This might save you some time if you’re building apps with Flex 4 Beta (Gumbo). In Spark Applications there is no backgroundAlpha property within you main MXML application file. To create a transparent Flex 4 background you’ll need to do the following:
1) Create a custom application skin MXML file.
2) Set the skinClass of your application MXML to the custom skin file: you can do this in the MXML for your app – eg. skinClass=”com.mycompany.myproject.skins.MyAppSkin”.
Search for the default ApplicationSkin.mxml in the Flex 4 SDK. Duplicate this file, rename it and place it within your project source. In the new skin code I just added alpha=”0″ to the backgroundRect declaration.
That’s it 
Thanks! I use this so I can place my Gumbo app over the html. very useful as a CMS editing system.
seriously? flex 4 so far, has been a f****ucking pain in the a**ss!!!! they’ve gone back to splash 2.0 days. little things like this have been driving me insAn3.
why should I have to cutNpaste an old class, just to reuse existing functionality that used to be just 10 characters long?
this works btw, but it’s ludicrous. I feel like I’ve gone to plaid. And now I’m a sad panda.
Fuck you flex 4 sdk. BackgroundImage not found :/
Yeah… they changed Flex UI Components and lost the background image style property in most (or all) cases which I think was a really bad move on Adobe’s part…
Could someone just send me a complete example.
I don’t seem to get this together.
Thanxs
You don’t need to copy the whole ApplicationSkin.mxml file, you can extend it.
Just create MySkin.as and do this:
package
{
import spark.skins.spark.ApplicationSkin;
public class MySkin extends ApplicationSkin
{
public function MySkin()
{
backgroundRect.alpha = 0;
}
}
}
Thanks a lot for the hint anyway : )
J
They have changed alsmot every thing ! Its a new start for every one… just a lot of waste of time ;-(
Here’s a good application transparency that works: http://kbala.com/how-to-create-transparent-application-background-spark-sdk/