feat: allow creating custom qualifier annotations

[skyecodes]

Following #108, I was wondering if it was possible to improve class-based qualifiers even more by being able to make our own qualifier annotations like so:

@Named
annotation class InMemoryLogger

@Named
annotation class DatabaseLogger

@Single
@InMemoryLogger
class LoggerInMemoryDataSource : LoggerDataSource

@Single
@DatabaseLogger
class LoggerLocalDataSource(private val logDao: LogDao) : LoggerDataSource

These dependencies would be injected that way:

val logger: LoggerDataSource by inject(named<InMemoryLogger>())

They would also be injected into constructors using the same annotation.

I think the syntax looks better that way but not everyone may think the same. Regardless, I feel like it wouldn't be a bad idea to give the developer the ability to choose between whatever option they prefer to declare a qualifier.

I don't have a lot of experience with KSP but I've managed to implement the feature (it works with either @Named or @Qualifier). I have no idea of the impact on compilation speed, but I'd expect that there is at least some impact because in order to find the qualifier for a definition, we need to resolve each of its annotations until we find one whose declaration is annotated with @Named or @Qualifier.

This could be extended to Scopes as well, I don't personally use them a lot, but I could try to implement the same feature for Scopes.

[arnaudgiuliani]

Sounds smart improvement to let people define their qualifier annotations 👍

[arnaudgiuliani]

Other point we could explore from there is if we can use directly the annotation, instead of named(). The idea is to generate named() at the end, but avoid us to write named() but directly use the newly defined annotations

[skyecodes]

Other point we could explore from there is if we can use directly the annotation, instead of named(). The idea is to generate named() at the end, but avoid us to write named() but directly use the newly defined annotations

I don't really know how that would be implemented, to be fair. As far as I understand KSP can only generate new code, not edit source code, so I don't know how we could make it so that the named() parameter gets automatically replaced.

If I understood your suggestion correctly we could use this syntax for example?

val loggerInject1: @InMemoryLogger LoggerDataSource by inject()
val loggerInject2 by inject<@InMemoryLogger LoggerDataSource>()
val loggerGet1: @InMemoryLogger LoggerDataSource = get()
val loggerGet2 = get<@InMemoryLogger LoggerDataSource>()

After processing (if KSP allowed to edit source code):

val loggerInject1: LoggerDataSource by inject(named<InMemoryLogger>())
val loggerInject2 by inject<LoggerDataSource>(named<InMemoryLogger>())
val loggerGet1: LoggerDataSource = get(named<InMemoryLogger>())
val loggerGet2 = get<LoggerDataSource>(named<InMemoryLogger>())

I don't know how we could do that unless we create specific inject() / get() methods for each annotation type (example: injectInMemoryLogger()), but creating new methods based on the annotation name doesn't seem like the best idea; I also lack experience with KSP so please let me know if you have a better idea.