我如何添加逻辑到现有的依赖属性的回调？ [英] How can I add logic to an existing dependency-property callback?

问题描述

我想要一个PropertyChangedCallback添加到UIElement.RenderTransformOriginProperty。当我尝试重写PropertyMetadata，则抛出异常。

我寻觅MSDN和谷歌，而我已经能够拿出的<一个href="http://social.msdn.microsoft.com/Forums/en-US/wpf/thread/d2aa5151-901e-47b7-9a0d-955214737b7f">this. DependencyPropertyDescriptor.AddValueChanged建议在该职位的某个时刻，但不会解决我的问题，因为这不是每个实例的回调。

我不明白这是什么异常意味着在所有。有谁知道我在做什么错了？

 公共类Foo：FrameworkElement的
{
    私有静态无效Origin_Changed（DependencyObject的研发，
                                        DependencyPropertyChangedEventArgs五）
    {}

    静态FOO（）
    {
        PropertyMetadata OriginalMetaData =
            UIElement.RenderTransformOriginProperty.GetMetadata（
                typeof运算（FrameworkElement的））;



/ *当执行这一行，则抛出异常：
 无法更改属性的元数据后，它已经与物业相关的* /
        OriginalMetaData.PropertyChangedCallback + =
            新PropertyChangedCallback（Origin_Changed）;



        UIElement.RenderTransformOriginProperty.OverrideMetadata（
            typeof运算（富），OriginalMetaData）;
    }
}
 

解决方案

WPF将合并属性的元数据为你，当你调用OverrideMetadata，没有必要通过它原来的元数据对象。因此，所有你需要做的就是

  UIElement.RenderTransformOriginProperty.OverrideMetadata（typeof运算（富），新PropertyMetadata（新PropertyChangedCallback（Origin_Changed）））;
 

有一点需要注意的是有时候code以上抛出一个异常。这两种情况下，出现这种情况是

1 原来的元数据是PropertyMetadata的一个子类 - 我见过FrameworkPropertyMetadata和UIPropertyMetadata。你只需要使用适当的一个在每种情况下。

2 的依赖项属性是只读的，你不能做任何事情。

I'm trying to add a PropertyChangedCallback to UIElement.RenderTransformOriginProperty. An exception is thrown when I try to override the PropertyMetadata.

I have searched MSDN and Google, and all I have been able to come up with is this. DependencyPropertyDescriptor.AddValueChanged is suggested at some point in that post, but that won't solve my problem since this is not a per-instance callback.

I don't understand what this exception means at all. Does anyone know what I am doing wrong?

public class foo : FrameworkElement
{
    private static void Origin_Changed( DependencyObject d,
                                        DependencyPropertyChangedEventArgs e)
    { }

    static foo()
    {
        PropertyMetadata OriginalMetaData =
            UIElement.RenderTransformOriginProperty.GetMetadata(
                typeof(FrameworkElement));



/*An exception is thrown when this line is executed:
 "Cannot change property metadata after it has been associated with a property"*/
        OriginalMetaData.PropertyChangedCallback +=
            new PropertyChangedCallback(Origin_Changed);



        UIElement.RenderTransformOriginProperty.OverrideMetadata(
            typeof(foo), OriginalMetaData);
    }
}

解决方案

WPF will merge the property metadata for you when you call OverrideMetadata, no need to pass it the original Metadata object. So all you have to do is

UIElement.RenderTransformOriginProperty.OverrideMetadata(typeof(foo), new PropertyMetadata(new PropertyChangedCallback(Origin_Changed)));

One thing to be aware of is sometimes the code above throws an exception. The two cases where that happens are

1. The original metadata is a subclass of PropertyMetadata - I have seen FrameworkPropertyMetadata and UIPropertyMetadata. You just have to use the appropriate one in each case.

2. The dependency property is read only and you can't do anything about it.

这篇关于我如何添加逻辑到现有的依赖属性的回调？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！