Implement abstract function - Extend to support external methods (Java standard library etc.)

[themkat]

General information

The current implementation only supports extending classes or implementing interfaces in the same project it seems. I often work with Java libraries where I have to implement functions from abstract classes or interfaces. This can be very tedious to do manually, so having it as a quickfix is really beneficial. The current implementation is based on KtClass, which to my understanding is only made for Kotlin classes. Java classes have their own PSI elements in the compiler. In the current implementation, we are also shown that a quickfix exists, but the edit-parts of it are empty/null.

The new implementation works with the DeclarationDescriptor objects directly, which seems like a very flexible approach.

Quirks

I originally did not see the QuickFixesTests file, so I originally had a different file with tests. That is why it seems like there is some overlap between some of the tests. I later merged them, but decided to keep the tests. I think the set of tests verifies a lot of different things; same file definitions, workspace definitions in other files, and Java standard library ones. Let me know if I instead should delete some of them.

I blame late evening coding after a beer for not noticing the QuickFixesTests file 😆

Future work

There are some useful things that should be added to this functionality. To not make this PR too big, I think it is better to have them as part of a future PR. Will Make some issues for them so people can pick them up if they want to (or I end up doing it when I have time after work).

• Add abstract variables to the quickfix as well. AbstractList in the Kotlin standard library is an example of a place that has abstract variables. Extend the Implement Abstract Functions Quick Fix to include abstract variables #358
• Automatic import of symbols. There might be symbols that need importing when we add the interface/class methods to our class body. This can be handled with the add missing imports quickfix for now. Automatic import of classes/functions/constants on Implement Abstract Functions Quick Fix #360
• Not Quickfix, but override related: A override-method/member operation like many Java plugins has (e.g, lsp-java for Emacs). Not just abstract/unimplemented ones, but making it possible to override methods/members in general. Some interfaces have default implementations now, so these will probably not show up as abstract. General override/implement member/function/variable operation #359

[themkat]

@fwcd , do you have time to look at this PR in the near future? Would be really awesome to have as part of the language server 🙂 Sorry for nagging you about this 🙁

[fwcd]

Sorry for the delay. Great work, thanks for digging into this, definitely very useful considering how interfaces/base classes often come from libraries!

An idea that I had was that it would be cool to have auto-completions for missing overridable methods, similar to how the Java/Python/Rust/etc language servers handle this, but definitely that is out-of-scope for this PR.