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.

Gradey

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)

Comitative

So Spark is not an alternate theme, but a new approach - that suggests I'd be better using Spark in my Flex 4 application if it's not a big amount of work, right? – Gradey

That's right. You should focus on the spark UI components for new projects. – Comitative

Disagree. You should choose based on the project requirements. Sometimes Spark makes things (which were simple in Halo) needlessly more complicated. Spark is better if you're doing heavy UI skinning & customization. If you don't do custom skin work, but want something nicer looking than Spark's boring default look, Halo is a nice alternative. Our users tend to like Halo UIs better than Spark UIs, which look like a step back to some (out of the box). – Fabien