如何解析 MarkupExtension 中数据绑定的值? [英] How do I resolve the value of a databinding inside a MarkupExtension?

问题描述

我制作了一个标记扩展，用于基于键翻译字符串.示例

I've made a markup extension for translating strings based on a key. Example

<TextBlock Text="{Translate myKey}" />

现在我希望能够使用嵌套绑定来提供我的密钥.示例:

Now I want to be able to use nested bindings for providing my keys. Example:

<TextBlock Text="{Translate {Binding KeyFromDataContext}}" />

当我这样做时，我得到一个 System.Windows.Data.Binding 对象.通过调用 ProvideValue 并传递 ServiceProvider，我可以获得 BindingExpression:

When I do this I get a System.Windows.Data.Binding object. By calling ProvideValue and passing down the ServiceProvider I can get a BindingExpression:

var binding = Key as Binding;
if (binding == null) {
    return null;
}
var bindingExpression = binding.ProvideValue(_serviceProvider) as BindingExpression;
if (bindingExpression == null) {
    return null;
}
var bindingKey = bindingExpression.DataItem;

我可以得到这个bindingExpression，但是DataItem 属性为空.我已经像这样测试了我的绑定

I can get this bindingExpression, but the DataItem property is null. I've tested my binding like this

<TextBlock Text="{Binding KeyFromDataContext}" />

它工作正常.

有什么想法吗?

推荐答案

toxvaerd 的答案 并不普遍.如果原始绑定已经有一个转换器，它就会中断.或者当无法编写转换器时.

The toxvaerd's answer is not universal. It breaks if the original binding already had a converter. Or when writing a converter is not possible.

有一个更好的解决方案.我们可以声明两个构造函数.当使用绑定时，XAML 将调用第二个接受 BindingBase 的方法.为了解析绑定的值，我们可以声明一个私有附加属性.为此，我们需要知道标记扩展的目标元素.

There's a better solution. We can declare two constructors. The second one accepting BindingBase will be called by XAML when a binding is used. To resolve the value of the binding, we can declare a private attached property. For this to work we need to know the target element of the markup extension.

有一个问题:当在模板中使用标记扩展时，没有目标元素(显然).在这种情况下，您是 应该在ProvideValue()中使用return this - 这样在应用模板时会再次调用扩展.

There's a catch: when the markup extension is used inside a template, there is no target element (obviously). In this case you are supposed to use return this in ProvideValue() - this way the extension will be called again when the template is applied.

public class TranslateExtension : MarkupExtension
{
    private readonly BindingBase _binding;

    public TranslateExtension(BindingBase binding)
    {
        _binding = binding;
    }

    public TranslateExtension(string key)
    {
        Key = key;
    }

    [ConstructorArgument("key")]
    public string Key { get; set; }

    public override object ProvideValue(IServiceProvider serviceProvider)
    {
        if (_binding != null)
        {
            var pvt = (IProvideValueTarget)serviceProvider.GetService(typeof(IProvideValueTarget));
            var target = pvt.TargetObject as DependencyObject;

            // if we are inside a template, WPF will call us again when it is applied
            if (target == null)
                return this; 

            BindingOperations.SetBinding(target, ValueProperty, _binding);
            Key = (string)target.GetValue(ValueProperty);
            BindingOperations.ClearBinding(target, ValueProperty);
        }

        // now do the translation using Key
        return ...;
    }

    private static readonly DependencyProperty ValueProperty = 
        DependencyProperty.RegisterAttached("Value", typeof(string), typeof(TranslateExtension));
}

这篇关于如何解析 MarkupExtension 中数据绑定的值?的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！