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[update 19.12.2010]
For several months I’ve used fb4linux and I would say that it works a lot better than the bellow described solution.
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At last after a loooong several hours digging and (re)searching how to setup my ubuntu(9.04 32bit) workstation to be able to develop in somehow _avarage_ speed I found a way doing it so with Flex Builder for Linux alpha 5 (and hopefully there will be alpha 6 supporting fully Flex4
.
I’ve lost around 2 hours to understand why Flexbuilder can not debug Flex4 based AIR project, when suddenly I’ve opened project’s “.actionScriptProperties” file and found what really was happening preventing me with all of the possible errors found during such setup…

So the problem is in the target version which was used (Flex Builder for Linux is generating air template projects for flex 4.0.0 sdk using ‘old’ .actionScriptProperties file, a template one you can get from the attached template-workspace-0.1-bla.tar)
And here are the essential parts which needed me quite a lot of time to figure out (may be one day this all will be one click setup?)
0. Ubuntu 9.04 32 bit with java installed (there are a lot of tutorials, mine is just to select its as package via Applications-> Add/Remove UI)
1. You need 32bit Eclipse 3.3.x (extract, mine is /home/outbounder/eclipse)
2. You need Flex Builder of course (run as normal console application ala “ALT+F2 – > xterm”)
3. After that you need all the sdks for development:
3.1 flex 4.0
3.1.2 (optional) I still work on flex 3 projects, so sdk 3.4 is needed)
3.3 Air SDK is needed, where strangely for one day I encountered several changes resulting in such download page at this moment (2010.03.17, 2:07 am)
4. Extract flex 4 sdk to install location of flex builder (mine destination is at /home/outoubnder/flexbuilder/sdks/4.0.0)
5. Extract AIR sdk on top of flex 4 sdk destination folder
6. Rename adl & adt (those are located at the ./bin folder of the flex 4 sdk destination folder) to adl_lin & adt_lin
7. Fire up eclipse, add flex 4 sdk (more info here)
8. You can use the above mentioned template workspace for flex3/4 +/- AIR projects
9. You’ll need to follow also the instructions for installing Adobe Flash Player 10 — Debugger Version. as well and for even faster coding -> snippets
And now I can dream of abstract GUIs managing complex data relationships…
