WPF

Question

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

我很好奇如何将 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 仍然是 returns 默认值。通过提供的 Binding AddInteger 依赖性属性没有变化的原因是什么？

脚注： MultiBinding 对我没有帮助，因为 ConvertBack 方法仅包含控件提供的值。这玩意在ViewModel中应该也能解决，不过我很好奇有转换器的解决方法

Answer 1

问题是，首先，转换器无法在其所在的位置继承 DataContext，因此绑定无法工作：如果放置跟踪，您将在 VS 输出中看到 "Framework mentor not found"在你的绑定上（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：您超出了可视化树。没有祖先。此外，即使有框架导师，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 classes 才能继承 DataContext。

所以我偷了一个 BindingProxy class (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，本质上是赋予它 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

WPF – 作为 DependencyObject 的转换器

WPF – Converter as DependencyObject

binding

wpf-controls

converters

附录 A