MVVM WPF Fire图像加载完成后的命令（加载事件）

Question

我在我的视图中有一个WPF图像控件。当图像被加载时，我想要触发一个在我的模型视图中定义和执行的事件。我怎样才能做到这一点？我不知道如何使用命令执行此操作。

<Image Grid.Row="0" 
     Source="{Binding Path=ImageSrc, NotifyOnTargetUpdated=True, Converter={StaticResource imgToSrcConverter}}" 
     Visibility="{Binding ImgVisibility}" 
     RenderTransformOrigin="0,0" 
     SnapsToDevicePixels="True" 
     OverridesDefaultStyle="False" 
     TargetUpdated="targetUpdated" 
     Cursor="Hand" 
     RenderOptions.BitmapScalingMode="LowQuality" 
     RenderOptions.EdgeMode="Aliased"> 
<Image.Effect> 
     <DropShadowEffect Opacity="0.8" BlurRadius="8">     
     </DropShadowEffect> 
    </Image.Effect> 
</Image> 

转换：

public class ImgToSrcConverter : IValueConverter 
{ 
    public object Convert(object value, Type targetType, object parameter, 
      System.Globalization.CultureInfo culture) 
    { 
     Image image = value as Image; 
     if (image != null) 
     { 
      MemoryStream ms = new MemoryStream(); 
      image.Save(ms, image.RawFormat); 
      ms.Seek(0, SeekOrigin.Begin); 
      BitmapImage bi = new BitmapImage(); 
      bi.BeginInit(); 
      bi.StreamSource = ms; 
      bi.EndInit(); 
      return bi; 
     } 
     return null; 
    } 

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

视图模型：

 public Image ImageSrc 
     { 
      get 
      { 
       MemoryStream ms = new MemoryStream(GetImageAsByteArray()); 
       Image img = Image.FromStream(ms); 

       // Here I call method to hide splash screen "Loading" 

       return img; 
      } 
     } 

Answer 1

您可以在后台加载图像，并称之为 “NotifyPropertyChanged” 的形象已经被加载时：

private static Image SplashScreenImage = GetSplashScreenImage(); 
private Image ActualImage = null; 
private bool IsLoading = false; 
public Image ImageSrc 
{ 
    get 
    { 
     if (ActualImage != null) 
      return ActualImage; 

     if (!IsLoading) 
     { 
      IsLoading = true; 
      // start loading image in background 
      Task.Run(() => 
      { 
       MemoryStream ms = new MemoryStream(GetImageAsByteArray()); 
       ActualImage = Image.FromStream(ms); 
      }).ContinueWith(t => PropertyChanged("ImageSrc"), TaskScheduler.FromCurrentSynchronizationContext()); 
     } 
     return SplashScreenImage; 
    } 
} 

Answer 2

您可以使用异步无需转换器绑定：

<Image Source="{Binding ImageSrc, IsAsync=True}"/> 

的ImageSrc属性的getter直接创建一个BitmapFrame或BitmapImage的是这样的：

public ImageSource ImageSrc 
{ 
    get 
    { 
     ImageSource image; 

     using (var stream = new MemoryStream(GetImageAsByteArray())) 
     { 
      image = BitmapFrame.Create(
       stream, BitmapCreateOptions.None, BitmapCacheOption.OnLoad); 
     } 

     SplashScreenEnabled = false; 
     return image; 
    } 
} 

public bool SplashScreenEnabled { get; set; } // firing PropertyChanged omitted 

Answer 3

如果你想调用指令时， Loaded事件为Image元素触发，您可以使用交互触发器：

<Image Grid.Row="0" 
      Source="{Binding Path=ImageSrc, NotifyOnTargetUpdated=True, Converter={StaticResource imgToSrcConverter}}" 
      Visibility="{Binding ImgVisibility}" 
      RenderTransformOrigin="0,0" 
      SnapsToDevicePixels="True" 
      OverridesDefaultStyle="False" 
      TargetUpdated="targetUpdated" 
      Cursor="Hand" 
      RenderOptions.BitmapScalingMode="LowQuality" 
      RenderOptions.EdgeMode="Aliased" 
      xmlns:i="clr-namespace:System.Windows.Interactivity;assembly=System.Windows.Interactivity"> 
    <i:Interaction.Triggers> 
     <i:EventTrigger EventName="Loaded"> 
      <i:InvokeCommandAction Command="{Binding YourCommand}" /> 
     </i:EventTrigger> 
    </i:Interaction.Triggers> 
    <Image.Effect> 
     <DropShadowEffect Opacity="0.8" BlurRadius="8"> 
     </DropShadowEffect> 
    </Image.Effect> 
</Image> 

有关如何通过执行视图模型命令来处理事件的更多信息，请参阅以下博文：https://blog.magnusmontin.net/2013/06/30/handling-events-in-an-mvvm-wpf-application/。

另一种选择可能是简单地从源setter方法执行命令：

public Image ImageSrc 
{ 
    get 
    { 
     MemoryStream ms = new MemoryStream(GetImageAsByteArray()); 
     Image img = Image.FromStream(ms); 

     YourCommandProperty.Execute(null); 

     return img; 
    } 
}