Flash XML Compressor

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Flash movies are known for their small file size and fast download. The movie file itself is compressed, external graphics and sound elements are also often heavily compressed using for example jpeg and mp3. XML files loaded into a Flash movie however are not. This is not a big problem most of the time, but it could be. I will explain one simple way to reduce the file size of XML files to about 30-50% of the original size (in some extreme cases you can reduce them to about 10% of the original file size). If you have a lot of XML files, this could save quite a bit of bandwidth. This technique requires Flash MX or later.

How does it work?

The XML file is compressed using a variation of the LZ77 algorithm. This method is pretty effective and is very fast to decode (which is good since we are dealing with Flash). The encoding is quite CPU intensive though, and it is not ideally done using ActionScript. The Flash XML Compressor is therefore available as a separate program (Windows executable).

ActionScript Prototype

In order for your Flash movie to understand compressed XML files, you need to add the following prototype:

XML.prototype.onData = function(i) {
        if (i.charAt(0) != '<') {
                var ecPos = i.indexOf(" ")+1;
                var eC = i.charAt(ecPos);
                i = i.substr(ecPos+1);
                var o = "";
                var iL = i.length;
                for (var n=0;n<iL;n++) {
                        if (i.charAt(n) == eC) {
                                var p = i.charCodeAt(n+1)*114 + i.charCodeAt(n+2) - 1610;
                                var l = i.charCodeAt(n+3)-14;
                                o += o.substr(-p, l);
                                n += 3;
                        } else {
                                o += i.charAt(n);
        } else {

        this.onLoad(i != undefined);

The code above is preferably placed on frame 1 in the movie. You can now load compressed XML files in the same way you load normal XML files. You can still load normal XML files, just make sure you don't have any white space before the <?xml version = "1.0"?>.

Example .fla

Here is an example you can look at: exampleFLA.zip (22Kb)
It includes a test xml file (48Kb), and the same xml file compressed (14Kb). The xml file contains map data for a game if you were wondering :-)

The Flash XML Compressor - Windows application

Download Version 1.1:

New in version 1.1:


  1. Simply drag one or more xml files to the compressor window, or select File -> Compress XML File(s).
  2. By default, new compressed files will be created in the same directory as the original XML files. If you compress a file called myXMLFile.xml, a file called myXMLFileCompressed.xml will be created. If it already exists, it will be overwritten.
  3. Done! The file is compressed and hopefully much smaller! :-)

    You can also convert a compressed file back to the original XML file. Select File -> Decode Compressed File. When the window with the decoded data appears, select File -> Save As to save the decoded data to a file.



Place a short cut in Window's "Send To" Context menu. This way you can right click on XML files and choose Send To -> Flash XML Compressor. Here's how:

  1. Click Start, and then click Run.
  2. In the Open box, type sendto, and then click OK.
  3. Place a short cut to Flash XML Compressor in the sendto folder.


Flash XML Compressor in Java

Java Class (12Kb zipped)

Flash XML Compressor in PHP

Ian Turgeon has ported the encoder to PHP.


File Format and Source Code

Flash XML Compressed File Format 1.0:

C# Encoder Function:


Q: The decoding works fine on my computer, but when I upload the compressed xml file to a server the decoded xml data is corrupted.
A: When uploading the compressed xml file, use binary transfer instead of ASCII to make sure the text file format is not converted when uploading.

Other approaches

This is an interesting tool which converts the xml file to a compressed swf (Flash MX and later). You use loadMovie to load the xml data. When the movie has loaded, you can read the xml data which is located in the loaded movie. This is a really neat solution!

HTTP Compression
This is something which will make this tool a bit redundant. It is already supported in many browsers. The content is compressed using gzip before it is sent from the server. The browser uncompresses the data, and the Flash movie doesn't know the difference, but the xml file will load a lot faster!