SourceDirectories.addGeneratedSourceDirectory works the same way as the artifact API and takes over the output for the tasks. For example in the addCustomSourceFolders recipe, even though we set the output to a specific location (toml/gen), running the verification task shows that the location is changed to build/generated/toml/debugAddCustomSources.

It does in fact work if you pass this to multiple variants. For instance changing the recipe to something like this:

val addSourceTaskProvider =  project.tasks.register<AddCustomSources>("addCustomSources") {
    outputFolder.set(File(project.layout.buildDirectory.asFile.get(), "toml/gen"))
}

androidComponents.onVariants { variant ->

    variant.sources.getByName("toml").also {
        // adding custom folders (static and generated) to `toml` source type
        it.addGeneratedSourceDirectory(addSourceTaskProvider, AddCustomSources::outputFolder)
    }
}

will in fact works. However the look of the API make it looks like it would not work.

Part of this is the way some of our other APIs work (for example transforms on Artifact are very variant specific).

At the very least we should document that it can in fact be used for multiple variantss. We should also consider making it more obvious (maybe the API just needs to receive a Provider<Directory>, though I think there are potentially issue as the location should be in build/?)