WPF 错误:找不到目标元素的管理 FrameworkElement [英] WPF Error: Cannot find governing FrameworkElement for target element

问题描述

我有一个带有图像的行的 DataGrid.该图像与某个状态的触发器绑定.当状态改变时，我想改变图像.

I've got a DataGrid with a row that has an image. This image is bound with a trigger to a certain state. When the state changes I want to change the image.

模板本身设置在 DataGridTemplateColumn 的 HeaderStyle 上.这个模板有一些绑定.第一个绑定日显示它是哪一天，状态通过触发器更改图像.

The template itself is set on the HeaderStyle of a DataGridTemplateColumn. This template has some bindings. The first binding Day shows what day it is and the State changes the image with a trigger.

这些属性在 ViewModel 中设置.

These properties are set in a ViewModel.

属性:

public class HeaderItem
{
    public string Day { get; set; }
    public ValidationStatus State { get; set; }
}

this.HeaderItems = new ObservableCollection<HeaderItem>();
for (int i = 1; i < 15; i++)
{
    this.HeaderItems.Add(new HeaderItem()
    {
        Day = i.ToString(),
        State = ValidationStatus.Nieuw,
    });
}

数据网格:

<DataGrid x:Name="PersoneelsPrestatiesDataGrid" HorizontalAlignment="Stretch" VerticalAlignment="Stretch"
              AutoGenerateColumns="False" SelectionMode="Single" ItemsSource="{Binding CaregiverPerformances}" FrozenColumnCount="1" >

    <DataGridTemplateColumn HeaderStyle="{StaticResource headerCenterAlignment}" Header="{Binding HeaderItems[1]}" Width="50">
        <DataGridTemplateColumn.CellEditingTemplate>
            <DataTemplate>
                <TextBox Text="{ Binding Performances[1].Duration,Converter={StaticResource timeSpanConverter},Mode=TwoWay}"/>
            </DataTemplate>
        </DataGridTemplateColumn.CellEditingTemplate>

        <DataGridTemplateColumn.CellTemplate>
            <DataTemplate>
                <TextBlock TextAlignment="Center" Text="{ Binding Performances[1].Duration,Converter={StaticResource timeSpanConverter}}"/>
            </DataTemplate>
        </DataGridTemplateColumn.CellTemplate>
    </DataGridTemplateColumn> 
</DataGrid>

Datagrid HeaderStyleTemplate:

Datagrid HeaderStyleTemplate:

<Style x:Key="headerCenterAlignment" TargetType="{x:Type DataGridColumnHeader}">
    <Setter Property="HorizontalContentAlignment" Value="Center"/>

    <Setter Property="Template">
        <Setter.Value>
            <ControlTemplate TargetType="{x:Type DataGridColumnHeader}">
                <Grid>
                    <Grid.RowDefinitions>
                        <RowDefinition />
                        <RowDefinition />
                    </Grid.RowDefinitions>

                    <TextBlock Grid.Row="0" Text="{Binding Day}" />
                    <Image x:Name="imageValidation" Grid.Row="1" Width="16" Height="16" Source="{StaticResource imgBevestigd}" />
                </Grid>

                <ControlTemplate.Triggers>
                    <MultiDataTrigger >
                        <MultiDataTrigger.Conditions>
                            <Condition Binding="{Binding State}" Value="Nieuw"/>                                 
                        </MultiDataTrigger.Conditions>
                        <Setter TargetName="imageValidation" Property="Source" Value="{StaticResource imgGeenStatus}"/>
                    </MultiDataTrigger>
                </ControlTemplate.Triggers>
            </ControlTemplate>
        </Setter.Value>
    </Setter>
</Style>

现在，当我启动项目时，图像不显示，并且出现此错误:

Now when I startup the project the images doesn't show and I get this error:

System.Windows.Data 错误:2:找不到目标元素的管理 FrameworkElement 或 FrameworkContentElement.BindingExpression:Path=HeaderItems[0];数据项=空；目标元素是DataGridTemplateColumn"(HashCode=26950454)；目标属性是标题"(类型对象")

System.Windows.Data Error: 2 : Cannot find governing FrameworkElement or FrameworkContentElement for target element. BindingExpression:Path=HeaderItems[0]; DataItem=null; target element is 'DataGridTemplateColumn' (HashCode=26950454); target property is 'Header' (type 'Object')

为什么会显示此错误?

推荐答案

遗憾的是，任何 DataGridColumn 托管在 DataGrid.Columns 下都不是 Visual 的一部分> 树，因此未连接到数据网格的数据上下文.因此绑定不适用于它们的属性，例如 Visibility 或 Header 等(尽管这些属性是有效的依赖属性！).

Sadly any DataGridColumn hosted under DataGrid.Columns is not part of Visual tree and therefore not connected to the data context of the datagrid. So bindings do not work with their properties such as Visibility or Header etc (although these properties are valid dependency properties!).

现在你可能想知道这怎么可能?他们的 Binding 属性不应该绑定到数据上下文吗?那么它只是一个黑客.绑定并没有真正起作用.实际上是数据网格单元复制/克隆这个绑定对象并用它来显示自己的内容！

Now you may wonder how is that possible? Isn't their Binding property supposed to be bound to the data context? Well it simply is a hack. The binding does not really work. It is actually the datagrid cells that copy / clone this binding object and use it for displaying their own contents!

现在回到解决您的问题，我假设 HeaderItems 是设置为父视图的 DataContext 的对象的属性.我们可以将视图的DataContext连接到任何DataGridColumn，通过我们称之为ProxyElement强>.

So now back to solving your issue, I assume that HeaderItems is a property of the object that is set as the DataContext of your parent View. We can connect the DataContext of the view to any DataGridColumn via something we call a ProxyElement.

下面的例子说明了如何将逻辑子视图(例如ContextMenu 或DataGridColumn)连接到父视图的DataContext

The example below illustrates how to connect a logical child such as ContextMenu or DataGridColumn to the parent View's DataContext

 <Window x:Class="WpfApplicationMultiThreading.Window5"
         xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
         xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"        
         xmlns:vb="http://schemas.microsoft.com/wpf/2008/toolkit"
         Title="Window5" Height="300" Width="300" >
  <Grid x:Name="MyGrid">
    <Grid.Resources>
        <FrameworkElement x:Key="ProxyElement" DataContext="{Binding}"/>
    </Grid.Resources>
    <Grid.DataContext>
         <TextBlock Text="Text Column Header" Tag="Tag Columne Header"/>
    </Grid.DataContext>
    <ContentControl Visibility="Collapsed"
             Content="{StaticResource ProxyElement}"/>
    <vb:DataGrid AutoGenerateColumns="False" x:Name="MyDataGrid">
        <vb:DataGrid.ItemsSource>
            <x:Array Type="{x:Type TextBlock}">
                <TextBlock Text="1" Tag="1.1"/>
                <TextBlock Text="2" Tag="1.2"/>
                <TextBlock Text="3" Tag="2.1"/>
                <TextBlock Text="4" Tag="2.2"/>
            </x:Array>
        </vb:DataGrid.ItemsSource>
        <vb:DataGrid.Columns>
            <vb:DataGridTextColumn
                       Header="{Binding DataContext.Text,
                                     Source={StaticResource ProxyElement}}"
                       Binding="{Binding Text}"/>
            <vb:DataGridTextColumn
                       Header="{Binding DataContext.Tag,
                                     Source={StaticResource ProxyElement}}"
                       Binding="{Binding Tag}"/>
        </vb:DataGrid.Columns>
    </vb:DataGrid>
  </Grid>
</Window>

如果我没有实现 ProxyElement hack，上面的视图遇到了相同的绑定错误.ProxyElement 是从主视图窃取 DataContext 并将其提供给逻辑子视图的任何 FrameworkElement，例如 ContextMenu 或 DataGridColumn.为此，它必须作为 Content 托管在同一视图下的不可见 ContentControl 中.

The view above encountered the same binding error that you have found if I did not have implemented the ProxyElement hack. The ProxyElement is any FrameworkElement that steals the DataContext from the main View and offers it to the logical child such as ContextMenu or DataGridColumn. For that it must be hosted as a Content into an invisible ContentControl which is under the same View.

我希望这能引导您朝着正确的方向前进.

I hope this guides you in correct direction.

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