Xaml 用指针构建一个框架

我想构建一个框架，如下图所示，带有指针，在本例中为右侧的三角形：

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如何做到这一点？

使用

网格创建两个堆叠的
BoxView
s，并将顶部旋转45度：

<StackLayout>
    <Grid HorizontalOptions="Center" VerticalOptions="CenterAndExpand" WidthRequest="100" HeightRequest="100">
        <BoxView HorizontalOptions="Center" VerticalOptions="CenterAndExpand" WidthRequest="100" HeightRequest="80" BackgroundColor="Red"/>
        <BoxView HorizontalOptions="End" VerticalOptions="CenterAndExpand" 
                 WidthRequest="50" HeightRequest="50" BackgroundColor="Red" Rotation="45"/>
        <StackLayout VerticalOptions="Center" HorizontalOptions="Center" Spacing="0">
            <Label HorizontalTextAlignment="Center" CharacterSpacing="2" FontSize="13" TextColor="White" Text="DECEMBER"/>
            <Label HorizontalTextAlignment="Center" FontAttributes="Bold" FontSize="22" TextColor="White" Text="25"/>
            <Label HorizontalTextAlignment="Center" CharacterSpacing="2" FontSize="13" TextColor="White" Text="WEDNESDAY"/>
        </StackLayout> 
    </Grid>
</StackLayout>