There are already APIs for getting a relative product path from an absolute source path, or getting a relative source path for an *existing* source file, but there were no APIs for getting a relative source path for a *new* source file. Prefabs will need this ability to be able to correctly generate a relative source path inside the prefab file before the file has been saved.
The logic for relative source paths is a little bit tricky because the paths are relative to the watch folders, and the watch folders can be nested, with different priorities to explain which should take precedence. The input paths can also include specifiers like "." and "..", which need to be reconciled before creating the final correct relative path. The included unit tests test all of the tricky edge cases that I was able to identify.
Mike Balfour
alexpete