为什么我不能在这种情况下设置不透明度?

<TabControl>
    <TabItem Header="item 1">
        <ListBox>
            <ListBoxItem>sub item 1</ListBoxItem>
            <ListBoxItem>sub item 2</ListBoxItem>
            <ListBoxItem>sub item 3</ListBoxItem>
        </ListBox>
    </TabItem>
    <TabItem Header="item 2">
        <ListBox>
            <ListBoxItem>sub item 1</ListBoxItem>
            <ListBoxItem>sub item 2</ListBoxItem>
        </ListBox>
    </TabItem>
</TabControl>

<Style TargetType="ListBox">
    <Setter Property="ItemsPanel">
        <Setter.Value>
            <ItemsPanelTemplate>
                <StackPanel Orientation="Horizontal"
                      VisibleChanged="onStackPanelVisibilityChange"
                      Loaded="onStackPanelLoaded"
                      VerticalAlignment="Center" HorizontalAlignment="Center" />
            </ItemsPanelTemplate>
        </Setter.Value>
    </Setter>
</Style>

你会注意到我在堆栈面板上有一些事件处理程序。这些是为堆栈面板中的项目设置动画,以便它们按顺序淡入视图。事件处理程序实现为:

void onStackPanelLoaded(object sender, RoutedEventArgs e)
{
    StackPanel panel = sender as StackPanel;

    applySubItemAnimations(panel);
}

void onStackPanelVisibilityChange(object sender, DependencyPropertyChangedEventArgs e)
{
    StackPanel panel = sender as StackPanel;

    if (panel.IsVisible)
    {
        applySubItemAnimations(panel);
    }
}

private void applySubItemAnimations(StackPanel panel)
{
    DoubleAnimation fadeIn = new DoubleAnimation();
    fadeIn.DecelerationRatio = 0.1;
    fadeIn.Duration = new Duration(new TimeSpan(0, 0, 0, 0, 500));
    fadeIn.From = 0.0;
    fadeIn.To = 1.0;

    for (int i = 0; i < panel.Children.Count; i++)
    {
        panel.Children[i].Opacity = 0.0;
        fadeIn.BeginTime = new TimeSpan(0, 0, 0, 0, 200 * i + 50);
        panel.Children[i].BeginAnimation(UIElement.OpacityProperty, fadeIn);
    }
}

在大多数情况下,这非常有效。第一次单击(或加载)选项卡时,“堆栈”面板中的子项会依次淡入视图。问题是,当您单击返回到之前已经显示过一次的选项卡时(即,您在“VisibleChanged”事件处理程序中,而不是在“Loaded”处理程序中),所有项目都已显示,并且它们 眨眼 为了有序,而不是一开始就被隐藏起来 整齐

panel.Children[i].Opacity = 0.0;

... 什么也不做。如果我在调试器中单步执行代码,并在“panel.Children[I].Opacity”上放置一个手表,它将保持在1.0。没有例外。它只是。。。不起作用。