sbt: generating shared sources in cross-platform project - scala

Question

Building my project on Scala with sbt, I want to have a task that will run prior to actual Scala compilation and will generate a Version.scala file with project version information. Here's a task I've came up with:
lazy val generateVersionTask = Def.task {
// Generate contents of Version.scala
val contents = s"""package io.kaitai.struct
|
|object Version {
| val name = "${name.value}"
| val version = "${version.value}"
|}
|""".stripMargin
// Update Version.scala file, if needed
val file = (sourceManaged in Compile).value / "version" / "Version.scala"
println(s"Version file generated: $file")
IO.write(file, contents)
Seq(file)
}
This task seems to work, but the problem is how to plug it in, given that it's a cross project, targeting Scala/JVM, Scala/JS, etc.
This is how build.sbt looked before I started touching it:
lazy val root = project.in(file(".")).
aggregate(fooJS, fooJVM).
settings(
publish := {},
publishLocal := {}
)
lazy val foo = crossProject.in(file(".")).
settings(
name := "foo",
version := sys.env.getOrElse("CI_VERSION", "0.1"),
// ...
).
jvmSettings(/* JVM-specific settings */).
jsSettings(/* JS-specific settings */)
lazy val fooJVM = foo.jvm
lazy val fooJS = foo.js
and, on the filesystem, I have:
shared/ — cross-platform code shared between JS/JVM builds
jvm/ — JVM-specific code
js/ — JS-specific code
The best I've came up so far with is adding this task to foo crossProject:
lazy val foo = crossProject.in(file(".")).
settings(
name := "foo",
version := sys.env.getOrElse("CI_VERSION", "0.1"),
sourceGenerators in Compile += generateVersionTask.taskValue, // <== !
// ...
).
jvmSettings(/* JVM-specific settings */).
jsSettings(/* JS-specific settings */)
This works, but in a very awkward way, not really compatible with "shared" codebase. It generates 2 distinct Version.scala files for JS and JVM:
sbt:root> compile
Version file generated: /foo/js/target/scala-2.12/src_managed/main/version/Version.scala
Version file generated: /foo/jvm/target/scala-2.12/src_managed/main/version/Version.scala
Naturally, it's impossible to access contents of these files from shared, and this is where I want to access it.
So far, I've came with a very sloppy workaround:
There is a var declared in singleton object in shared
in both JVM and JS main entry points, the very first thing I do is that I assign that variable to match constants defined in Version.scala
Also, I've tried the same trick with sbt-buildinfo plugin — the result is exactly the same, it generated per-platform BuildInfo.scala, which I can't use directly from shared sources.
Are there any better solutions available?

Answer 1

Consider pointing sourceManaged to shared/src/main/scala/src_managed directory and scoping generateVersionTask to the root project like so
val sharedSourceManaged = Def.setting(
baseDirectory.value / "shared" / "src" / "main" / "scala" / "src_managed"
)
lazy val root = project.in(file(".")).
aggregate(fooJS, fooJVM).
settings(
publish := {},
publishLocal := {},
sourceManaged := sharedSourceManaged.value,
sourceGenerators in Compile += generateVersionTask.taskValue,
cleanFiles += sharedSourceManaged.value
)
Now sbt compile should output something like
Version file generated: /Users/mario/IdeaProjects/scalajs-cross-compile-example/shared/src/main/scala/src_managed/version/Version.scala
...
[info] Compiling 3 Scala sources to /Users/mario/IdeaProjects/scalajs-cross-compile-example/js/target/scala-2.12/classes ...
[info] Compiling 1 Scala source to /Users/mario/IdeaProjects/scalajs-cross-compile-example/target/scala-2.12/classes ...
[info] Compiling 3 Scala sources to /Users/mario/IdeaProjects/scalajs-cross-compile-example/jvm/target/scala-2.12/classes ...