为什么附加属性属性更改事件只触发一次？

I have a listbox binded to a list of objects. For each list item I wanted to have a rectangle whose fill color is determined by a few properties of the binded object. So I did the following:

我有一个绑定到对象列表的列表框。对于每个列表项,我想要一个矩形,其填充颜色由绑定对象的一些属性确定。所以我做了以下事情:

Made sure INotifyPropertyChanged was implemented on my object.

确保在我的对象上实现了INotifyPropertyChanged。

Created a class to expose the properties I am interested in as attached properties.

创建了一个类来公开我感兴趣的属性作为附加属性。

Binded the properties of the object to the attached properties of the rectangle

将对象的属性绑定到矩形的附加属性

Created a style that uses triggers to set the rectangle fill based on attached properties.

创建了一个样式,该样式使用触发器根据附加属性设置矩形填充。

This works, but only the first time the property of the object changes. After that, the attached properties do not seem to be receiving notification when the data object's property changes. I have double checked and my data object is raising the INotifyPropertyChanged event. What could be the problem?

这是有效的,但只是第一次对象的属性发生变化。之后,附加属性似乎在数据对象的属性更改时不会收到通知。我已经仔细检查过,我的数据对象正在引发INotifyPropertyChanged事件。可能是什么问题呢?

<Rectangle Style="{StaticResource RecordStateRectangleStyle}" 
                Width="10" Height="10" Stroke="Black"
                local:RecordAttachment.RecordState="{Binding Path=RecordState}"
                local:RecordAttachment.IsDeleted="{Binding Path=IsDeleted}" />

The Style:

   <Style x:Key="RecordStateRectangleStyle" TargetType="Rectangle">
        <Style.Resources>
            <SolidColorBrush x:Key="AddedStateBrush" Color="LightGreen" Opacity=".8" />
            <SolidColorBrush x:Key="ModifiedStateBrush" Color="Orange"  Opacity=".8" />
            <SolidColorBrush x:Key="DeletedStateBrush" Color="Red" Opacity=".8" />
        </Style.Resources>
        <Style.Triggers>
            <Trigger Property="local:RecordAttachment.RecordState" Value="{x:Static model:RecordState.Added}">
                <Setter Property="Fill" Value="{StaticResource AddedStateBrush}" />
            </Trigger>
            <Trigger Property="local:RecordAttachment.RecordState" Value="{x:Static model:RecordState.Modified}">
                <Setter Property="Fill" Value="{StaticResource ModifiedStateBrush}" />
            </Trigger>
            <Trigger Property="local:RecordAttachment.IsDeleted" Value="true">
                <Setter Property="Fill" Value="{StaticResource DeletedStateBrush}" />
            </Trigger>
        </Style.Triggers>
    </Style>

Attached Properties Class:

附属物类:

Public Class RecordAttachment
Public Shared ReadOnly RecordStateProperty As DependencyProperty
Public Shared ReadOnly IsDeletedProperty As DependencyProperty

Shared Sub New()
    RecordStateProperty = DependencyProperty.RegisterAttached("RecordState", _
                                                               GetType(Model.RecordState), _
                                                               GetType(RecordAttachment), _
                                                               New PropertyMetadata(Model.RecordState.Unchanged, AddressOf RecordStatePropertyChanged))
    IsDeletedProperty = DependencyProperty.RegisterAttached("IsDeleted", _
                                                          GetType(Boolean), _
                                                          GetType(RecordAttachment), _
                                                          New PropertyMetadata(AddressOf DeletedPropertyChanged))
End Sub

Public Shared Sub SetRecordState(ByVal element As UIElement, ByVal state As Model.RecordState)
    element.SetValue(RecordStateProperty, state)
End Sub
Public Shared Function GetRecordState(ByVal element As UIElement) As Model.RecordState
    Return CType(element.GetValue(RecordStateProperty), Model.RecordState)
End Function

Public Shared Sub SetIsDeleted(ByVal element As UIElement, ByVal value As Boolean)
    element.SetValue(IsDeletedProperty, value)
End Sub

Public Shared Function GetIsDeleted(ByVal element As UIElement) As Boolean
    Return CType(element.GetValue(IsDeletedProperty), Boolean)
End Function

Public Shared Sub RecordStatePropertyChanged(ByVal sender As DependencyObject, ByVal e As DependencyPropertyChangedEventArgs)
    If sender IsNot Nothing Then
        sender.SetValue(RecordStateProperty, e.NewValue)
    End If
End Sub
Public Shared Sub DeletedPropertyChanged(ByVal sender As DependencyObject, ByVal e As DependencyPropertyChangedEventArgs)
    If sender IsNot Nothing Then
        sender.SetValue(IsDeletedProperty, e.NewValue)
    End If
End Sub
End Class

Someone suggested I post C# version, so here it is:

有人建议我发布C#版本,所以这里是:

    public class RecordAttachment
{
    public static readonly DependencyProperty RecordStateProperty;
    public static readonly DependencyProperty IsDeletedProperty;

    static RecordAttachment()
    {
        RecordStateProperty = DependencyProperty.RegisterAttached("RecordState",
                                                                  typeof(model.RecordState),
                                                                  typeof(RecordAttachment),
                                                                  new PropertyMetadata(model.RecordState.Unchanged, RecordStatePropertyChanged));
        IsDeletedProperty = DependencyProperty.RegisterAttached("IsDeleted",
                                                                 typeof(bool),
                                                                 typeof(RecordAttachment),
                                                                 new PropertyMetadata(DeletedPropertyChanged));
    }

    public static void SetRecordState(UIElement element, model.RecordState state)
    {
        element.SetValue(RecordStateProperty, state);
    }
    public static model.RecordState GetRecordState(UIElement element)
    {
        return (model.RecordState)element.GetValue(RecordStateProperty);
    }
    public static void SetIsDeleted(UIElement element, bool value)
    {
        element.SetValue(IsDeletedProperty, value);
    }
    public static bool GetIsDeleted(UIElement element)
    {
        return (bool)element.GetValue(IsDeletedProperty);
    }

    public static void RecordStatePropertyChanged(DependencyObject sender, DependencyPropertyChangedEventArgs e)
    {
        if (sender != null)
            sender.SetValue(RecordStateProperty, e.NewValue);
    }
    public static void DeletedPropertyChanged(DependencyObject sender, DependencyPropertyChangedEventArgs e)
    {
        if (sender != null)
            sender.SetValue(IsDeletedProperty, e.NewValue);
    }
}

UPDATE I solved my underlying problem of needing to change the color of the rectangle fill by using datatriggers instead of using the attached properties and regular triggers. I would still like to know why the attached property 'propertychanged' event is only fired once though.

更新我通过使用数据触发器而不是使用附加属性和常规触发器解决了我需要更改矩形填充颜色的基本问题。我仍然想知道为什么附加属性'propertychanged'事件只被触发一次。

I did some more googling and I came across this link where Josh Smith says 'An attached property can only be set on an element once.'. I've looked around and I can't find any explanation...

我做了一些谷歌搜索,我遇到了这个链接,其中约什史密斯说'附加属性只能在元素上设置一次。'。我环顾四周,找不到任何解释......

2 个解决方案

#1

The problem is caused by these lines of code in the property change handlers:

问题是由属性更改处理程序中的这些代码行引起的:

sender.SetValue(RecordStateProperty, e.NewValue)

and

sender.SetValue(IsDeletedProperty, e.NewValue)

By calling SetValue, you are setting a new local value on the target. Setting a local value replaces any data binding that might previously have been in place.

通过调用SetValue,您将在目标上设置新的本地值。设置本地值将替换先前可能已存在的任何数据绑定。

In short, your property change handler removes the data binding for that property.

简而言之,您的属性更改处理程序会删除该属性的数据绑定。

Since you are effectively removing the binding, your property will no longer change when the data source changes because it is no longer the data source for that property.

由于您实际上正在删除绑定,因此当数据源发生更改时,您的属性将不再更改,因为它不再是该属性的数据源。

A property change notification is just that - it tells you that the property's value is changing. You do not need to do anything in response to that if you don't want to, and in particular, it's not your responsibility to make the property change. It's going to change anyway.

属性更改通知就是这样 - 它告诉您属性的值正在发生变化。如果您不愿意,您不需要做任何响应,特别是,您不负责更改属性。无论如何它会改变。

#2

In addition to selected answer , this can also be resolved by using

除了选择的答案,这也可以通过使用来解决

Sender.SetCurrentValue(IsDeletedProperty, e.NewValue)

This will change the value of the dependency without changing the source

这将在不更改源的情况下更改依赖项的值

#1