Add task to only check formatting of Scala filesscalafmt-check

3 files changed, 93 insertions, 31 deletions

diff --git a/docs/pages/2 - Configuring Mill.md b/docs/pages/2 - Configuring Mill.md
index 97fd1a37..f0b03790 100644
--- a/docs/pages/2 - Configuring Mill.md
+++ b/docs/pages/2 - Configuring Mill.md
@@ -259,9 +259,10 @@ object foo extends ScalaModule with ScalafmtModule {
 }
 ```
 
-Now you can reformat code with `mill foo.reformat` command.
+Now you can reformat code with `mill foo.reformat` command, or only check for misformatted files with `mill checkFormat`.
 
-You can also reformat your project's code globally with `mill mill.scalalib.scalafmt.ScalafmtModule/reformatAll __.sources` command.
+You can also reformat your project's code globally with `mill mill.scalalib.scalafmt.ScalafmtModule/reformatAll __.sources` command,
+or only check the code's format with `mill mill.scalalib.scalafmt.ScalafmtModule/checkFormatAll __.sources`.
 It will reformat all sources that matches `__.sources` query.
 
 If you add a `.scalafmt.conf` file at the root of you project, it will be used
diff --git a/scalalib/src/scalafmt/ScalafmtModule.scala b/scalalib/src/scalafmt/ScalafmtModule.scala
index ea254e6d..ec67ff18 100644
--- a/scalalib/src/scalafmt/ScalafmtModule.scala
+++ b/scalalib/src/scalafmt/ScalafmtModule.scala
@@ -15,6 +15,15 @@ trait ScalafmtModule extends JavaModule {
       )
   }
 
+  def checkFormat(): Command[Unit] = T.command {
+    ScalafmtWorkerModule
+      .worker()
+      .checkFormat(
+        filesToFormat(sources()),
+        scalafmtConfig().head
+      )
+  }
+
   def scalafmtConfig: Sources = T.sources(os.pwd / ".scalafmt.conf")
 
   protected def filesToFormat(sources: Seq[PathRef]) = {
@@ -39,6 +48,17 @@ object ScalafmtModule extends ExternalModule with ScalafmtModule {
         )
     }
 
+  def checkFormatAll(sources: mill.main.Tasks[Seq[PathRef]]): Command[Unit] =
+    T.command {
+      val files = Task.sequence(sources.value)().flatMap(filesToFormat)
+      ScalafmtWorkerModule
+        .worker()
+        .checkFormat(
+          files,
+          scalafmtConfig().head
+        )
+    }
+
   implicit def millScoptTargetReads[T] = new mill.main.Tasks.Scopt[T]()
 
   lazy val millDiscover = Discover[this.type]
diff --git a/scalalib/src/scalafmt/ScalafmtWorker.scala b/scalalib/src/scalafmt/ScalafmtWorker.scala
index f9c7e9b4..25b58f24 100644
--- a/scalalib/src/scalafmt/ScalafmtWorker.scala
+++ b/scalalib/src/scalafmt/ScalafmtWorker.scala
@@ -8,6 +8,7 @@ import mill.api.Ctx
 import org.scalafmt.interfaces.Scalafmt
 
 import scala.collection.mutable
+import mill.api.Result
 
 object ScalafmtWorkerModule extends ExternalModule {
   def worker: Worker[ScalafmtWorker] = T.worker { new ScalafmtWorker() }
@@ -21,44 +22,84 @@ private[scalafmt] class ScalafmtWorker {
 
   def reformat(input: Seq[PathRef],
                scalafmtConfig: PathRef)(implicit ctx: Ctx): Unit = {
-    val toFormat =
-      if (scalafmtConfig.sig != configSig) input
-      else
-        input.filterNot(ref => reformatted.get(ref.path).contains(ref.sig))
-
-    if (toFormat.nonEmpty) {
-      ctx.log.info(s"Formatting ${toFormat.size} Scala sources")
-      reformatAction(toFormat.map(_.path),
-                     scalafmtConfig.path)
-      reformatted ++= toFormat.map { ref =>
-        val updRef = PathRef(ref.path)
-        updRef.path -> updRef.sig
-      }
-      configSig = scalafmtConfig.sig
+    reformatAction(input, scalafmtConfig, dryRun = false)
+  }
+
+  def checkFormat(input: Seq[PathRef],
+                  scalafmtConfig: PathRef)(implicit ctx: Ctx): Result[Unit] = {
+
+    val misformatted = reformatAction(input, scalafmtConfig, dryRun = true)
+    if (misformatted.isEmpty) {
+      Result.Success(())
     } else {
-      ctx.log.info(s"Everything is formatted already")
+      val out = ctx.log.outputStream
+      for (u <- misformatted) {
+        out.println(u.path.toString)
+      }
+      Result.Failure(s"Found ${misformatted.length} misformatted files")
     }
   }
 
-  private val cliFlags = Seq("--non-interactive", "--quiet")
+  // run scalafmt over input files and return any files that changed
+  // (only save changes to files if dryRun is false)
+  private def reformatAction(
+    input: Seq[PathRef],
+    scalafmtConfig: PathRef,
+    dryRun: Boolean
+  )(implicit ctx: Ctx): Seq[PathRef] = {
+
+    // only consider files that have changed since last reformat
+    val toConsider =
+      if (scalafmtConfig.sig != configSig) input
+      else input.filterNot(ref => reformatted.get(ref.path).contains(ref.sig))
+
+    if (toConsider.nonEmpty) {
 
-  private def reformatAction(toFormat: Seq[os.Path],
-                             config: os.Path)(implicit ctx: Ctx) = {
-    val scalafmt =
-      Scalafmt
+      if (dryRun) {
+        ctx.log.info(s"Checking format of ${toConsider.size} Scala sources")
+      } else {
+        ctx.log.info(s"Formatting ${toConsider.size} Scala sources")
+      }
+
+      val scalafmt = Scalafmt
         .create(this.getClass.getClassLoader)
         .withRespectVersion(false)
 
-    val configPath =
-      if (os.exists(config))
-        config.toNIO
-      else
-        JPaths.get(getClass.getResource("default.scalafmt.conf").toURI)
+      val configPath =
+        if (os.exists(scalafmtConfig.path))
+          scalafmtConfig.path.toNIO
+        else
+          JPaths.get(getClass.getResource("default.scalafmt.conf").toURI)
+
+      // keeps track of files that are misformatted
+      val misformatted = mutable.ListBuffer.empty[PathRef]
+
+      def markFormatted(path: PathRef) = {
+        val updRef = PathRef(path.path)
+        reformatted += updRef.path -> updRef.sig
+      }
+
+      toConsider.foreach { pathToFormat =>
+        val code = os.read(pathToFormat.path)
+        val formattedCode = scalafmt.format(configPath, pathToFormat.path.toNIO, code)
+
+        if (code != formattedCode) {
+          misformatted += pathToFormat
+          if (!dryRun) {
+            os.write.over(pathToFormat.path, formattedCode)
+            markFormatted(pathToFormat)
+          }
+        } else {
+          markFormatted(pathToFormat)
+        }
 
-    toFormat.foreach { pathToFormat =>
-      val code = os.read(pathToFormat)
-      val formatteCode = scalafmt.format(configPath, pathToFormat.toNIO, code)
-      os.write.over(pathToFormat, formatteCode)
+      }
+      configSig = scalafmtConfig.sig
+      misformatted.toList
+    } else {
+      ctx.log.info(s"Everything is formatted already")
+      Nil
     }
   }
+
 }