将XML绑定到ItemsSource时应用动态XPath表达式 [英] Apply a dynamic XPath expression when binding XML to ItemsSource

问题描述

我有一个XML文档描述如何为用户输入构建UI元素，并且我有包含一些数据和XPath表达式的数据对象。我有一个数据对象类型的DataTemplate，它使用HierarchicalDataTemplate来构建基于XML的UI，但是我只想使用XML的一个子集。如果XPath表达式是静态的，我可以写：

 < TreeView ItemsSource ={Binding Source = {StaticResource dataProvider} ，
 XPath = / Static / XPath / Expression / *}/> 
  

由于XPath表达式来自数据对象，因此使用数据绑定会很方便： p>

 < TreeView ItemsSource ={Binding Source = {StaticResource dataProvider}，
 XPath = {Binding Path = Data.XPathExpression }}/> 
  

不幸的是，绑定标记扩展不会从DependencyObject继承，所以它的属性不是DependencyProperty和don不支持数据绑定。

如何在绑定到XML数据时应用动态XPath表达式？

解决方案

您可以创建一个IMultiValueConverter，将XML数据和XPath表达式转换为新的XML数据：

  public object Convert（object [] values，Type targetType，object parameter，
 CultureInfo culture）
 {
 try 
 {
 if（values.Length< 2）{return null; } 
 
 var data = values [0] as IEnumerable; 
 if（data == null）{return null; } 
 
 var xPathExpression = values [1] as string; 
 if（xPathExpression == null）{return null; 
 
 XmlDocument xmlDocument = data.Cast< XmlNode>（）
 .Where（node => node.OwnerDocument！= null）
 .Select（node => node .OwnerDocument）
 .FirstOrDefault（）; 
 
 return（xmlDocument == null）？ null：
 xmlDocument.SelectNodes（xPathExpression）; 
} 
 catch（异常）{return null; } 
 $ b public object [] ConvertBack（object value，Type [] targetTypes，
 object parameter，System.Globalization.CultureInfo culture）
 {
抛出新的NotImplementedException（）; 
} 
  

然后，使用MultiBinding将ItemsSource绑定到XML数据和动态XPathExpression与转换器：

 < TreeView> 
< TreeView.Resources> 
< this：XmlXPathConverter x：Key =xmlXPathConverter/> 
< /TreeView.Resources> 
< TreeView.ItemsSource> 
< MultiBinding Converter ={StaticResource xmlXPathConverter}> 
< Binding Source ={StaticResource dataProvider}/> 
< Binding Path =Data.XPathExpression/> 
< / MultiBinding> 
< /TreeView.ItemsSource> 
< / TreeView> 
  

I have an XML document that describes how to build UI elements for user input, and I have data objects that contain some data and an XPath expression. I have a DataTemplate for the data object type that uses a HierarchicalDataTemplate to build the UI based on the XML, but I only want to use a subset of the XML. If the XPath expression were static, I could just write:

<TreeView ItemsSource="{Binding Source={StaticResource dataProvider},
    XPath=/Static/XPath/Expression/*}" />

Since the XPath expression comes from a data object, it would be convenient to use data binding:

<TreeView ItemsSource="{Binding Source={StaticResource dataProvider},
    XPath={Binding Path=Data.XPathExpression}}" />

Unfortunately, the Binding MarkupExtension doesn't inherit from DependencyObject, so it's properties aren't DependencyProperty and don't support data binding.

How can I apply a dynamic XPath expression when binding to XML data?

解决方案

You can create a IMultiValueConverter that converts the XML data and XPath expression into new XML data:

public object Convert(object[] values, Type targetType, object parameter,
    CultureInfo culture)
{
    try
    {
        if (values.Length < 2) { return null; }

        var data = values[0] as IEnumerable;
        if (data == null) { return null; }

        var xPathExpression = values[1] as string;
        if (xPathExpression == null) { return null; }

        XmlDocument xmlDocument = data.Cast<XmlNode>()
            .Where(node => node.OwnerDocument != null)
            .Select(node => node.OwnerDocument)
            .FirstOrDefault();

        return (xmlDocument == null) ? null : 
            xmlDocument.SelectNodes(xPathExpression);
    }
    catch (Exception) { return null; }
}

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

Then, use MultiBinding to bind the ItemsSource to the XML data and a dynamic XPathExpression with the converter:

<TreeView>
    <TreeView.Resources>
        <this:XmlXPathConverter x:Key="xmlXPathConverter" />
    </TreeView.Resources>
    <TreeView.ItemsSource>
        <MultiBinding Converter="{StaticResource xmlXPathConverter}">
            <Binding Source="{StaticResource dataProvider}" />
            <Binding Path="Data.XPathExpression" />
        </MultiBinding>
    </TreeView.ItemsSource>
</TreeView>

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