Generator.kt

package work.beltran.sample

import com.google.auto.service.AutoService
import com.squareup.kotlinpoet.FileSpec
import com.squareup.kotlinpoet.FunSpec
import com.squareup.kotlinpoet.TypeSpec
import java.io.File
import javax.annotation.processing.*
import javax.lang.model.SourceVersion
import javax.lang.model.element.TypeElement

@AutoService(Processor::class)
class Generator: AbstractProcessor() {

    override fun getSupportedAnnotationTypes(): MutableSet<String> {
        println("getSupportedAnnotationTypes")
        return mutableSetOf(GenName::class.java.name)
    }

    override fun getSupportedSourceVersion(): SourceVersion {
        return SourceVersion.latest()
    }

    override fun process(set: MutableSet<out TypeElement>, roundEnv: RoundEnvironment): Boolean {
        println("process")
        roundEnv.getElementsAnnotatedWith(GenName::class.java)
                .forEach {
                    val className = it.simpleName.toString()
                    println("Processing: $className")
                    val pack = processingEnv.elementUtils.getPackageOf(it).toString()
                    generateClass(className, pack)
                }
        return true
    }

    private fun generateClass(className: String, pack: String) {
        val fileName = "Generated_$className"
        val file = FileSpec.builder(pack, fileName)
                .addType(TypeSpec.classBuilder(fileName)
                        .addFunction(FunSpec.builder("getName")
                                .addStatement("return \"World\"")
                                .build())
                        .build())
                .build()

        val kaptKotlinGeneratedDir = processingEnv.options[KAPT_KOTLIN_GENERATED_OPTION_NAME]
        file.writeTo(File(kaptKotlinGeneratedDir, "$fileName.kt"))
    }

    companion object {
        const val KAPT_KOTLIN_GENERATED_OPTION_NAME = "kapt.kotlin.generated"
    }
}