如何将绑定中的FallbackValue设置为外部图像文件的路径？

Question

我正在尝试设置FallbackValue，以防我的转换器无法被调用，但我不确定该如何做。

<Image Source="{Binding FallbackValue="Pictures/Unknown.png", Path=LatestPosition.DeviceFamily, Converter={x:Static conv:ConverterSet.DeviceTypeToImageSourceconverter}}" Name="image1" Stretch="Fill" Margin="5,8" Width="150" Height="150" Grid.RowSpan="4" />

转换器中外部图像的路径看起来就像这样，当LatestPosition!=null时，图像以正确的方式设置。

private static readonly ImageSource Dev1 = new BitmapImage(new Uri("/Pictures/dev1.png", UriKind.Relative));
private static readonly ImageSource Dev2 = new BitmapImage(new Uri("/Pictures/dev2.png", UriKind.Relative));

Answer 1

对于Image控件，当您将Source属性与URI字符串绑定时，它会自动将URI转换为BitmapImage。但是如果您将FallbackValue和TargetNullValue设置为URI字符串，它将不会显示。

您需要将其设置为BitmapImage：

<Window.Resources>
    <BitmapImage x:Key="DefaultImage" UriSource="/Resources;component/Images/Default.jpg" />
</Window.Resources>

<Image Width="128"
               Height="128"
               HorizontalAlignment="Left"
               VerticalAlignment="Top"
               Source="{Binding Photo,FallbackValue={StaticResource DefaultImage},
                                TargetNullValue={StaticResource DefaultImage}}" />

当我们将FallbackValue和TargetNullValue设置为BitmapImage的StaticResource时，它就可以工作了。

Answer 2

对于我自己，我创建了以下示例：

<!-- xmlns:sys="clr-namespace:System;assembly=mscorlib" -->

<Window.Resources>
    <!-- Test data -->
    <local:TestDataForImage x:Key="MyTestData" />

    <!-- Image for FallbackValue -->
    <sys:String x:Key="ErrorImage">pack://application:,,,/NotFound.png</sys:String>

    <!-- Image for NULL value -->
    <sys:String x:Key="NullImage">pack://application:,,,/NullImage.png</sys:String>
</Window.Resources>

<!-- Grid using DataContext -->
<Grid DataContext="{StaticResource MyTestData}">
    <Image Name="ImageNull" Width="100" Height="100" HorizontalAlignment="Left" VerticalAlignment="Bottom" Source="{Binding Path=NullString, TargetNullValue={StaticResource NullImage}}" />

    <Image Name="ImageNotFound" Width="100" Height="100" HorizontalAlignment="Left" VerticalAlignment="Top" Source="{Binding Path=NotFoundString, FallbackValue={StaticResource ErrorImage}}" />
</Grid>

通向图像的路径，我在参考资料中宣布。图像存储在项目的根目录中。MyTestData列表

public class TestDataForImage : DependencyObject
{
    public string NotFoundString
    {
        get
        {
            return (string)GetValue(NotFoundStringProperty);
        }

        set
        {
            SetValue(NotFoundStringProperty, value);
        }
    }

    public static readonly DependencyProperty NotFoundStringProperty = DependencyProperty.Register("NotFoundString", typeof(string), typeof(TestDataForImage), new PropertyMetadata(""));

    public string NullString
    {
        get
        {
            return (string)GetValue(NullStringProperty);
        }

        set
        {
            SetValue(NullStringProperty, value);
        }
    }

    public static readonly DependencyProperty NullStringProperty = DependencyProperty.Register("NullString", typeof(string), typeof(TestDataForImage), new PropertyMetadata(""));

    public TestDataForImage()
    {
        NotFoundString = "pack://application:,,,/NotExistingImage.png";
        NullString = null;
    }
}