Simple builder for Play run hooks: https://www.playframework.com/documentation/2.5.x/SBTCookbook#Hooking-into-Plays-dev-mode

ProcessRunHookBuilder

package build

import java.net.InetSocketAddress
import play.sbt.PlayRunHook
import sbt._

object ProcessRunHookBuilder {
  def apply(
    base:    File,
    before:  Option[String] = None,
    after:   Option[String] = None,
    stopped: Option[String] = None,
    error:   Option[String] = None
  ): PlayRunHook = {
    object ProcessRunHook extends PlayRunHook {
      val processes: Seq[Option[Process]] = Nil

      override def beforeStarted() = {
        processes :+ before.map(Process(_).run())
      }

      override def afterStarted(addr: InetSocketAddress) = {
        processes :+ after.map(Process(_).run())
      }

      override def afterStopped() = {
        processes.map(_.map(_.destroy()))
        stopped.map(Process(_).run())
      }

      override def onError() = {
        processes :+ error.map(c => Process(c).run())
      }
    }

    ProcessRunHook
  }
}

// Enable in your build.sbt:
// import build.ProcessRunHookBuilder
// playRunHooks <+= baseDirectory.map(f => ProcessRunHookBuilder(f, Some("npm run start")))

Looks good! Would you consider submitting a pull request to Play to add this to the documentation page? It should be at
https://github.com/playframework/playframework/edit/2.5.x/documentation/manual/working/commonGuide/build/SBTCookbook.md