All Your Base Are Belong To Us

from Yahoo! Flash(R) Blog by Alaric Cole

You’be probably heard of the many available SWF embed parameters, such as allowNetworking, wmode, allowScriptAccess, and of course height and width. But there a few less well-known ones that may be incredibly useful.

One of these optional attributes you can specify when embedding a SWF is named base. Adobe’s LiveDocs give the following description of the attribute:

base - Specifies the base directory or URL used to resolve relative path statements in ActionScript.

This is important when dealing with relative paths (URLs) in Flash. By default, the base URL of a SWF is its parent HTML page. This means if your SWF is loaded from another domain and has assets such as images or XML files, you’re in trouble.

Say, for instance, that yahoo.com loads a SWF ad from ads.com/car.swf. That SWF, in turn, tries to load “images/convertible.jpg”, using a relative path to an images directory that resides on ads.com. Given the default settings, this will not work, unless “yahoo.com/images/convertible.jpg” exists. To get around this, developers have often used absolute paths, such as “http://ads.com/images/convertible.jpg”. The base parameter solves this problem, allowing you to continue to use relative paths.

To allow relative paths, all you need to do is “reset” the base directory. You can even use base="." to mean “make all paths relative to the swf itself.”

Here’s an example of a full embed statement:

<object classid=”clsid:d27cdb6e-ae6d-11cf-96b8-444553540000″
width=”500″
height=”300″
codebase=”http://active.macromedia.com/flash7/cabs/swflash.cab#version=9,0,2,8″>
<param name=”base” value=”.”></param>
<param name=”movie” value=”someSWF.swf”></param>
<param name=”play” value=”true”></param>
<embed base=”.” src=”someSWF.swf”
width=”500″ height=”300″ play=”true”
pluginspage=”http://www.macromedia.com/shockwave/download/index.cgi?P1_Prod_Version=ShockwaveFlash”>
</embed></object>

If you are fond of the popular SWFObject JavaScript library for embedding Flash content, here’s how to use the base parameter with it:

var swfobj = new SWFObject(value, “someSWF”, 500, 300, “9.0.28″, “#FFFFFF”);
swfobj.addParam(”base“, “.“);
swfobj.write(”swfDiv”);

Note that if you have a SWF that loads another SWF within itself, the base path of the loaded SWF will always be the loading SWF.