how to cache the results of an sbt TaskKey?

Question

I have an expensive task that I need to reference in my tests

lazy val exampleSources = TaskKey[Seq[File]]("exampleSources", "for use in tests")

exampleSources := (updateClassifiers in Test).value.select(
  artifact = artifactFilter(classifier = "sources")
)

(and then I can pass exampleSources.value as a parameter to my forked tests)

However, every time I run a test, this task is called, and updateClassifiers (expensive) is called. But I'm happy caching the value on first call and then using that for the session.

Without writing the cache myself, is there any way to do this using built-in sbt objects?

UPDATE: this doesn't work. Second evaluation has CACHE=true but the resolution tasks still run.

lazy val infoForTests = TaskKey[Seq[String]]("infoForTests", "for use in tests")

val infoForTestsCache = collection.mutable.Buffer[String]()

infoForTests := {
  println("CACHE=" + infoForTestsCache.nonEmpty)
  if (infoForTestsCache.isEmpty) {
    infoForTestsCache ++= Seq[String](
      "-Densime.compile.jars=" + jars((fullClasspath in Compile).value),
      "-Densime.test.jars=" + jars((fullClasspath in Test).value),
      "-Densime.compile.classDirs=" + classDirs((fullClasspath in Compile).value),
      "-Densime.test.classDirs=" + classDirs((fullClasspath in Test).value),
      "-Dscala.version=" + scalaVersion.value,
      // sorry! this puts a source/javadoc dependency on running our tests
      "-Densime.jars.sources=" + (updateClassifiers in Test).value.select(
        artifact = artifactFilter(classifier = "sources")
      ).mkString(",")
    )
    println("CACHE=" + infoForTestsCache.nonEmpty)
  }
  infoForTestsCache
}

score 2 · Answer 1 · edited May 23 '17 at 10:33

You can use FileFunction.cached to cache the results and doing work only if input changes.

Just an example from linked sbt documentation:

// define a task that takes some inputs
//   and generates files in an output directory
myTask := {
    // wraps a function taskImpl in an uptodate check
    //   taskImpl takes the input files, the output directory,
    //   generates the output files and returns the set of generated files
    val cachedFun = FileFunction.cached(cacheDirectory.value / "my-task") { (in: Set[File]) =>
      taskImpl(in, target.value) : Set[File]
    }
    // Applies the cached function to the inputs files
    cachedFun(inputs.value)
}

You may also want to check my answer to a similar question : How to list files output by incremental recompilation.

hmm, well I don't have any inputs so I guess my input is `()`. I'll take a closer look at this shortly. — fommil, Aug 20 '14 at 17:14
This doesn't work for me because I'm not reading or writing `File`s — fommil, Oct 25 '14 at 14:36

how to cache the results of an sbt TaskKey?

1 Answers1

Linked