WPF - 转换器作为 DependencyObject

Question

无法将值绑定到Binding 的ConverterParametr。 Binding 只能设置在 DependencyProperty 或 DependencyObject 上。

我很好奇将IValueConverter 转换器实现为DependencyObject。

public class AddConverter : DependencyObject, IValueConverter
{
    public static readonly DependencyProperty AddIntegerProperty =
        DependencyProperty.Register(nameof(AddInteger),
            typeof(int),
            typeof(AddConverter),
            new PropertyMetadata(0));

    public int AddInteger
    {
        get => (int)GetValue(AddIntegerProperty);
        set => SetValue(AddIntegerProperty, value);
    }

    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
    {
        if (!(value is int intValue)) return 0;
        return intValue + AddInteger;
    }

    public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
    {
        int intValue;
        try
        {
            intValue = System.Convert.ToInt32(value);
        }
        catch (Exception)
        {
            return 0;
        }

        return intValue - AddInteger;
    }
}

让我们在示例视图中使用它。

<TextBox>
    <TextBox.Text>
        <Binding Path="MyIntegerProperty">
            <Binding.Converter>
                <local:AddConverter AddInteger="{Binding MyAddIntegerProperty, Mode=OneWay}" />
            </Binding.Converter>
        </Binding>
    </TextBox.Text>
</TextBox>

结果是AddInteger 仍然返回默认值。通过提供的Binding 没有改变AddInteger 的依赖属性是什么原因？

---

脚注：MultiBinding 对我没有帮助，因为ConvertBack 方法仅包含控件提供的值。这东西也应该在 ViewModel 中解决，但我很好奇有转换器的解决方案。

Answer 1

问题是，首先，转换器无法继承它所在的 DataContext，因此绑定无法工作：如果您在 VS 输出中添加跟踪，您将看到“未找到框架导师”您拥有的绑定（参见附录 A）。这也是您不能仅从 FrameworkElement 派生并使用 RelativeSource={RelativeSource AncestorType=Whatever} 的原因：您已经脱离了可视化树。没有祖先。此外，即使有框架指导者，DependencyObject 也无法为绑定提供源。来源必须是明确的。只有从 FrameworkElement 继承的类才能继承 DataContext。

所以我偷了一个 BindingProxy 类 (from this answer) 并用它来提供绑定源。这有点笨拙，但我想到的另一种选择是从 Freezable 继承转换器，本质上赋予它 BindingProxy 的属性，并在资源中创建转换器。它有效，但我更喜欢这种组织方式。

public class BindingProxy : Freezable
{
    #region Overrides of Freezable

    protected override Freezable CreateInstanceCore()
    {
        return new BindingProxy();
    }

    #endregion

    #region Data Property
    public Object Data
    {
        get { return (Object)GetValue(DataProperty); }
        set { SetValue(DataProperty, value); }
    }

    public static readonly DependencyProperty DataProperty =
        DependencyProperty.Register(nameof(Data), typeof(Object), typeof(BindingProxy),
            new PropertyMetadata(null));
    #endregion Data Property
}

XAML

<StackPanel.Resources>
    <local:BindingProxy
        x:Key="VMProxy"
        Data="{Binding}"
        />
</StackPanel.Resources>
<TextBlock>
    <TextBlock.Text>
        <Binding Path="MyIntegerProperty">
            <Binding.Converter>
                <local:AddConverter
                    AddInteger="{Binding Data.MyAddIntegerProperty, Source={StaticResource VMProxy}}" 
                    />
            </Binding.Converter>
        </Binding>
    </TextBlock.Text>
</TextBlock>

---

附录 A

AddInteger="{Binding MyAddIntegerProperty, Mode=OneWay, 
    PresentationTraceSources.TraceLevel=High}"

System.Windows.Data Warning: 56 : Created BindingExpression (hash=14964341) for Binding (hash=21653700)
System.Windows.Data Warning: 58 :   Path: 'MyAddIntegerProperty'
System.Windows.Data Warning: 60 : BindingExpression (hash=14964341): Default mode resolved to OneWay
System.Windows.Data Warning: 61 : BindingExpression (hash=14964341): Default update trigger resolved to PropertyChanged
System.Windows.Data Warning: 62 : BindingExpression (hash=14964341): Attach to WpfApp2.AddConverter2.AddInteger (hash=57434139)
System.Windows.Data Warning: 64 : BindingExpression (hash=14964341): Use Framework mentor <null>
System.Windows.Data Warning: 67 : BindingExpression (hash=14964341): Resolving source 
System.Windows.Data Warning: 69 : BindingExpression (hash=14964341): Framework mentor not found
System.Windows.Data Warning: 65 : BindingExpression (hash=14964341): Resolve source deferred
System.Windows.Data Warning: 67 : BindingExpression (hash=14964341): Resolving source 
System.Windows.Data Warning: 69 : BindingExpression (hash=14964341): Framework mentor not found