具有依赖属性的 WPF ValidationRule

Question

假设你有一个继承自 ValidationRule 的类:

public class MyValidationRule : ValidationRule
{
    public string ValidationType { get; set; }
    
    public override ValidationResult Validate(object value, CultureInfo cultureInfo) {}
}

在 XAML 中，您正在像这样进行验证:

<ComboBox.SelectedItem>
    <Binding Path="MyPath" Mode="TwoWay" UpdateSourceTrigger="PropertyChanged" NotifyOnValidationError="True">
        <Binding.ValidationRules>
            <qmvalidation:MyValidationRule  ValidationType="notnull"/>
        </Binding.ValidationRules>
    </Binding>
</ComboBox.SelectedItem>

哪个有效，一切正常。
但假设现在，你想要 ValidationType="{Binding MyBinding}"在哪里 MyBinding来自 DataContext .
为此，我需要制作 MyValidationRule作为 DependencyObject并添加 依赖属性 .
我试图写一个类是 DependencyObject , 并绑定(bind)它。不过有两个问题.. ValidationRule没有 DataContext从组合框/项目。
你有什么想法，如何解决这个问题？

Answer 1

由于ValidationRule不继承自 DependencyObject您不能创建 DependecyProperty在您的自定义验证类中。

然而正如 this link 中所解释的那样你可以在你的验证类中拥有一个继承自 DependecyObject 的类型的普通属性。并创建一个 DependencyProperty在那个类(class)。

例如这里是一个自定义 ValidationRule支持可绑定(bind)属性的类:

[ContentProperty("ComparisonValue")]
public class GreaterThanValidationRule : ValidationRule
{
    public ComparisonValue ComparisonValue { get; set; }

    public override ValidationResult Validate(object value, CultureInfo cultureInfo)
    {
        string s = value?.ToString();
        int number;

        if (!Int32.TryParse(s, out number))
        {
            return new ValidationResult(false, "Not a valid entry");
        }

        if (number <= ComparisonValue.Value)
        {
            return new ValidationResult(false, $"Number should be greater than {ComparisonValue}");
        }

        return ValidationResult.ValidResult;
    }
}

ComparisonValue是一个简单的类，继承自 DependencyObject并且有一个 DependencyProperty :

public class ComparisonValue : DependencyObject
{
    public int Value
    {
        get { return (int)GetValue(ValueProperty); }
        set { SetValue(ValueProperty, value); }
    }
    public static readonly DependencyProperty ValueProperty = DependencyProperty.Register(
        nameof(Value),
        typeof(int),
        typeof(ComparisonValue),
        new PropertyMetadata(default(int));

这解决了原来的问题，但不幸的是又带来了两个问题:

自 ValidationRules 以来，绑定(bind)无法正常工作不是可视树的一部分，因此无法正确获取绑定(bind)属性。例如，这种天真的方法行不通:

<TextBox Name="TextBoxToValidate">
    <TextBox.Text>
        <Binding Path="ViewModelProperty" UpdateSourceTrigger="PropertyChanged">
            <Binding.ValidationRules>
                <numbers:GreaterThanValidationRule>
                    <numbers:ComparisonValue Value="{Binding Text, ElementName=TextBoxToValidate}"/>
                </numbers:GreaterThanValidationRule>
            </Binding.ValidationRules>
        </Binding>
    </TextBox.Text>
</TextBox>

相反，应该使用代理对象，如 this 中所述。回答:

<TextBox Name="TextBoxToValidate">
    <TextBox.Resources>
        <bindingExtensions:BindingProxy x:Key="TargetProxy" Data="{Binding Path=Text, ElementName=TextBoxToValidate}"/>
    </TextBox.Resources>
    <TextBox.Text>
        <Binding Path="ViewModelProperty" UpdateSourceTrigger="PropertyChanged">
            <Binding.ValidationRules>
                <numbers:GreaterThanValidationRule>
                    <numbers:ComparisonValue Value="{Binding Data, Source={StaticResource TargetProxy}}"/>
                </numbers:GreaterThanValidationRule>
            </Binding.ValidationRules>
        </Binding>
    </TextBox.Text>
</TextBox>

BindingProxy是一个简单的类:

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

    public object Data
    {
        get { return GetValue(DataProperty); }
        set { SetValue(DataProperty, value); }
    }
    public static readonly DependencyProperty DataProperty = DependencyProperty.Register(nameof(Data), typeof(object), typeof(BindingProxy), new UIPropertyMetadata(null));
}

如果自定义属性ValidationRule绑定(bind)到另一个对象的属性，当另一个对象的属性更改时，原始属性的验证逻辑将不会触发。

为了解决这个问题，我们应该在 ValidationRule 时更新绑定(bind)。的绑定(bind)属性已更新。首先我们应该将该属性绑定(bind)到我们的ComparisonValue。类(class)。然后，我们可以在 Value 时更新绑定(bind)源。属性变化:

public class ComparisonValue : DependencyObject
{
    public int Value
    {
        get { return (int)GetValue(ValueProperty); }
        set { SetValue(ValueProperty, value); }
    }
    public static readonly DependencyProperty ValueProperty = DependencyProperty.Register(
        nameof(Value),
        typeof(int),
        typeof(ComparisonValue),
        new PropertyMetadata(default(int), OnValueChanged));

    private static void OnValueChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        ComparisonValue comparisonValue = (ComparisonValue) d;
        BindingExpressionBase bindingExpressionBase = BindingOperations.GetBindingExpressionBase(comparisonValue, BindingToTriggerProperty);
        bindingExpressionBase?.UpdateSource();
    }

    public object BindingToTrigger
    {
        get { return GetValue(BindingToTriggerProperty); }
        set { SetValue(BindingToTriggerProperty, value); }
    }
    public static readonly DependencyProperty BindingToTriggerProperty = DependencyProperty.Register(
        nameof(BindingToTrigger),
        typeof(object),
        typeof(ComparisonValue),
        new FrameworkPropertyMetadata(default(object), FrameworkPropertyMetadataOptions.BindsTwoWayByDefault));
}

第一种情况下同样的代理问题也存在这里。因此我们应该创建另一个代理对象:

<ItemsControl Name="SomeCollection" ItemsSource="{Binding ViewModelCollectionSource}"/>

<TextBox Name="TextBoxToValidate">
    <TextBox.Resources>
        <bindingExtensions:BindingProxy x:Key="TargetProxy" Data="{Binding Path=Items.Count, ElementName=SomeCollection}"/>
        <bindingExtensions:BindingProxy x:Key="SourceProxy" Data="{Binding Path=Text, ElementName=TextBoxToValidate, Mode=TwoWay}"/>
    </TextBox.Resources>
    <TextBox.Text>
        <Binding Path="ViewModelProperty" UpdateSourceTrigger="PropertyChanged">
            <Binding.ValidationRules>
                <numbers:GreaterThanValidationRule>
                    <numbers:ComparisonValue Value="{Binding Data, Source={StaticResource TargetProxy}}" BindingToTrigger="{Binding Data, Source={StaticResource SourceProxy}}"/>
                </numbers:GreaterThanValidationRule>
            </Binding.ValidationRules>
        </Binding>
    </TextBox.Text>
</TextBox>

在这种情况下，Text TextBoxToValidate 的属性(property)根据 Items.Count 进行验证SomeCollection 的属性(property).当列表中的项目数发生变化时，Text 的验证属性将被触发。