegisterAttached(
            "SafePath",
            typeof(string),
            typeof(SafePicture),
            new PropertyMetadata(OnSourceWithCustomRefererChanged));

    public string SafePath
    {
        get { return (string)GetValue(SafePathProperty); }
        set { SetValue(SafePathProperty, value); }
    }

    private static void OnSourceWithCustomRefererChanged(DependencyObject o, DependencyPropertyChangedEventArgs e)
    {
        if (e.NewValue == null) // New value here
            return;
    }


    public SafePicture()
    {
        Content = new Image();
        Loaded += OnLoaded;
        Unloaded += OnUnloaded;
    }

    private void OnLoaded(object _sender, RoutedEventArgs _routedEventArgs)
    {
        var image = Content as Image;

        if (image == null)
            return;

        var path = (string)GetValue(SafePathProperty); // Also, tried SafePath (debugger cant catch setter and getter calls), but same result.

        image.Source = null;
        {
            var request = WebRequest.Create(path) as HttpWebRequest;
            request.AllowReadStreamBuffering = true;
            request.BeginGetResponse(result =>
            {
                try
                {
                    Stream imageStream = request.EndGetResponse(result).GetResponseStream();
                    DispatcherHelper.CheckBeginInvokeOnUI(() =>
                    {
                        if (imageStream == null)
                        {
                            image.Source = new BitmapImage { UriSource = new Uri(path, UriKind.Relative) };
                            return;
                        }

                        var bitmapImage = new BitmapImage();
                        bitmapImage.CreateOptions = BitmapCreateOptions.BackgroundCreation;
                        bitmapImage.SetSource(imageStream);
                        image.Source = bitmapImage;
                    });
                }
                catch (WebException)
                {
                }
            }, null);
        }
    }


    private void OnUnloaded(object sender, RoutedEventArgs e)
    {
        var image = Content as Image;

        if (image == null)
            return;

        var bitmapImage = image.Source as BitmapImage;
        if (bitmapImage != null)
            bitmapImage.UriSource = null;
        image.Source = null;
    }
}

用法：

<wpExtensions:SafePicture SafePath="{Binding ImageUrl}"/>

因此，乍一看，它工作得很好，但是如果要向下滚动并返回向上，图像就会随机改变。

编辑:在本例中，目前我只使用纯ListBox，不使用虚拟化(但在其他情况下需要使用)。

EDIT2:重现此问题的示例项目。我相信，它会在一段时间内包含解决方案：https://simca.codeplex.com/

Answer 1

问题是，当使用虚拟化时，用于每个项的ui元素将被回收并重用于其他对象(因此包括图像对象)，并且由于您在足够快的滚动速度时异步加载图像，因此您将在图像上设置一个位图，该位图已经被用于另一个项。

快速修复的方法是检查路径值是否仍然相同，如果没有，只需返回，因为图像已经被用于另一个对象：

...
var request = WebRequest.Create(path) as HttpWebRequest;
        request.AllowReadStreamBuffering = true;
        request.BeginGetResponse(result =>
        {

            try
            {

                Stream imageStream = request.EndGetResponse(result).GetResponseStream();
                DispatcherHelper.CheckBeginInvokeOnUI(() =>
                {
                if (path!=SafePath){
                  //Item has been recycled
                  return;
                }
                 ....

编辑:代码中有几个问题：-Switch RegisterAttached to Register，RegisterAttached用于附加属性，而不是OnSourceWithCustomRefererChanged中的普通依赖属性-call OnLoaded，因为在加载元素之后，更改的SafePath属性实际上会在onLoaded开始时发生-Add清除uri和源，以便在路径为空时清除图像。

下面是一个完整的工作代码：

public class SafeImage : ContentControl
{
    private SynchronizationContext uiThread;

    public static readonly DependencyProperty SafePathProperty =
        DependencyProperty.Register("SafePath", typeof (string), typeof (SafeImage),
        new PropertyMetadata(default(string), OnSourceWithCustomRefererChanged));

    public string SafePath
    {
        get { return (string) GetValue(SafePathProperty); }
        set { SetValue(SafePathProperty, value); }
    }


    private static void OnSourceWithCustomRefererChanged(DependencyObject o, DependencyPropertyChangedEventArgs e)
    {
        SafeImage safeImage = o as SafeImage;
        safeImage.OnLoaded(null, null);
        //OnLoaded(null, null);
        if (e.NewValue == null)
            return;
    }





    public SafeImage()
    {
        Content = new Image();
        uiThread = SynchronizationContext.Current;

        Loaded += OnLoaded;
        Unloaded += OnUnloaded;
    }

    private void OnLoaded(object _sender, RoutedEventArgs _routedEventArgs)
    {
        var image = Content as Image;

        if (image == null)
            return;

        var path = SafePath; //(string)GetValue(SafePathProperty);
        //image.Source = new BitmapImage(new Uri(SafePath));
        Debug.WriteLine(path);

        var bitmapImage = image.Source as BitmapImage;
        if (bitmapImage != null)
            bitmapImage.UriSource = null;
        image.Source = null;

        if (String.IsNullOrEmpty(path))
        {
            //image.Source = new BitmapImage { UriSource = new Uri(Constants.RESOURCE_IMAGE_EMPTY_PRODUCT, UriKind.Relative) };
            return;
        }

        // If local image, just load it (non-local images paths starts with "http")
        if (path.StartsWith("/"))
        {
            image.Source = new BitmapImage { UriSource = new Uri(path, UriKind.Relative) };
            return;
        }



        {
            var request = WebRequest.Create(path) as HttpWebRequest;
            request.AllowReadStreamBuffering = true;
            request.BeginGetResponse(result =>
            {
                try
                {
                    Stream imageStream = request.EndGetResponse(result).GetResponseStream();
                    uiThread.Post(_ =>
                    {

                        if (path != this.SafePath)
                        {
                            return;
                        }
                        if (imageStream == null)
                        {
                            image.Source = new BitmapImage { UriSource = new Uri(path, UriKind.Relative) };
                            return;
                        }

                        bitmapImage = new BitmapImage();
                        bitmapImage.CreateOptions = BitmapCreateOptions.BackgroundCreation;
                        bitmapImage.SetSource(imageStream);
                        image.Source = bitmapImage;
                        //imageCache.Add(path, bitmapImage);
                    }, null);
                }
                catch (WebException)
                {
                    //uiThread.Post(_ =>
                    //{
                    //    image.Source = new BitmapImage { UriSource = new Uri(Constants.RESOURCE_IMAGE_EMPTY_PRODUCT, UriKind.Relative) };
                    //}, null);
                }
            }, null);
        }
    }


    private void OnUnloaded(object sender, RoutedEventArgs e)
    {
        var image = Content as Image;

        if (image == null)
            return;

        var bitmapImage = image.Source as BitmapImage;
        if (bitmapImage != null)
            bitmapImage.UriSource = null;
        image.Source = null;
    }
}

最后，windows默认使用虚拟化和回收( ItemPanel是一个VirtualisedStackPanel)。