ListView的ItemsPanelTemplate抛出一个例外 [英] ItemsPanelTemplate of a ListView throws apperently worngly an exception

问题描述

我创建了一个包含具有自定义ItemsPanelTemplate的ListView的用户控件。

I created a user control which contains a ListView with a custom ItemsPanelTemplate.

<UserControl x:Class="..."
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
Name="thisUserControl">
<ListView ItemsSource="{Binding Source={StaticResource cvs}}"
          Name="mainListView">
    <ListView.GroupStyle>
        <GroupStyle>
            ...
        </GroupStyle>
    </ListView.GroupStyle>
    <ListView.ItemsPanel>
        <ItemsPanelTemplate>
            <cd:TimeLinePanel UnitsPerSecond="{Binding ElementName=thisUserControl,Path=DataContext.UnitsPerSecond}" Start="{Binding ElementName=thisUserControl, Path=DataContext.Start}"/>
        </ItemsPanelTemplate>
    </ListView.ItemsPanel>
</ListView>

UserControl的DataContext有两个属性Start和UnitsPerSecond。当我使用组时，我不能简单地写

The DataContext of the UserControl has two properties Start and UnitsPerSecond. As I am using groups I cannot simply write

Start={Binding Path=.Start}

因此，我使用了上面的代码。但是，如果我将开始的绑定更改为此，我会得到一个例外：

Thus I used the code above. But if I change the binding of Start to this I get an exception:

ItemsTanelTemplate的VisualTree必须是单个元素。

显然，ItemsPanelTemplate只有一个元素。

Obviously the ItemsPanelTemplate has got just a single element.

那么可能是什么问题？我的自定义面板不创建任何元素。

So what could be the problem? My custom panel does not create any elements. It just arranges them.

提前感谢

推荐答案

你正在收到此异常，可能是因为您尝试将Children添加到TimeLinePanel（或者您将覆盖面板的可视化树，并在VisualChildrenCount中返回1以外的内容）。可悲的是，如果由于ItemsPanelTemplate而在ItemsControl中创建，则不能修改面板的Children属性。在ItemsControl之外，没有问题。

You are receiving this exception probably because you are trying to add "Children" to TimeLinePanel (or you are overriding the visual tree of the panel and returning something other than 1 in "VisualChildrenCount"). Sadly, you cannot modify the "Children" property of a panel if it is created inside an ItemsControl because of an ItemsPanelTemplate. Outside of an ItemsControl, there is no problem.

或者，您可以覆盖ListView的模板，如下所示 - 它适用于我的例子，并摆脱了异常。

Alternatively, you can override the template of ListView to something like the following - it works in my case, and gets rid of the exception.

<ListView.Template>
    <ControlTemplate>
        <cd:TimeLinePanel IsItemsHost="True" UnitsPerSecond="..."/>
    <ControlTemplate>
</ListView.ItemsPanel>

有一个CodeProject文章（ http://www.codeproject.com/KB/WPF/ConceptualChildren.aspx ），其中提供了有关此行为的一些细节，这可能有助于您了解为什么就是这样。

There is a CodeProject article (http://www.codeproject.com/KB/WPF/ConceptualChildren.aspx) which gives some details on this behavior, which may help you understand why it is the case.

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