WPF列表框與圖片 - 如何檢測如果圖像沒有下載

Question

我有一個WPF列表框，並且每個項目都有該列表需要從服務器上下載的圖像 - 列表定義看起來像這樣：

<ListBox x:Name="List" BorderThickness="0" AlternationCount="2" ItemContainerStyle="{StaticResource alternatingWithBinding}" 
    HorizontalContentAlignment="Stretch" ScrollViewer.HorizontalScrollBarVisibility="Hidden"> 
<ListBox.ItemTemplate> 
    <DataTemplate> 
     <Grid x:Name="itemsGrid" Margin="3" ShowGridLines="False" > 
      <Grid.RowDefinitions> 
       <RowDefinition Height="59"/> 
      </Grid.RowDefinitions> 
      <Grid.ColumnDefinitions> 
       <ColumnDefinition Width="45" /> 
       <ColumnDefinition Width="60" /> 
       <ColumnDefinition Width="*" /> 
       <ColumnDefinition Width="150" /> 
      </Grid.ColumnDefinitions> 

      <Button x:Name="btn" Grid.Column="0" HorizontalAlignment="Left" VerticalAlignment="Center" Tag="{Binding}" 
       CommandParameter="{Binding RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type ListBoxItem}}}" /> 
      <Image x:Name="Thumb" Grid.Column="1" Stretch="Uniform" Opacity="1.0" Source="{Binding Path=Image, Converter={StaticResource ImageConverter}}" Height="65" VerticalAlignment="Center"/> 
      <TextBlock x:Name="Name" Grid.Column="2" Padding="2" Margin="17,0" VerticalAlignment="Center" Text="{Binding Path=Name}" 
         Tag="{Binding}" /> 
     </Grid> 
     <DataTemplate.Triggers> 
      ... 
     </DataTemplate.Triggers> 
    </DataTemplate> 
</ListBox.ItemTemplate> 

和轉換器的樣子：

public class ImageConverter : IValueConverter 
{ 
    public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture) 
    { 
      if (value is string) 
      { 
       value = new Uri((string)value); 
      } 

      if (value is Uri) 
      { 
       BitmapImage bi = new BitmapImage(); 
       bi.BeginInit(); 
       bi.DecodePixelWidth = 150; 
       bi.UriSource = value; 
       bi.DownloadFailed += new EventHandler<ExceptionEventArgs>(bi_DownloadFailed); 
       bi.EndInit(); 
       return bi; 
      } 

      return null; 
    } 

這樣做是爲了顯示默認圖像時，sourceUrl返回從服務器什麼。但是因爲我在XAML代碼中使用了轉換器，所以我不知道如何攔截這種情況。我不知道如何攔截這種情況。我看到BitmapImage具有對我來說很完美的DownloadFailed事件，我只是不知道如何在這種情況下使用它。

Answer 1

嗯....我相信下載發生在EndInit上，這很重要。由於您正在爲轉換器使用靜態資源，因此每個應用程序最終會有一個實例。這意味着，如果您創建了一個變量作用於類級別並用於存儲下載結果的變量，則在調用轉換之間不注意變量狀態時可能會出現問題。但是，由於UI是在單個線程中呈現的，因此您不必擔心太多。

public class ImageConverter : IValueConverter 
{ 
    private static readonly BitmapImage _default = MakeDefault(); 
    private bool _downloadFailed; 

public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture) 
{ 
     if (value is string) 
     { 
      value = new Uri((string)value); 
     } 

     if (value is Uri) 
     { 
      _downloadFailed = false; 
      BitmapImage bi = new BitmapImage(); 
      bi.BeginInit(); 
      bi.DecodePixelWidth = 150; 
      bi.UriSource = value; 
      // the event handler sets _downloadFailed to true! 
      bi.DownloadFailed += bi_DownloadFailed; 
      bi.EndInit(); 
      // unhook so we don't unintentionally keep the instance alive 
      // this is important; you will leak BI instances otherwise! 
      bi.DownloadFailed -= bi_DownloadFailed; 
      if(_downloadFailed)     
       return _default; 
      return bi; 
     } 

     return null; 
} 

Answer 2

你看過Binding類的TargetNullValue屬性嗎？

如果您無法下載文件，我會讓Converter返回null。

---

public object Convert(object value, Type targetType, 
    object parameter, System.Globalization.CultureInfo culture) 
{ 

    if (parameter.ToString()=="blue") 
    { 
    return new Uri("Butterfly1.png", UriKind.RelativeOrAbsolute); 
    } 
    return null; 
} 

---

然後安裝在你的綁定指定TargetNullValue在XAML

<Window.Resources> 
<my:ImageConverter x:Key='ImageConverter1' /> 
<BitmapImage x:Key='defaultImage' 
      UriSource='/WpfApplication1;component/default.png' /> 

---

然後默認圖像。

---

<Image Source='{Binding Converter={StaticResource ImageConverter1}, 
    ConverterParameter="red",TargetNullValue={StaticResource defaultImage}}' 
     Height='100' /> 
<Image Source='{Binding Converter={StaticResource ImageConverter1}, 
    ConverterParameter="blue",TargetNullValue={StaticResource defaultImage}}' 
     Height='100' /> 

---