Make PBXFileReference return an optional full path

[mbernson]

The filesystem path to a PBXReference may not always exist in the fullFilePaths
dictionary, for example when the user incorrectly edits (or merges) their Xcode project file.

I would like to catch this error in R.swift, but can't because this dictionary lookup in Xcode.swift is already a fatal error.

[mac-cain13]

Little bit of background; We crashed into this forced unwrap in a project we worked on. It was an incorrectly merged file, but R.swift now exited with some sort of segfault complaining about a "mac architecture thing". Very confusing.

I don't feel Xcode.swift should support all kind of corrupt Xcodeproj files, but by making this an optional R.swift could indeed catch nil values here and warn the user about them.

@tomlokhorst: Do you agree this is an okay change?

[tomlokhorst]

As you may have noticed in the source code; This isn't the only !, there's a lot of them!

In practice, I've never seen Xcode.swift crash on any of them. Except for when the xcodeproj is corrupt, like you have here.

In general, I think it's a bad idea to "hide" errors like this.
By making fullPath optional, the user who has a non-corrupt xcodeproj now has to deal with an extra case that will never occur.

A different approach would be to have the XCProjectFile initializer do a bunch of checks to see if the xcodeproj is internally consistent.
It would be really nice if it would throw ProjectFileError.InternalInconsistency with something like:

- BuildFile (46880B8819C43A87006E1F66) references missing fileRef 3EACC98E19EE6D4300EB3C5E
- FileReference `AppDelegate.cpp` (288765A40DF7441C002DB57D) is not used in any BuildFile.

Perhaps a merge conflict?

[mac-cain13]

I agree with @tomlokhorst. After some IRL discussion I think a constructor that validates the project and throws clear errors is better than just making this an optional.

@mbernson do you have the time and would you like to take this up and PR that? Would be awesome if you have the time to pick this up. :)

[mbernson]

Yes, I'd like to pick this up! But I can't give any guarantees for when it will be done.

[tomlokhorst]

I've created an issue for this: #13