Flex 3 to 4

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One of the major themes in the Flex 4 SDK is "Design in Mind".

Differences

http://www.adobe.com/devnet/flex/articles/flex3and4_differences.html
http://oreilly.com/insideria/index.html

Transitioning

About You have arrived at this page because InsideRIA is no longer an active website, as of January 31, 2011. While we're sad to see the change, we think you will find the resources and community here at oreilly.com helpful. An archive of the material previously posted on InsideRIA can be found at DevelopRIA.com .
By Gaurav, on December 19th, 2009 In Flex 3, the styles defined using the [Style] metadata were validated by the flex compiler, but styles defined in css files were not validated.. Flex 4 has multiple themes: http://gauravj.com/blog/2009/12/styles-themes-and-the-stricter-compiler/

Styles & Themes

If you want to compile a Flex 3 project with a Flex 4 compiler your are willing to set "report-invalid-styles-as-warnings" to true (otherwise it will raise errors). by nicolas Sep 16