WPF Popup弹出框箭头自动定位效果

在WPF中如何做到，点击按钮，弹出一个带箭头的消息框，箭头对准按钮，效果如图所示。

 XAML代码

        <Button Grid.Row="1" Grid.Column="1" x:Name="button" Content="..." HorizontalAlignment="Right" Margin="0,0,100,0" VerticalAlignment="Top" Click="button_Click" Width="10"/>
        <Popup x:Name="Popup1" IsOpen="False" StaysOpen="True" PlacementTarget="{Binding ElementName=button}" AllowsTransparency="True" Placement="Center" HorizontalOffset="0" VerticalOffset="36">
            <Grid Width="200" Background="Transparent" >
                <Grid.RowDefinitions>
                    <RowDefinition Height="15"></RowDefinition>
                    <RowDefinition Height="50"></RowDefinition>
                </Grid.RowDefinitions>
                <Border Grid.Row="0" Background="Transparent">
                    <Canvas Background="Transparent">
                        <TextBlock x:Name="TextBlock1" Background="Transparent" Canvas.Left="0" Canvas.Top="0" Padding="0 1.1 0 0" FontSize="20" Foreground="Blue">^</TextBlock>
                    </Canvas>
                </Border>
                <Border Grid.Row="1" Background="BurlyWood" BorderThickness="1" BorderBrush="#ccc">
                    <TextBlock>123456789abcdefghijklmnopqrstuvw</TextBlock>
                </Border>
            </Grid>
        </Popup>

　　CS代码

        public MainWindow()
        {
            InitializeComponent();
            Popup1.Opened += Popup1_Opened;
            Popup1.Closed += Popup1_Closed;
        }

        private void Popup1_Closed(object sender, EventArgs e)
        {
            TextBlock1.SetValue(Canvas.LeftProperty, (double)0);
        }

        private void Popup1_Opened(object sender, EventArgs e)
        {
            Point point = button.TranslatePoint(new Point(0, 0), this);
            Point point2 = TextBlock1.TranslatePoint(new Point(0, 0), this);
            TextBlock1.SetValue(Canvas.LeftProperty, point.X - point2.X);
        }

        private void button_Click(object sender, RoutedEventArgs e)
        {
            Popup1.IsOpen = true;
        }

　　看代码其实很简单，只是没想到办法，突然想到了就写一个简单的例子做演示。