WPF PlacementTarget技巧

<Window x:Class="WpfApplication1.Window1"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="Window1" Height="300" Width="300">
    <Grid>
        <Button x:Name="btn1"  Content="Button" HorizontalAlignment="Left" Margin="42,77,0,0" VerticalAlignment="Top" Width="75" Click="btn1_Click"/>

        <!--AllowsTransparency="True"
           PopupAnimation="Fade"
           HorizontalOffset="5"
           VerticalOffset="-120"-->
        
        <Popup x:Name="popusBottom2"   PlacementTarget="{Binding ElementName=btn1}"  StaysOpen="False" >
            <Border Height="200" Width="200" Background="AliceBlue"></Border>
        </Popup>
    </Grid>
</Window>

private void btn1_Click(object sender, RoutedEventArgs e)
        {
            popusBottom2.IsOpen = false;
            popusBottom2.IsOpen = true;
            popusBottom2.PopupAnimation = System.Windows.Controls.Primitives.PopupAnimation.Scroll;

            if (popusBottom2.IsOpen == false)
            {
                popusBottom2.IsOpen = true;
            }
        }