I am working on a Swift playground that contains multiple files in the Sources folder alongside the main file. When I type something, for instance, "UI", in the main file, I get the expected autocomplete suggestions. However, when I do the same in files in the Sources folder, I get a "dumb" autocomplete menu as seen in this question: Xcode 9 Autocomplete Not Working 100% - Partially Working
I have tried deleting DerivedData as suggested in the above question, and have even gone to the lengths of reinstalling Xcode entirely. However, the problem has not gone away. I have created a new playground to ensure it is not specific to this project, and the same problem occurs.
Edit: I just tried the same thing on a different Mac and the same problem was observed. I suppose this is simply a bug in Xcode that has nothing to do with this particular installation or project.
In order to enable autocompletion, you can embed your Playground in a regular Xcode project (e.g. an iOS application). I recommend creating a dummy project for that purpose. Simply drag and drop your playground in this dummy project and make sure to check "Add to target".
Then you can navigate to
Target -> Build Phases -> Compile Sources -> + -> Add other
and add all the files from your source folder. Please note, that you don't need to actually copy the files, a reference is enough for this purpose.
After this process all your source files are built against this dummy target and you can use autocompletion as usual. As far as I know, this is the best practice for debugging Playgrounds right now. Anyway I am curious, if there is an easier way to achieve that.