如何从附加的依赖属性为属性设置值？

Question

I'm trying to show selected items of treeview in textblock. This is my XAML code

<Style TargetType="{x:Type TreeViewItem}">
   <Style.Triggers>
     <Trigger Property="IsSelected" Value="true">
       <Setter Property="vm:HLViewModel.SelectedNode" Value="{Binding ElementName="tree",Path=SelectedItem}"/>
     </Trigger>
   </Style.Triggers>
</Style>

Here is my textblock where I'm trying to show selected Item

<TextBlock Text="{Binding myText}"/>

I have created attached dependencyproperty which will be set when Treeview's IsSelected property is triggered. How Can I set the value of myText in the callback function?

public class HLViewModel : DependencyObject
    {
        public myText{get;set;}

        public static object GetSelectedNode(DependencyObject obj)
        {
            return (object)obj.GetValue(SelectedNodeProperty);
        }

        public static void SetSelectedNode(DependencyObject obj, object value)
        {
            obj.SetValue(SelectedNodeProperty, value);
        }

        public static readonly DependencyProperty SelectedNodeProperty =
            DependencyProperty.RegisterAttached("SelectedNode", typeof(object), typeof(HLViewModel), new PropertyMetadata("def",SelectedNode_changed));

        private static void SelectedNode_changed(DependencyObject d, DependencyPropertyChangedEventArgs e)
        {
            // wanna set of myText property value here
        }

Answer 1

Here's the simple way to have a viewmodel use the selected item from a TreeView:

XAML:

    <TreeView
        x:Name="MyTreeView"
        SelectedItemChanged="MyTreeView_SelectedItemChanged"

Codebehind:

private void MyTreeView_SelectedItemChanged(object sender, RoutedPropertyChangedEventArgs<object> e)
{
    (DataContext as MyViewModel).SelectedRoomLevelItem = e.NewValue;
}

Here's a much more sophisticated way of doing the same thing. This example demonstrates how attached properties are used in WPF. Note that setting TreeViewAttached.SelectedItem will not set the selected item of the tree view -- if you want to do that, it's doable, but troublesome. All this attached property does is allow you to write bindings which receive the selected item from the treeview when the selection changes.

public static class TreeViewAttached
{
    #region TreeViewAttached.SelectedItem Attached Property
    public static Object GetSelectedItem(TreeView obj)
    {
        return (Object)obj.GetValue(SelectedItemProperty);
    }

    public static void SetSelectedItem(TreeView obj, Object value)
    {
        obj.SetValue(SelectedItemProperty, value);
    }

    public static readonly DependencyProperty SelectedItemProperty =
        DependencyProperty.RegisterAttached("SelectedItem", typeof(Object), typeof(TreeViewAttached),
            new FrameworkPropertyMetadata(null) {
                BindsTwoWayByDefault = true
            });
    #endregion TreeViewAttached.SelectedItem Attached Property

    #region TreeViewAttached.MonitorSelectedItem Attached Property
    public static bool GetMonitorSelectedItem(TreeView obj)
    {
        return (bool)obj.GetValue(MonitorSelectedItemProperty);
    }

    public static void SetMonitorSelectedItem(TreeView obj, bool value)
    {
        obj.SetValue(MonitorSelectedItemProperty, value);
    }

    public static readonly DependencyProperty MonitorSelectedItemProperty =
        DependencyProperty.RegisterAttached("MonitorSelectedItem", typeof(bool), typeof(TreeViewAttached),
            new PropertyMetadata(false, MonitorSelectedItem_PropertyChanged));

    private static void MonitorSelectedItem_PropertyChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        if ((bool)e.NewValue)
        {
            (d as TreeView).SelectedItemChanged += TreeViewAttached_SelectedItemChanged;
        }
        else
        {
            (d as TreeView).SelectedItemChanged -= TreeViewAttached_SelectedItemChanged;
        }
    }

    private static void TreeViewAttached_SelectedItemChanged(object sender, RoutedPropertyChangedEventArgs<object> e)
    {
        SetSelectedItem(sender as TreeView, e.NewValue);
    }
    #endregion TreeViewAttached.MonitorSelectedItem Attached Property
}

XAML:

<TreeView
    local:TreeViewAttached.MonitorSelectedItem="True"
    local:TreeViewAttached.SelectedItem="{Binding SelectedRoomLevelItem}"
    ItemsSource="{Binding Items}"
    >
    <TreeView.ItemTemplate>
        <HierarchicalDataTemplate ItemsSource="{Binding Items}">
            <Label Content="{Binding HeaderText}" />
        </HierarchicalDataTemplate>
    </TreeView.ItemTemplate>
</TreeView>
<Label Content="{Binding Path=SelectedRoomLevelItem.HeaderText}" />

My example uses a quickie treeview item datacontext class with Items and HeaderText properties. Your use of this code will have to adapt to the particular viewmodel classes in your project.