Text on A Path for Silverlight

• Download source code - 192 KB

Introduction

Currently, with Silverlight, there is no built-in capability to display a text string that will follow the path of a given geometry (see picture below). Similarly, Windows Presentation Foundation does not have such a capability either, but WPF does provides a number of helper methods (e.g., path flattening) that provide a good start. I wrote an article a while ago that provides text on a path for WPF. The article here takes the text on a path class written for WPF and makes it available for Silverlight.

Background

To display text on a path, the fundamental problem that must be solved is to figure out how to translate and rotate each character of the string to fit the specified geometry. A big help in solving the problem is the WPF method GetFlattenedPathGeometry() provided by the Geometry base class. This method returns a PathGeometry that approximates the geometry by a series of lines (PolyLineSegment and LineSegment). For example, the red lines below depict the flattened path of the path geometry (black):

Currently, Silverlight 2 has no such path flattening method! Charles Petzold developed a path flatting method in his article and gave permission to reuse the code. The code is included here in the ‘PathGeometryHelper’ project. Charles’ method was written for WPF, and so it needed to be ported to Silverlight. In porting this code to Silverlight, more missing functionality was found. Namely, Silverlight lacks some methods in the Matrix class. Also, Silverlight has no Vector class. Equivalent functionality for these classes (and a few others) has been provided in the ‘MatrixMathHelpers’ project.

The next step in the problem is to take the flattened path geometry and figure out how to transform the characters of the string to follow the flattened path. The figure below depicts the angle that must be calculated so the letter “A” will follow the flattened path. In TextOnAPath, the method ‘GetIntersectionPoints’ in the GeometryHelper class calculates the points along the flattened path where the characters intersect with the path. Knowing the intersection points, the angle (and translation) values can then be calculated.

Using the code

In the attached solution, there are two projects for the TextOnAPath control, one for Silverlight (TextOnAPathSilverlight) and another for WPF (TextOnAPathWPF). The source code in the Silverlight version is linked over to the WPF version. In this way, both versions of the control share a common code base. The compiler directive ‘#if SILVERLIGHT’ is used to denote code differences. Notice that the XAML is not linked between the two controls because currently there is no equivalent for the ‘#if’ compiler directive in XAML.

The Silverlight version of the TextOnAPath, like the WPF version, is used just like any normal UIElement. The XAML below will display the text string along the curved path centered in a grid. Notice how the path is defined using the mini-language by the ‘MyPath’ string variable (for more info on path mini-language, see this article). Currently, Silverlight has no built-in functionality to convert this mini-language string into a PathGeometry, but thankfully, a converter has been provided in this article. The code is included here in the ‘PathConverter’ project. Using a binding converter, the TextPath attribute is able to get the desired PathGeometry value from the String value.

<UserControl x:Class="TextOnAPathTestAppSilverlight.PageTest"
   xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" 
   xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
   xmlns:core="clr-namespace:System;assembly=mscorlib"
   xmlns:PathConverter="clr-namespace:PathConverter;
       assembly=PathConverter"
   xmlns:TextOnAPath="clr-namespace:TextOnAPath;
       assembly=TextOnAPathSilverlight"
   Width="500" Height="500">

 <Grid>
   <Grid.Resources>

     <core:String x:Key="MyPath">M0,0 C120,361 230.5,276.5 
230.5,276.5 L308.5,237.50001 C308.5,237.50001 419.5,179.5002 
367.5,265.49993 315.5,351.49966 238.50028,399.49924 
238.50028,399.49924 L61.500017,420.49911</core:String>

     <PathConverter:StringToPathGeometryConverter 
         x:Key="MyPathConverter"/>

   </Grid.Resources>

   <TextOnAPath:TextOnAPath TextPath="{Binding 
     Source={StaticResource MyPath}, 
     Converter={StaticResource MyPathConverter}}" 
     FontSize="25" DrawLinePath="True" 
     Text="The quick brown fox jumped over the lazy dog."/>
 
 </Grid>

</UserControl>

Parameters of the TextOnAPath control:

• Text (string): the string to be displayed. If the Text string is longer than the geometry path, then text will be truncated.
• TextPath (Geometry): the geometry for the text to follow.
• DrawPath (Boolean) (only in WPF version): if true, draws the geometry below the text string.
• DrawLinePath (Boolean): if true, draws the flattened path geometry below the text string.
• ScaleTextPath (Boolean): if true, the geometry will be scaled to fit the size of the control.

Points of interest

• Additional optimization of the code is needed to improve performance.
• Other similar work.