我在运行时通过使用ItemsControl生成UI元素。 UI会成功生成，但是如果无法获取所生成的UI项的任何属性，例如标签的“ Content”或SelectedItem的ComboBox。我尝试使用this tutorial和these answers来获取这些属性，但是我总是得到NullReferenceException。
XAML中的ItemsControl看起来像这样：
<ItemsControl Name="ListOfVideos">
<ItemsControl.Background>
<SolidColorBrush Color="Black" Opacity="0"/>
</ItemsControl.Background>
<ItemsControl.ItemTemplate>
<DataTemplate>
<Grid Margin="0,0,0,10">
<Grid.ColumnDefinitions>
<ColumnDefinition Width="180"/>
<ColumnDefinition Width="400"/>
<ColumnDefinition Width="200"/>
</Grid.ColumnDefinitions>
<Image HorizontalAlignment="Left" Height="100" Width="175" x:Name="VideoThumbnailImage" Stretch="Fill" Source="{Binding VideoThumbnailURL}" Grid.Column="0"></Image>
<Label x:Name="VideoTitleLabel" Content="{Binding VideoTitleText}" Foreground="White" Grid.Column="1" VerticalAlignment="Top" FontSize="16" FontWeight="Bold"></Label>
<Label x:Name="VideoFileSizeLabel" Content="{Binding VideoTotalSizeText}" Foreground="White" FontSize="14" Grid.Column="1" Margin="0,0,0,35" VerticalAlignment="Bottom"></Label>
<Label x:Name="VideoProgressLabel" Content="{Binding VideoStatusText}" Foreground="White" FontSize="14" Grid.Column="1" VerticalAlignment="Bottom"></Label>
<ComboBox x:Name="VideoComboBox" SelectionChanged="VideoComboBox_SelectionChanged" Grid.Column="2" Width="147.731" Height="20" VerticalAlignment="Bottom" HorizontalAlignment="Center" Margin="0,0,0,50" ItemsSource="{Binding VideoQualitiesList}"></ComboBox>
<Label Content="Video Quality" Foreground="White" FontSize="14" VerticalAlignment="Top" Grid.Column="2" HorizontalAlignment="Center"></Label>
<Label Content="Audio Quality" Foreground="White" FontSize="14" VerticalAlignment="Bottom" HorizontalAlignment="Center" Margin="0,0,0,27" Grid.Column="2"></Label>
<Slider x:Name="VideoAudioSlider" Grid.Column="2" HorizontalAlignment="Center" VerticalAlignment="Bottom" Width="147.731" Maximum="{Binding AudioCount}"></Slider>
</Grid>
</DataTemplate>
</ItemsControl.ItemTemplate>
</ItemsControl>
这是我生成UI元素的方式
public class VideoMetadataDisplay
{
public string VideoTitleText { get; set; }
public int AudioCount { get; set; }
public string VideoThumbnailURL { get; set; }
public string VideoStatusText { get; set; }
public string VideoTotalSizeText { get; set; }
public List<string> VideoQualitiesList { get; set; }
}
public partial class PlaylistPage : Page
{
private void GetPlaylistMetadata()
{
List<VideoMetadataDisplay> newList = new List<VideoMetadataDisplay>();
//populate the list
ListOfVideos.ItemsSource = newList;
}
}
这就是我试图获取UI元素的属性的方式
public class Utils
{
public childItem FindVisualChild<childItem>(DependencyObject obj)
where childItem : DependencyObject
{
for (int i = 0; i < VisualTreeHelper.GetChildrenCount(obj); i++)
{
DependencyObject child = VisualTreeHelper.GetChild(obj, i);
if (child != null && child is childItem)
{
return (childItem)child;
}
else
{
childItem childOfChild = FindVisualChild<childItem>(child);
if (childOfChild != null)
return childOfChild;
}
}
return null;
}
}
private void VideoComboBox_SelectionChanged(object sender, SelectionChangedEventArgs e)
{
UIElement CurrentItem = (UIElement)ListOfVideos.ItemContainerGenerator.ContainerFromItem(ListOfVideos.Items.CurrentItem);
Utils utils = new Utils();
ContentPresenter CurrentContentPresenter = utils.FindVisualChild<ContentPresenter>(CurrentItem);
DataTemplate CurrentDataTemplate = CurrentContentPresenter.ContentTemplate;
Label VideoTitle = (Label)CurrentDataTemplate.FindName("VideoTitleLabel", CurrentContentPresenter);
string VideoTitleText = VideoTitle.Content.ToString();
MessageBox.Show(VideoTitleText);
}
[每次我尝试运行此命令时，FindVisualChild总是返回标签之一（VideoTitleLabel），而不是返回当前活动项目的ContentPresenter。 CurrentDataTemplate为空，因此我无法从中获取任何UI元素。
FindVisualChild<ContentPresenter>不可能返回Label实例。 FindVisualChild将结果转换为ContentPresenter。由于Label是not一个ContentPresenter，因此将抛出一个InvalidCastException。
仅为了获取其绑定数据而访问DataTemplate或查找控件总是太复杂了。直接访问数据源总是更容易。ItemsControl.SelectedItem将返回所选项目的数据模型。您通常对容器不感兴趣。
private void VideoComboBox_SelectionChanged(object sender, SelectionChangedEventArgs e)
{
var listView = sender as ListView;
var item = listView.SelectedItem as VideoMetadataDisplay;
MessageBox.Show(item.VideoTitleText);
}
您的版本（已改进）
FindVisualChild的实现很弱。如果遍历遇到没有子节点的子节点，即参数obj为null，它将失败并引发异常。您必须在调用obj之前检查null的参数VisualTreeHelper.GetChildrenCount(obj)，以避免引用null。
而且您也不需要通过访问模板来搜索元素。您可以直接在视觉树中查找它。我已经修改了您的FindVisualChild方法以按名称搜索元素。为了方便起见，我也将其转换为扩展方法：
扩展方法
public static class Utils
{
public static bool TryFindVisualChildByName<TChild>(
this DependencyObject parent,
string childElementName,
out TChild childElement)
where TChild : FrameworkElement
{
childElement = null;
if (parent == null)
{
return false;
}
for (int i = 0; i < VisualTreeHelper.GetChildrenCount(parent); i++)
{
DependencyObject child = VisualTreeHelper.GetChild(parent, i);
if (child is TChild resultElement && resultElement.Name.Equals(childElementName, StringComparison.Ordinal))
{
childElement = resultElement;
return true;
}
if (child.TryFindVisualChild(childElementName, out childElement))
{
return true;
}
}
return false;
}
}
示例
private void VideoComboBox_SelectionChanged(object sender, SelectionChangedEventArgs e)
{
var listView = sender as ListView;
object item = listView.SelectedItem;
var itemContainer = listView.ItemContainerGenerator.ContainerFromItem(item) as ListViewItem;
if (itemContainer.TryFindVisualChild("VideoTitleLabel", out Label label))
{
var videoTitleText = label.Content as string;
MessageBox.Show(videoTitleText);
}
}