It was recently pointed out to me that some applications, which shall remain nameless here, are using File.applicationStorageDirectory to cache hundreds of megabytes of files. Please, don’t do this.
The File.applicationStorageDirectory property is intended to provide applications with a guaranteed writable, unique location in which they can save important information, regardless of which platform they’re running on. The information is saved to locations that are, on typical machines, backed up. On Windows, it’s saved under “<user>/Application Data”, which also roams with the user, if roaming profiles are enabled. There’s an implicit assumption here that the amount of data being stored is relatively small–megabytes at most.
Large sets of cached files should be stored elsewhere, in locations that aren’t backed up and don’t roam. AIR doesn’t include a handy property for finding such a location, but don’t let that stop you; the filesystem API lets you access any location on disk, after all. On Windows, you can use the “<user>\Local Settings\Application Data” directory (note the “Local Settings”); on Mac OS, use “<user>/Library/Caches”.