Experiment to add reformatOnCompile settings to scalafmt sbt >v0.5

AutomateScalafmtPlugin.scala

// Works for scalafmt 0.5.5
// From https://gist.github.com/hseeberger/03677ef75bfadb7663c3b41bb58c702b
// Thank you @hseeberger!

import org.scalafmt.bootstrap.ScalafmtBootstrap
import org.scalafmt.sbt.ScalafmtPlugin
import sbt._
import sbt.Keys._
import sbt.inc.Analysis

object AutomateScalafmtPlugin extends AutoPlugin {

  object autoImport {
    def automateScalafmtFor(configurations: Configuration*): Seq[Setting[_]] =
      configurations.flatMap { c =>
        inConfig(c)(
          Seq(
            compileInputs.in(compile) := {
              scalafmtInc.value
              compileInputs.in(compile).value
            },
            sourceDirectories.in(scalafmtInc) := Seq(scalaSource.value),
            scalafmtInc := {
              val cache   = streams.value.cacheDirectory / "scalafmt"
              val include = includeFilter.in(scalafmtInc).value
              val exclude = excludeFilter.in(scalafmtInc).value
              val sources =
                sourceDirectories
                  .in(scalafmtInc)
                  .value
                  .descendantsExcept(include, exclude)
                  .get
                  .toSet
              def format(handler: Set[File] => Unit, msg: String) = {
                def update(handler: Set[File] => Unit, msg: String)(in: ChangeReport[File],
                                                                    out: ChangeReport[File]) = {
                  val label = Reference.display(thisProjectRef.value)
                  val files = in.modified -- in.removed
                  Analysis
                    .counted("Scala source", "", "s", files.size)
                    .foreach(count => streams.value.log.info(s"$msg $count in $label ..."))
                  handler(files)
                  files
                }
                FileFunction.cached(cache)(FilesInfo.hash, FilesInfo.exists)(update(handler, msg))(
                  sources
                )
              }
              def formattingHandler(files: Set[File]) =
                if (files.nonEmpty) {
                  val filesArg = files.map(_.getAbsolutePath).mkString(",")
                  ScalafmtBootstrap.main(List("--quiet", "-i", "-f", filesArg))
                }
              format(formattingHandler, "Formatting")
              format(_ => (), "Reformatted") // Recalculate the cache
            }
          )
        )
      }
  }

  private val scalafmtInc = taskKey[Unit]("Incrementally format modified sources")

  override def requires = ScalafmtPlugin

  override def trigger = allRequirements

  override def projectSettings =
    (includeFilter.in(scalafmtInc) := "*.scala") +: autoImport.automateScalafmtFor(Compile, Test)
}

See https://gist.github.com/pettyjamesm/ed6351ac2054a76ee6c223e216804298 for an extension of this plugin that handles empty list of files.

This always recompile on some file.
I did this ugly hack to get changed files

 scalafmtIncremental := Def.taskDyn {
      val cache = streams.value.cacheDirectory / "scalafmt"
      val include = (includeFilter in scalafmtIncremental).value
      val exclude = (excludeFilter in scalafmtIncremental).value
      val files: Set[File] =
        (sourceDirectories in scalafmtIncremental).value
          .descendantsExcept(include, exclude)
          .get
          .toSet
      val label = Reference.display(thisProjectRef.value)
      var changed  = Set.empty[File]
      def handleUpdate(in: ChangeReport[File],
                       out: ChangeReport[File]): Set[File] = {
        val files = in.modified -- in.removed
        import sbt._
        inc.Analysis
          .counted("Scala source", "", "s", files.size)
          .foreach(count =>
          streams.value.log.info(s"Formatting $count $label..."))
        changed = files
        files
      }
      val toFormat = FileFunction.cached(cache)(
        FilesInfo.hash,
        FilesInfo.exists
      )(handleUpdate)(files)
      if (changed.nonEmpty) {
        val filesFlag = changed.map(_.getAbsolutePath).mkString(",")
        val args = Seq("", "org.scalafmt.cli.Cli", "-i", "-f", filesFlag)
        (runMain in ScalafmtPlugin.scalafmtStub).toTask(args.mkString(" "))
      } else {
        Def.task[Unit](())
      }
    }.value

Please update to the following which works (properly) for version 0.5.4 and hopefully higher:

import org.scalafmt.bootstrap.ScalafmtBootstrap
import org.scalafmt.sbt.ScalafmtPlugin
import sbt._
import sbt.Keys._
import sbt.inc.Analysis

object AutomateScalafmtPlugin extends AutoPlugin {

  object autoImport {
    def automateScalafmtFor(configurations: Configuration*): Seq[Setting[_]] =
      configurations.flatMap { c =>
        inConfig(c)(
          Seq(
            compileInputs.in(compile) := {
              scalafmtInc.value
              compileInputs.in(compile).value
            },
            sourceDirectories.in(scalafmtInc) := Seq(scalaSource.value),
            scalafmtInc := {
              val cache   = streams.value.cacheDirectory / "scalafmt"
              val include = includeFilter.in(scalafmtInc).value
              val exclude = excludeFilter.in(scalafmtInc).value
              val sources =
                sourceDirectories
                  .in(scalafmtInc)
                  .value
                  .descendantsExcept(include, exclude)
                  .get
                  .toSet
              def format(handler: Set[File] => Unit, msg: String) = {
                def update(handler: Set[File] => Unit, msg: String)(
                    in: ChangeReport[File], out: ChangeReport[File]) = {
                  val label = Reference.display(thisProjectRef.value)
                  val files = in.modified -- in.removed
                  Analysis
                    .counted("Scala source", "", "s", files.size)
                    .foreach(count => streams.value.log.info(s"$msg $count in $label ..."))
                  handler(files)
                  files
                }
                FileFunction.cached(cache)(FilesInfo.hash,
                                           FilesInfo.exists)(update(handler, msg))(sources)
              }
              def formattingHandler(files: Set[File]) =
                if (files.nonEmpty) {
                  val filesArg = files.map(_.getAbsolutePath).mkString(",")
                  ScalafmtBootstrap.main(List("--non-interactive", "-i", "-f", filesArg))
                }
              format(formattingHandler, "Formatting")
              format(_ => (), "Reformatted") // Recalculate the cache
            }
          )
        )
      }
  }

  private val scalafmtInc = taskKey[Unit]("Incrementally format modified sources")

  override def requires = ScalafmtPlugin

  override def trigger = allRequirements

  override def projectSettings =
    (includeFilter.in(scalafmtInc) := "*.scala") +: autoImport.automateScalafmtFor(Compile, Test)
}

There seems to be a race condition for multi projects in this version. Please see my changes in akka/alpakka@15228be

PS, thanks for providing this :)

@stephennancekivell Thanks for the fix!