Unpack schema files

build.sbt

val avro = project
  .settings(
    avroSchemaFiles := target.value / "avro_src",
    avroSchemaJar := "com.example" %% "schema" % "0.0.0",
    Compile / avroSourceDirectories += avroSchemaFiles.value,
    Compile / sourceGenerators +=
      Def.sequential(unpackSchemaFilesTask, Compile / avroScalaGenerate).taskValue
  )

UnpackSchemaFiles.scala

import sbt.Keys._
import sbt.{Def, _}

/**
  * We want to generate case classes from schema files in a jar. We download 
  * and unpack the jar into a directory for the sbt avro plugin to generate 
  * the case classes from.
  */
trait UnpackSchemaFiles {

  val avroSchemaFiles = settingKey[File]("Avro schema files")
  val avroSchemaJar   = settingKey[ModuleID]("Jar containing avro schema")

  val unpackSchemaFilesTask = Def.task {

    val log       = streams.value.log
    val avro      = avroSchemaFiles.value
    val schemaJar = avroSchemaJar.value

    log.info(s"Downloading $schemaJar to ${target.value}")

    dependencyResolution.value.retrieve(
      schemaJar,
      None,
      target.value,
      streams.value.log
    ) match {
      case Left(err) => log.error(s"$err")
      case Right(files) =>
        files.foreach { jar =>
          log.info(s"Unpacking $jar")

          IO.unzip(jar, avro, filter = _.endsWith(".avsc")).foreach { avsc =>
            log.info(s"Unpacked $avsc to $avro")
          }
        }
    }
  }

}