This doesn't require a custom control. You can create a normal TextBox or TextBlock and add a triangle underneath (or above) it. Use a template selector to choose left or right based on which person is chatting:
Xaml:
<Page.Resources>
<DataTemplate x:Key="ChatTemplateR">
<StackPanel Margin="30,2,0,2">
<Border Background="{Binding Fill}" >
<TextBlock MinWidth="200" Text="{Binding Text}" TextWrapping="Wrap" Margin="5"/>
</Border>
<Path x:Name="DownRightTri"
HorizontalAlignment="Right"
Margin="0,0,10,0"
Fill="{Binding Fill}"
Data="M0,0 H10 V10" />
</StackPanel>
</DataTemplate>
<DataTemplate x:Key="ChatTemplateL">
<StackPanel Margin="0,2,30,2" >
<Path x:Name="UpLeftTri"
HorizontalAlignment="Left"
Margin="10,0,0,0"
Fill="{Binding Fill}"
Data="M0,-5 V5 H10 " />
<Border Background="{Binding Fill}" >
<TextBlock MinWidth="200" Text="{Binding Text}" TextWrapping="Wrap" Margin="5"/>
</Border>
</StackPanel>
</DataTemplate>
<local:ChatTemplateSelector x:Key="ChatSelector" LeftTemplate="{StaticResource ChatTemplateL}" RightTemplate="{StaticResource ChatTemplateR}"/>
</Page.Resources>
<Grid>
<ListView x:Name="lv" ItemTemplateSelector="{StaticResource ChatSelector}"/>
</Grid>
TemplateSelector:
class ChatTemplateSelector: DataTemplateSelector
{
public DataTemplate LeftTemplate { get; set; }
public DataTemplate RightTemplate { get; set; }
protected override DataTemplate SelectTemplateCore(object item, DependencyObject container)
{
DataItem di = (DataItem)item;
DataTemplate dt = di.IsLeft ? this.LeftTemplate : this.RightTemplate;
return dt;
}
}