What's the deal with Halo and Spark in Flex 4?

Question

I used Flex 2/3 for a couple of years and have just come back to it, upgrading to Flex 4. I'm seeing some references to Halo and Spark themes but I never got into themes in my previous work so I'm a bit confused what's going on. Am I correct that Halo was the default theme, or is this all new?

Also, I took a simple Flex 3 MXML and tried to make it work in Flex4:

<?xml version="1.0" encoding="utf-8"?>
<mx:Application
    xmlns:fx="http://ns.adobe.com/mxml/2009"
    xmlns:mx="library://ns.adobe.com/flex/mx"
    layout="absolute" borderStyle="none" borderColor="#000000" cornerRadius="0" 
    backgroundGradientAlphas="[1.0, 1.0]" backgroundGradientColors="[#000000, #FF0000]"
    width="800" height="600"
    frameRate="20">
</mx:Application>

But I get errors, like "Error: The style 'backgroundGradientColors' is only supported by type 'mx.core.Application' with the theme(s) 'halo'."

I tried a few things but each one introduced its own errors.

Answer 1

Flex 4 introduced a new component architecture, called "Spark". "Halo" refers to the Flex 3 component architecture.

• an overview of Spark vs Halo components
• instructions to use Halo theme in a project (which is what you need to make your example compile)