Automatically run scalafmtSbt on reload about sbt-scalafmt

[rtoomey-coatue]

Crude POC using Global / onLoad with sbt 1.3.x - done in my own project as an autoplugin. Has "double pass" behavior if changes are detected:

import sbt.{ Def, _ }
import Keys._
import org.scalafmt.sbt.ScalafmtPlugin

// https://www.scala-sbt.org/1.x/docs/Howto-Startup.html
object ScalafmtOnLoad extends AutoPlugin {
  override def requires = ScalafmtPlugin
  override def trigger  = AllRequirements

  // This prepends the String you would type into the shell
  lazy val startupTransition: State => State = { s: State =>
    "scalafmtSbt" :: s
  }

  override def globalSettings: Seq[Def.Setting[_]] = Seq(
    // onLoad is scoped to Global because there's only one.
    Global / onLoad := {
      val old = (Global / onLoad).value
      // compose the new transition on top of the existing one
      // in case your plugins are using this hook.
      startupTransition.compose(old)
    }
  )
}

Note that I excluded ScalafmtOnLoad.scala using project.excludeFilters out of fear of infinite loop 😊

1. sbt detects changes, reloads
2. onLoad fires scalafmtSbt, build files are formatted (i intentionally added a changes that would get reformatted)
3. sbt detects changes, reloads a second time
4. this time, no changes detected, loop exits, life goes on, it’s all cool

sbt:foo> 
[info] build source files have changed
[info] modified files: 
[info]   /Users/rtoomey/workspace/foo/build.sbt
[info]   /Users/rtoomey/workspace/foo/project/ScalafmtOnLoad.scala
[info] Reloading sbt...
[info] Loading global plugins from /Users/rtoomey/.sbt/1.0/plugins
[info] Loading settings for project foo-build from plugins.sbt,build.sbt ...
[info] Loading project definition from /Users/rtoomey/workspace/foo/project
[info] Compiling 1 Scala source to /Users/rtoomey/workspace/foo/project/target/scala-2.12/sbt-1.0/classes ...
[info] Loading settings for project root from build.sbt ...
[info] Resolving key references (36593 settings) ...
[info] Set current project to foo (in build file:/Users/rtoomey/workspace/foo/)
[success] Total time: 3 s, completed Dec 6, 2019 12:24:11 PM
[info] build source files have changed
[info] modified files: 
[info]   /Users/rtoomey/workspace/foo/build.sbt
[info] Reloading sbt...
[info] Loading global plugins from /Users/rtoomey/.sbt/1.0/plugins
[info] Loading settings for project foo-build from plugins.sbt,build.sbt ...
[info] Loading project definition from /Users/rtoomey/workspace/foo/project
[info] Loading settings for project root from build.sbt ...
[info] Resolving key references (36593 settings) ...
[info] Set current project to foo (in build file:/Users/rtoomey/workspace/foo/)
[success] Total time: 3 s, completed Dec 6, 2019 12:24:17 PM

Further manual reload command is silent about formatting build files:

sbt:foo> reload
[info] Loading global plugins from /Users/rtoomey/.sbt/1.0/plugins
[info] Loading settings for project foo-build from plugins.sbt,build.sbt ...
[info] Loading project definition from /Users/rtoomey/workspace/foo/project
[info] Loading settings for project root from build.sbt ...
[info] Resolving key references (36593 settings) ...
[info] Set current project to foo (in build file:/Users/rtoomey/workspace/foo/)
[success] Total time: 3 s, completed Dec 6, 2019 12:27:53 PM

Seems to work! 🤞