在 Windows 8 应用程序中下载图像后设置图像的高度？

Question

我正在尝试在 Windows 8 应用程序中动态设置 Image 元素的高度。为此，我需要知道源图像的高度。通常这可以通过调用 BitmapImage.PixelHeight 或 BitmapImage.DecodePixelHeight 来访问，但由于图像尚未下载，它返回 0。我可以像这样设置回调：

image = new BitmapImage(new Uri(link.Url));
System.Diagnostics.Debug.WriteLine("height :" + image.PixelHeight) // returns 0
instance.Source = image;
instance.Height = image.PixelHeight // what I'd like to do, but cant.
image.DownloadProgress += image_DownloadProgress;

static void image_DownloadProgress(object sender, DownloadProgressEventArgs e)
{
    BitmapImage i = (BitmapImage)sender;

    // returns the actual height I need
    System.Diagnostics.Debug.WriteLine("post-download height: " + i.PixelHeight); 
}

我在该回调中得到了我需要的正确像素高度。问题是我需要将它传递给要添加该图像的用户控件的实例。有没有一种方法可以 a.) 将实例传递到回调的参数中，以便我可以在那里修改它，或者 b.) 以某种方式在没有回调的情况下异步获取高度？

Answer 1

我最终解决此类问题的方法是创建一个自定义用户控件来显示图像，它会在加载图像时自行调整大小。

这是我的 xaml：

<UserControl
    x:Name="userControl"
    x:Class="BoardsForWindows.Controls.BBPostImage"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="using:BoardsForWindows.Controls"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    mc:Ignorable="d"
    d:DesignHeight="300"
    d:DesignWidth="400">

    <StackPanel Orientation="Vertical">
        <Image x:Name="imageViewer" HorizontalAlignment="Stretch" VerticalAlignment="Stretch"/>
    </StackPanel> 
</UserControl>

这是我的 .xaml.cs（使用已删除的语句）：

namespace BoardsForWindows.Controls
{
    public sealed partial class BBPostImage : UserControl
    {
        private BitmapImage bitmapImage;

        public string Source
        {
            get { return (string)GetValue(SourceProperty); }
            set { SetValue(SourceProperty, value); }
        }

        // Using a DependencyProperty as the backing store for Source.  This enables animation, styling, binding, etc...
        public static readonly DependencyProperty SourceProperty =
            DependencyProperty.Register("Source", typeof(string), typeof(BBPostImage), new PropertyMetadata(null, SourceChanged));

        public BBPostImage()
        {
            this.InitializeComponent();
        }

        public static void SourceChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
        {
            BBPostImage image = d as BBPostImage;
            if (image == null)
            {
                return;
            }

            string url = (e.NewValue ?? string.Empty).ToString();
            Uri uri = null;
            if (String.IsNullOrEmpty(url) || !Uri.TryCreate(url, UriKind.Absolute, out uri))
            {
                image.imageViewer.Width = 200;
                image.imageViewer.Height = 40;
                image.imageViewer.Source = null;
                return;
            }

            image.bitmapImage = new BitmapImage();
            image.bitmapImage.ImageFailed += image.bitmapImage_ImageFailed;
            image.bitmapImage.ImageOpened += image.bitmapImage_ImageOpened;
            image.bitmapImage.UriSource = uri;

            image.imageViewer.Source = null;
            image.imageViewer.Source = image.bitmapImage;
        }

        void bitmapImage_ImageOpened(object sender, RoutedEventArgs e)
        {
            Width = bitmapImage.PixelWidth;
            Height = bitmapImage.PixelHeight;
        }

        void bitmapImage_ImageFailed(object sender, ExceptionRoutedEventArgs e)
        {
            // TODO
        }
    }
}