WPF Popup带箭头，自适应箭头位置在target中间的实现方式

如题，在WPF项目中，有时会涉及使用Popup控件实现一些弹出式菜单：

1.程序不靠屏幕边框时，箭头居于Popup中央，指向target中央

 

 2.程序靠边框时，Popup自动偏移，箭头仍然指向target控件的中央，这样比较美观和谐

 

并且，当target控件比较靠下时，Popup控件会自动翻转到target的上方，箭头向下

 这里使用了我自定义的一个Popup控件：CustomizePopup 

            <!--Popup-->
            <local:CustomizePopup StaysOpen="False" AllowsTransparency="True" IsOpen="{Binding IsPopupMore, ElementName=header}" PlacementTarget="{Binding ElementName=btnMore}" Placement="Bottom">
                <StackPanel Margin="2">
                    <StackPanel.Effect>
                        <DropShadowEffect Color="Gray" Opacity="0.4" ShadowDepth="0" Direction="270" />
                    </StackPanel.Effect>
                    <Path x:Name="arrowTop" StrokeThickness="0" Fill="{DynamicResource PopupBackgroundColor}" HorizontalAlignment="Left" Data="M11,0 0,10 22,10Z"/>
                    <Border Background="#FF242424" Width="135" CornerRadius="4">
                        <StackPanel Margin="0,0">
                            <local:ImageButton Content="修改密码" Height="35" Style="{StaticResource PopupMenuBtnStyle}" CornerRadius="4,4,0,0"/>
                            <local:ImageButton Content="管理后台" Height="35" Style="{StaticResource PopupMenuBtnStyle}" Margin="0,1,0,0"/>
                            <local:ImageButton Content="退出登录" Height="35" Style="{StaticResource PopupMenuBtnStyle}" Margin="0,1,0,0" CornerRadius="0,0,4,4"/>
                        </StackPanel>
                    </Border>
                    <Path x:Name="arrowBottom" StrokeThickness="0" Fill="{DynamicResource PopupBackgroundColor}" HorizontalAlignment="Left" Data="M0,0 22,0 11,10Z"/>
                </StackPanel>
            </local:CustomizePopup>