Update a timestamp SettingKey in an sbt 0.12 task - scala

Question

I need a way to update a SettingKey[String] in my sbt build each time the compile task executes. A SettingKey is necessary so it can be injected into application via sbt-buildinfo. Creating a command is one way to accomplish this per this question, but I want it to happen when the compile task executes, which can be caused by any number of other tasks. Any ideas how to accomplish this in sbt 0.12?

Answer 1

You cannot use SettingKey[String] as the SettingKeys are only computed once - during the project load, which is specified in the documentation:
For a setting, the value will be computed once at project load time.
For a task, the computation will be re-run each time the task is
executed.
How about following Build.scala
import sbt._
import Keys._
import sbtbuildinfo.Plugin._
object HelloBuild extends Build {
val helloSettings = Defaults.defaultSettings ++
buildInfoSettings ++ Seq (
sourceGenerators in Compile <+= buildInfo,
buildInfoKeys := Seq[BuildInfoKey](
name,
version,
scalaVersion,
// Here is the interesting part
BuildInfoKey.action("buildTime") {
System.currentTimeMillis
}
),
buildInfoPackage := "org.myorg.myapp"
)
lazy val project = Project (
id = "project",
base = file ("."),
settings = helloSettings
)
}