Monday, September 8, 2014

Dynamically Changing the Color of SVG Using an ASP.NET HttpHandler

I have an SVG file that I'm using as the background image of elements on the page (using sprites). I wanted to dynamically update the color of the shapes and paths when theming the application, but I could not find a way to do this using CSS when the SVG is loaded as a background image (I have heard that inline <svg> elements will inherit style rules).

The way I solved the issue was to use a custom HttpHandler in my ASP.NET application that allows me to replace parts of the SVG with other content. Here's the code I used:

using System;
using System.Collections.Generic;
using System.IO;
using System.Text;
using System.Web;

namespace MyNamespace
{
 public class DynamicSVGHandler : IHttpHandler
 {
  public bool IsReusable
  {
   get
   {
    return true;
   }
  }

  public void ProcessRequest( HttpContext context )
  {
   var request = context.Request;
   var response = context.Response;

   var file = context.Server.MapPath( Path.ChangeExtension( request.Url.LocalPath, ".svg" ) );

   if( File.Exists( file ) )
   {
    var lastWrite = File.GetLastWriteTimeUtc( file );
    response.Clear();
    response.ContentType = "image/svg+xml";
    response.Cache.SetExpires( DateTime.Now.AddMinutes( 5d ) );
    response.Cache.SetCacheability( HttpCacheability.Public );

    List<KeyValuePair<string, string>> replacements = new List<KeyValuePair<string, string>>();

    string value;

    foreach( string key in request.QueryString )
    {
     if( key.Length > 3 && !string.IsNullOrEmpty( value = request.QueryString[key] ) && value.Length > 3 )//Implement any custom validation here.
      replacements.Add( new List<KeyValuePair<string, string>>( key, value ) );
    }

    if( replacements.Count > 0 )
    {
     string line;
     using( StreamReader reader = new StreamReader( File.OpenRead( file ) ) )
     {
      while( (line = reader.ReadLine()) != null )
      {
       foreach( var replacement in replacements )
        line = line.Replace( replacement.Key, replacement.Value );

       response.Output.WriteLine( line );
      }
     }
    }
    else
     response.WriteFile( file );
   }
   else
   {
    response.Clear();
    response.StatusCode = 404;
    response.End();
   }
  }
 }
}

Make sure to register your handler in your web.config (This example is for IIS 7+, integrated mode and I used the extension .dsvg):

<configuration>
 <system.webServer>
  <handlers>
   <add name="*.dsvg" path="*.dsvg" preCondition="integratedMode" verb="*" type="MyNamespace.DynamicSVGHandler, MyAssembly"/>
  </handlers>
 </system.webServer>
</configuration>

You can now reference any svg file in your application with the extension .dsvg instead of .svg and specify replacements in the URL (e.g. mysvg.dsvg?white=red to change "white" to "red").

In my SVG, I found it easiest to define the color using a style section rather than setting the fill of each shape, then add the class="svg-shape" to all the shapes.