FallbackValue如何与MultiBinding一起使用？

Question

我问,因为它似乎不起作用.

假设我们绑定到以下对象:

public class HurrDurr
{
  public string Hurr {get{return null;}}
  public string Durr {get{return null;}}
}

好吧,看来如果我们使用MultiBinding来反映这一点,就会显示回退值,对吗？

<TextBlock>
    <TextBlock.Text>                                
        <MultiBinding StringFormat="{}{0} to the {1}"
                        FallbackValue="Not set!  It works as expected!)">
            <Binding Path="Hurr"/>
            <Binding Path="Durr"/>
        </MultiBinding>
    </TextBlock.Text>
</TextBlock>

然而,结果实际上是"对".即使强制绑定返回DependencyProperty.UnsetValue也不起作用:

<TextBlock xmnlns:base="clr-namespace:System.Windows;assembly=WindowsBase">
    <TextBlock.Text>                                
        <MultiBinding StringFormat="{}{0} to the {1}"
            FallbackValue="Not set!  It works as expected!)">
            <Binding Path="Hurr"
                FallbackValue="{x:Static base:DependencyProperty.UnsetValue}" />
            <Binding Path="Durr"
                FallbackValue="{x:Static base:DependencyProperty.UnsetValue}" />
        </MultiBinding>
    </TextBlock.Text>
</TextBlock>

尝试使用TargetNullValue,它也是一个半身像.

所以似乎MultiBinding永远不会使用FallbackValue.这是真的吗,还是我错过了什么？

---

更麻烦一点,我发现转换器可以返回我需要的UnsetValue:

class MultiValueFailConverter : IMultiValueConverter
{
    public object Convert(
        object[] values, 
        Type targetType, 
        object parameter, 
        System.Globalization.CultureInfo culture)
    {
        if (values == null || 
            values.Length != 2 ||
            values.Any(x=>x == null))
            return System.Windows.DependencyProperty.UnsetValue;
        return values;
    }

    public object[] ConvertBack(
        object value, 
        Type[] targetTypes, 
        object parameter, 
        System.Globalization.CultureInfo culture)
    {
        throw new NotImplementedException("Too complex hurt brain.");
    }
}

然而,这似乎是一个肮脏的污秽黑客.我认为这样的场景会在框架中加以说明.但是,我在Reflector中找不到任何东西.

Answer 1

这是一个古老的问题,但它可以使用一些解释.

从FallbackValue文档:

在以下情况下,绑定会成功返回值:

1. 绑定源的路径成功解析.
2. 值转换器(如果有)能够转换结果值.
3. 结果值对绑定目标(目标)属性有效.

如果1和2返回DependencyProperty.UnsetValue,则target属性设置为FallbackValue的值(如果有).如果没有FallbackValue,则使用target属性的默认值.

在提供的示例中,绑定成功解析为Hurr和Durr属性.Null是字符串的有效值,表示绑定有效.

换句话说,当绑定无法返回值时使用FallbackValue,并且在提供的示例中,绑定确实提供了有效值.

举例说明基于原始示例的以下每个片段:

示例1
Hurr和Durr属性绑定正确; null是一个有效值,永远不会看到FallbackValue.

<TextBlock>
    <TextBlock.Text>
        <MultiBinding FallbackValue="Binding is valid. I will never be seen." StringFormat="{}{0} to the {1}">
            <Binding Path="Hurr" />
            <Binding Path="Durr" />
        </MultiBinding>
    </TextBlock.Text>
</TextBlock>

示例2
Hurr和Durr属性未正确绑定; 将看到FallbackValue.

<TextBlock>
    <TextBlock.Text>
        <MultiBinding FallbackValue="Binding paths are invalid. Look at me." StringFormat="{}{0} to the {1}">
            <Binding Path="xHurr" />
            <Binding Path="xDurr" />
        </MultiBinding>
    </TextBlock.Text>
</TextBlock>

示例3
如果一个绑定路径无效,则将看到FallbackValue.

<TextBlock>
    <TextBlock.Text>
        <MultiBinding FallbackValue="One binding path is invalid. Look at me." StringFormat="{}{0} to the {1}">
            <Binding Path="xHurr" />
            <Binding Path="Durr" />
        </MultiBinding>
    </TextBlock.Text>
</TextBlock>

示例4
与前面的示例一样,绑定是正确的,因此不会使用FallbackValue.此外,父级的每个子Binding属性的MultiBindingFallbackValue应该引用要用于MultiBinding的target属性的FallbackValue,而不是用于子Bindings.

<TextBlock xmlns:base="clr-namespace:System.Windows;assembly=WindowsBase">
    <TextBlock.Text>
        <MultiBinding FallbackValue="Binding is valid. I will never be seen." StringFormat="{}{0} to the {1}">
            <Binding FallbackValue="{x:Static base:DependencyProperty.UnsetValue}" Path="Hurr" />
            <Binding FallbackValue="{x:Static base:DependencyProperty.UnsetValue}" Path="Durr" />
        </MultiBinding>
    </TextBlock.Text>
</TextBlock>

示例5
即使在Binding属性中未提供路径,绑定仍然有效,因为绑定将使用绑定的任何对象.

<TextBlock xmlns:base="clr-namespace:System.Windows;assembly=WindowsBase">
    <TextBlock.Text>
        <MultiBinding FallbackValue="Binding is still valid. I will never be seen." StringFormat="{}{0} to the {1}">
            <Binding FallbackValue="{x:Static base:DependencyProperty.UnsetValue}" />
            <Binding FallbackValue="{x:Static base:DependencyProperty.UnsetValue}" />
        </MultiBinding>
    </TextBlock.Text>
</TextBlock>

示例6
最后,如果将转换器添加到任何Binding属性以强制使用UnsetValue,则会看到MultiBinding FallbackValue:

变流器

internal class ForceUnsetValueConverter : IValueConverter
{
    #region Implementation of IValueConverter

    public object Convert( object value, Type targetType, object parameter, CultureInfo culture )
    {
        return DependencyProperty.UnsetValue;
    }

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

    #endregion
}

XAML

<TextBlock>
    <TextBlock.Text>
        <MultiBinding FallbackValue="Binding is valid, but look at me. I'm an UnsetValue." StringFormat="{}{0} to the {1}">
            <Binding Converter="{StaticResource ForceUnset}" Path="Hurr" />
            <Binding Path="Durr" />
        </MultiBinding>
    </TextBlock.Text>
</TextBlock>