About a month ago, the Flex team announced that nightly builds of the next version of Flex (code named Gumbo) were now available on the opensource Flex site. Along with early versions of the SDK, the team also posted a ton of other information, including specs and API docs.
Below is a listing of links and resources about Gumbo / Flex 4. Going through these will quickly get your up to speed on what is being worked on in Flex 4, and get your ready for playing around with the new builds.
- Official opensource project page
- Flex 4 Downloads
- Gumbo Themes
- An Introduction to the Gumbo Component Architecture
- Flex 4 Language / API Reference
- The Next Dimension of Rich Experiences (360 Flex Keynote) Mark Anders
- Flex SDK Next Plan (Matt Chotin)
- Flex 4 Skinning Architechture : Deepa Subramaniam
- Designer / Developer Workflow in Flex 4 (Ely Greenfield)
- FXG 1.0 (Graphic Interchange Format)
- Skinning and Skinnable Components
- Enchanced States Syntax
- Group Containter
- Items Component and Selector
- Layout Interface
- Application Component
- Text Primitives
- TextInput Component
- TextArea Component
- ScrollBar Component
- Range Class
- Track Base Class
- Slider Component
- Spinner Component
- Numeric Stepper Component
- MXML 2009
Sean Moore also has a good link round up on his blog.
If you have any more resources or links, post them in the comments.