I need to write a Spotlight Importer for an application that I've written in Swift, and am referring to the official Apple guide for Writing a Spotlight Importer.
It seems straightforward enough, however creating a Spotlight Importer project creates a default setup for an Objective-C implementation. Now, working with Objective-C isn't a huge problem (I've used it plenty of times in the past) but everything I've written for my application is in Swift, so I'd really I'd like to write the importer in Swift too to avoid switching between languages, and also so I can share some of the code that I've already done for reading/writing files.
Firstly, is it possible to write a Spotlight Importer using Swift instead of Objective-C? And if it is, where should I start (e.g- if I take the Objective-C starting point, what would I do to switch over to Swift instead)?
Since Apple introduced Swift as a language to be perfectly compatible with any existing Objective-C project I would suggest you just start with whatever makes things easier for you.
If you know Swift best then nothing keeps you from using that – for whatever project you might want. If you want to follow a tutorial that was written for Objective-C and not updated for Swift yet, I think you have two choices (I'd personally recommend going for the second option for now):
Write the same logic written in Objective-C within the tutorial now in Swift from scratch (nearly everything possible in Objective-C is easily possible with Swift, too). For that you need to understand the basics of Objective-C and the corresponding syntax and features in Swift though.
Start with Objective-C to follow the tutorial and keep things easier at the beginning (no need to really understand the tutorials details). Then use the great possibility of mix and matching Swift code alongside Objective-C code to customize the code for your needs or to extend it with your own pre-existing classes.
More specifically on the second option:
If you want to write new classes just use Swift – you can perfectly use everything written in Objective-C from within Swift and vice versa. If you feel you need to change classes already written in Objective-C you have these options: Extend the class written in Objective-C with a new Swift class, re-write that specific file in Swift or just edit the Objective-C file directly.
To learn more on how to mix and match Swift code alongside Objective-C I recommend reading Apples official documentation. It's part of the free iBook "Using Swift with Cocoa and Objective-C" written by Apple engineers for developers.
Unfortunately Apple actually does seem to provide their template for a Spotlight Importer from within XCode for Objective-C only at the moment. Don't know why this is though – I can't see anything stopping them from supporting Swift. We should probably report this with Apples Bug Reporter to stress the fact that people are actually asking for this.
Hope I didn't overlook anything here, otherwise my answer will be pointless. ^^
UPDATE (request) Here are some steps on where to begin to implement the first approach:
I hope this helps to get you started and is specific enough. I tried to do the necessary changes myself in order to provide an example project in Swift but unfortunately I couldn't get it working in a limited time. You may want to consider providing your code publicly though (e.g. on GitHub with a link posted here) in case you decide to port it yourself so others can profit from this, too.
Good luck!