如何知道使用 mvvm 单击了哪个树视图项

Question

我有一个 WPF MVVM 应用程序，它有一个 TreeView，所有静态项目都保存在 XAML 页面中。我如何知道在我的视图模型中单击了哪个 MenuItem，以便我可以相应地显示相应的页面。

    <TreeView Height="Auto" HorizontalAlignment="Stretch" Margin="0" Name="MyTreeViewMenu" 
                      VerticalAlignment="Stretch" Width="Auto" Opacity="1" 
                     BorderThickness="1" BorderBrush="Black" Grid.Row="2">

        <TreeViewItem Header="Country" Width="Auto" HorizontalAlignment="Stretch" 
                      ></TreeViewItem>

        <TreeViewItem Header="View Details" Width="Auto" HorizontalAlignment="Stretch" IsEnabled="False">
                <TreeViewItem Header="User" />
                <TreeViewItem Header="Group" />
                <TreeViewItem Header="User Group" />
            </TreeViewItem>
    </TreeView>

Answer 1

我想Selected 事件在您的情况下与click 具有相同的效果。要确定选择了哪一个TreeViewItem，您应该添加事件Trigger：

<TreeView Height="Auto" HorizontalAlignment="Stretch" Margin="0" Name="MyTreeViewMenu" 
                      VerticalAlignment="Stretch" Width="Auto" Opacity="1" 
                     BorderThickness="1" BorderBrush="Black" Grid.Row="2">

        <TreeViewItem Header="Country" Width="Auto" HorizontalAlignment="Stretch"></TreeViewItem>    
        <TreeViewItem Header="View Details" Width="Auto" HorizontalAlignment="Stretch" IsEnabled="False">
                <TreeViewItem Header="User" />
                <TreeViewItem Header="Group" />
                <TreeViewItem Header="User Group" />
            </TreeViewItem>
               <i:Interaction.Triggers>
                  <i:EventTrigger EventName="SelectedItemChanged">
                      <i:InvokeCommandAction 
                         Command="{Binding selectItemCommand}"
                         CommandParameter="{Binding SelectedItem, ElementName=MyTreeViewMenu}"/>
                  </i:EventTrigger>
              </i:Interaction.Triggers>
</TreeView>

因此，您可以使用并确定通过传递给Command 的参数选择了哪个项目。

ViewModel 应该如下所示：

private ICommand _selectItemCommand;
public ICommand selectItemCommand
{
    get
    {
        return _selectItemCommand ?? (_selectItemCommand = new RelayCommand(param => this.LoadPage(param)));
    }
}

private void LoadPage(object selectedMenuItem)
{
      ...
}

Answer 2

查看 MSDN 上的 TreeView.SelectedItem Property 页面。

您可以直接绑定到TreeView.SelectedItem 属性：

<TreeView ItemsSource="{Binding Items}" SelectedItem="{Binding Item, Mode=OneWay}" />

注意TreeView.SelectedItem 属性是只读的，所以您必须使用OneWay 绑定...这意味着您不能从视图模型中设置选定项。为此，您需要使用Attached Property 创建自己的双向选定项属性。

编辑>>>

抱歉@Scroog1，我通常使用AttachedProperty 来执行此操作。您是对的，即使使用OneWay 绑定，使用此方法也会出错。不幸的是，我的AttachedProperty 代码很长，但还有另一种方法。

我不一定推荐这样做，因为将 UI 属性放入数据对象从来都不是一个好主意，但如果您将 IsSelected 属性添加到数据对象，则可以将其直接绑定到 @987654333 @属性：

<TreeView ItemsSource="Items" HorizontalAlignment="Stretch" ... Name="MyTreeViewMenu">
    <TreeView.ItemContainerStyle>
        <Style TargetType="{x:Type TreeViewItem}">
            <Setter Property="IsSelected" Value="{Binding IsSelected}" />
        </Style>
    </TreeView.ItemContainerStyle>
</TreeView>

我刚刚在 StackOverflow 上的 WPF MVVM TreeView SelectedItem 帖子中搜索并为您找到了一个“更全面”的答案。

另外，还有另一种方法...您也可以使用TreeView.SelectedValue 和TreeView.SelectedValuePath 属性。基本思想是将TreeView.SelectedValuePath 属性设置为数据对象上的属性名称。当一个项目被选中时，TreeView.SelectedValue 属性将被设置为所选数据项的该属性的值。您可以从 MSDN 的 How to: Use SelectedValue, SelectedValuePath, and SelectedItem 页面了解有关此方法的更多信息。如果您具有唯一可识别的属性（例如某种标识符），这通常最有效。此代码示例来自 MSDN：

<TreeView ItemsSource="{Binding Source={StaticResource myEmployeeData}, 
XPath=EmployeeInfo}" Name="myTreeView" SelectedValuePath="EmployeeNumber" />

<TextBlock Margin="10">SelectedValuePath: </TextBlock>
<TextBlock Margin="10,0,0,0" Text="{Binding ElementName=myTreeView, 
Path=SelectedValuePath}" Foreground="Blue"/>

<TextBlock Margin="10">SelectedValue: </TextBlock>
<TextBlock Margin="10,0,0,0" Text="{Binding ElementName=myTreeView, 
Path=SelectedValue}" Foreground="Blue"/>

Answer 3

除了绑定到 TreeView.SelectedItem 属性：

在使用 MVVM 时，它帮助我停止思考 UI 中的事件并开始思考 UI 中的状态。

您可以将 ViewModel 绑定到 View 的属性。所以通常我会尝试将 SelectedItem 绑定到 ViewModel 上的属性，以便 ViewModel 知道选择了什么。

以同样的方式，您可以向正在显示的 ViewModel 项目添加一个名为 Selected 的属性，并将该属性绑定到视图中的复选框。这样您就可以在 ViewModel 中轻松启用多项选择并访问选定的项目。

Answer 4

为了完整起见，这里是附加属性和 TreeView 子类选项：

附加属性选项

public static class TreeViewSelectedItemHelper
{
    public static readonly DependencyProperty BindableSelectedItemProperty
        = DependencyProperty.RegisterAttached(
            "BindableSelectedItem",
            typeof (object),
            typeof (TreeViewSelectedItemHelper),
            new FrameworkPropertyMetadata(false,
                                          OnSelectedItemPropertyChanged)
                {
                    BindsTwoWayByDefault = true
                });

    public static object GetBindableSelectedItem(TreeView treeView)
    {
        return treeView.GetValue(BindableSelectedItemProperty);
    }

    public static void SetBindableSelectedItem(
        TreeView treeView, 
        object selectedItem)
    {
        treeView.SetValue(BindableSelectedItemProperty, selectedItem);
    }

    private static void OnSelectedItemPropertyChanged(
        DependencyObject sender,
        DependencyPropertyChangedEventArgs args)
    {
        var treeView = sender as TreeView;
        if (treeView == null) return;
        SetBindableSelectedItem(treeView, args.NewValue);
        treeView.SelectedItemChanged -= HandleSelectedItemChanged;
        treeView.SelectedItemChanged += HandleSelectedItemChanged;
        if (args.OldValue != args.NewValue)
            SetSelected(treeView, args.NewValue);
    }

    private static void SetSelected(ItemsControl treeViewItem,
                                    object itemToSelect)
    {
        foreach (var item in treeViewItem.Items)
        {
            var generator = treeViewItem.ItemContainerGenerator;
            var child = (TreeViewItem) generator.ContainerFromItem(item);
            if (child == null) continue;
            child.IsSelected = (item == itemToSelect);
            if (child.HasItems) SetSelected(child, itemToSelect);
        }
    }

    private static void HandleSelectedItemChanged(
        object sender,
        RoutedPropertyChangedEventArgs<object> args)
    {
        if (args.NewValue is TreeViewItem) return;
        var treeView = sender as TreeView;
        if (treeView == null) return;
        var binding = BindingOperations.GetBindingExpression(treeView,
            BindableSelectedItemProperty);
        if (binding == null) return;
        var propertyName = binding.ParentBinding.Path.Path;
        var property = binding.DataItem.GetType().GetProperty(propertyName);
        if (property != null)
            property.SetValue(binding.DataItem, treeView.SelectedItem, null);
    }
}

子类选项

public class BindableTreeView : TreeView
{
    public BindableTreeView()
    {
        SelectedItemChanged += HandleSelectedItemChanged;
    }

    public static readonly DependencyProperty BindableSelectedItemProperty =
        DependencyProperty.Register(
            "BindableSelectedItem",
            typeof (object),
            typeof (BindableTreeView),
            new FrameworkPropertyMetadata(
                default(object),
                OnBindableSelectedItemChanged) {BindsTwoWayByDefault = true});

    public object BindableSelectedItem
    {
        get { return GetValue(BindableSelectedItemProperty); }
        set { SetValue(BindableSelectedItemProperty, value); }
    }

    private static void OnBindableSelectedItemChanged(
        DependencyObject d,
        DependencyPropertyChangedEventArgs e)
    {
        var treeView = d as TreeView;
        if (treeView != null) SetSelected(treeView, e.NewValue);
    }

    private static void SetSelected(ItemsControl treeViewItem,
                                    object itemToSelect)
    {
        foreach (var item in treeViewItem.Items)
        {
            var generator = treeViewItem.ItemContainerGenerator;
            var child = (TreeViewItem) generator.ContainerFromItem(item);
            if (child == null) continue;
            child.IsSelected = (item == itemToSelect);
            if (child.HasItems) SetSelected(child, itemToSelect);
        }
    }

    private void HandleSelectedItemChanged(
        object sender,
        RoutedPropertyChangedEventArgs<object> e)
    {
        SetValue(BindableSelectedItemProperty, SelectedItem);
    }
}