In my recent efforts to create an MP3 downloader application I have been extensively investigating the Adobe Flex browser API to pass through download URLs and other meta data through to my app.

Any sensible person would ensure a web service or suitable server-based XML file was available to AIR requiring only an ID or two to get going but in this instance I had no such luxury.

Typically a flash/flex badge sits on a website and can detect, install and launch an AIR app while passing through simple arguments. It is not widely documented that these arguments may only be alpha-numeric and may not include special characters like dots, slashes, percents and ampersands. This counts simple encoding out.

The way the flash badge fetches content from the page is using the External Interface method whereby the flash badge can call JavaScript functions embedded on the page, which in turn allows XML, also embedded in the page, to be collected and encoded.

I came up with my own encoding mechanism so that XML could be passed through the flash badge and into my AIR app as a string and then decoded there. This meant replacing . with DOT and % with PCNT and / with SLSH etc etc

So everything is working a lot better now, although it would be helpful if Adobe included details about this restriction in their Flex documentation to save wasting time debugging why the flash badge fails to launch the AIR app for no apparent reason. It’s probably a fair restriction for various security reasons due to people running dubious scripts on even more dubious websites that would somehow take hold of your computer via AIR. I had never thought a percent sign could be so dangerous, exclamation marks are much more scary.