当我运行项目时出现运行时错误ocure: 错误:属性'UriSource'或属性'StreamSource'必须设置。 因为this.ImageUri为null,我不知道为什么this.ImageUri为null!帮我为WPF图像类提供初始图像占位符
我一直在使用WPF ListBox使用图像作为我的列表框项目。源图像路径指向托管这些图像的服务器。在快速网络中,图像没有任何明显的延迟。然而,慢速链接显示用户体验下降,我真的想在图像下载和解码时显示占位符图像。
令人惊讶的是,我没有在这个问题的博客上找到解决方案,所以我编写了一个派生类来解决这个问题。
下面的示例XAML来自我的项目容器样式。我用本地的本地类实现替换了Image:ImageLoader。
<Window.Resources>
<DataTemplate DataType="{x:Type local:MyData}">
...
<StackPanel Grid.Column="0" Margin="5">
<Border BorderThickness="0">
<MyControl:ImageLoader Width="50" Height="50" ImageUri="{Binding Path=profile_image_url_https, FallbackValue=profile_image_url_https}" InitialImage="/MyProject;component/Images/nopic.png" HorizontalAlignment="Left"></imgz:ImageLoader>
</Border>
</StackPanel>
...
</DataTemplate>
</Window.Resources>
<Grid>
<ListBox ItemsSource="{Binding Source = {StaticResource MyData}}" />
</Grid>
的处理为初始图像的心脏是在onLoaded()方法,其中,我使用的BitmapImage作为源和UriSource设置为派生类的ImageUri依赖属性,其允许数据绑定。当下载完成或接收到故障事件时,初始映像会更新为实际映像。该类还可以选择性地允许您指定“LoadFailedImage”。
public class ImageLoader : Image
{
public static readonly DependencyProperty ImageUriProperty = DependencyProperty.Register(
"ImageUri", typeof(Uri), typeof(ImageLoader), new PropertyMetadata(null, null));
private BitmapImage loadedImage;
public ImageLoader()
{
this.Loaded += this.OnLoaded;
}
public string LoadFailedImage
{
get;
set;
}
public Uri ImageUri
{
get {return this.GetValue(ImageUriProperty) as Uri;}
set {this.SetValue(ImageUriProperty, value);}
}
public string InitialImage
{
get;
set;
}
private new ImageSource Source
{
get {return base.Source;}
set {base.Source = value;}
}
private void OnLoaded(object sender, RoutedEventArgs e)
{
// Loading the specified image
this.loadedImage = new BitmapImage();
this.loadedImage.BeginInit();
this.loadedImage.CacheOption = BitmapCacheOption.OnDemand;
this.loadedImage.DownloadCompleted += this.OnDownloadCompleted;
this.loadedImage.DownloadFailed += this.OnDownloadFailed;
this.loadedImage.UriSource = this.ImageUri;
this.loadedImage.EndInit();
// The image may be cached, in which case we will not use the initial image
if (!this.loadedImage.IsDownloading)
{
this.Source = this.loadedImage;
}
else
{
// Create InitialImage source if path is specified
if (!string.IsNullOrWhiteSpace(this.InitialImage))
{
BitmapImage initialImage = new BitmapImage();
// Load the initial bitmap from the local resource
initialImage.BeginInit();
initialImage.UriSource = new Uri(this.InitialImage, UriKind.Relative);
initialImage.DecodePixelWidth = (int)this.Width;
initialImage.EndInit();
// Set the initial image as the image source
this.Source = initialImage;
}
}
e.Handled = true;
}
private void OnDownloadFailed(object sender, ExceptionEventArgs e)
{
if (!string.IsNullOrWhiteSpace(this.LoadFailedImage))
{
BitmapImage failedImage = new BitmapImage();
// Load the initial bitmap from the local resource
failedImage.BeginInit();
failedImage.UriSource = new Uri(this.LoadFailedImage, UriKind.Relative);
failedImage.DecodePixelWidth = (int)this.Width;
failedImage.EndInit();
this.Source = failedImage;
}
}
private void OnDownloadCompleted(object sender, EventArgs e)
{
this.Source = this.loadedImage;
}
}
当我运行该项目运行时错误ocured: 错误:属性“UriSource”或财产“的StreamSource”必须设置。 因为this.ImageUri为null,我不知道为什么this.ImageUri为null!帮我
不起作用,问题在于this.loadedImage.UriSource = this.ImageUri;线,ImageUri没有绑定 – ArMaN
好吧。请参阅http://msdn.microsoft.com/en-us/library/ms619218.aspx中的注释。 “属性更改只能在对象初始化期间发生”。我不相信绑定知道:) – LPL
我已经更新了我的答案。 – LPL