WPF ToolBarPanel宽度未更改

Question

我里面有一个Canvas和一个ToolBarPanel。

我想使用Thumb调整ToolBarPanel的大小，因此我将Thumb放在ToolBarPanel中。当我捕捉到Thumb的DragDelta时，宽度不会改变。以下是代码：

<Canvas x:Uid="userControlCanvas" Name="userControlCanvas" Width="300" Height="300">
    <ToolBarPanel x:Uid="categoriesToolBar" Name="categoriesToolBar" Visibility="Visible" 
                  Height="300" Width="10" Panel.ZIndex="100" 
                  Background="#CCDCDCDC" 
                  SizeChanged="toolbarSizeChanged">
            <Thumb x:Name="splitPane" Cursor="SizeWE" Height="300" Canvas.Left="2" Width="8" DragStarted="splitPaneDragStarted" DragDelta="splitPaneDragging">
            </Thumb>
    </ToolBarPanel>
</Canvas>

和cs代码：

    private void splitPaneDragging(object sender, DragDeltaEventArgs e) {
        categoriesToolBar.Width = categoriesToolBar.ActualWidth + e.HorizontalChange;
        //categoriesToolBar.InvalidateMeasure();
        categoriesToolBar.UpdateLayout();
        //categoriesToolBar.InvalidateArrange();
        //categoriesToolBar.InvalidateVisual();
        //categoriesToolBar.InvalidateMeasure();

        //Width is always the initial width (150)
        Console.WriteLine("ch=" + e.HorizontalChange + " tw=" + categoriesToolBar.Width + " aw=" + categoriesToolBar.ActualWidth);
    }

    private void toolbarSizeChanged(object sender, SizeChangedEventArgs e) {
        // this is not fired when I try to change the width in splitPaneDragging method
    }