begin Dotty support (WIP, still fails)

- RichWindow - we cannot override internal class InterfaceMixin, thus
  introduce InterfaceMixin2
- ScrollPane.BarPolicy cannot be an Enumeration because we cannot
  override internal class Value. Make it a case class instead
- Window#SuperMixin causes a compiler bug in Dotty
  ("dotc: wrong number of arguments at crossCast for (x$0: java.awt.GraphicsConfiguration): java.awt.Window: (java.awt.Window#<init> : (x$0: java.awt.GraphicsConfiguration): java.awt.Window
  ), expected: 1, found: 0"

Author: Sciss

build.sbt

@@ -1,29 +1,39 @@
+lazy val scalaTestVersion = "3.2.2"
+
+lazy val commonSettings = Seq(
+  crossScalaVersions += "0.27.0-RC1",
+  scalaVersion := "0.27.0-RC1", // while working in IntelliJ
+)
+
 lazy val swing = project.in(file("."))
   .settings(ScalaModulePlugin.scalaModuleSettings)
   .settings(ScalaModulePlugin.scalaModuleOsgiSettings)
+  .settings(commonSettings)
   .settings(
     name := "scala-swing",
     OsgiKeys.exportPackage := Seq(s"scala.swing.*;version=${version.value}"),
     scalaModuleMimaPreviousVersion := Some("2.1.0"),
     // set the prompt (for this build) to include the project id.
     ThisBuild / shellPrompt := { state => Project.extract(state).currentRef.project + "> " },
     libraryDependencies ++= Seq(
-      "org.scalatest" %% "scalatest-flatspec" % "3.2.0" % Test,
-      "org.scalatest" %% "scalatest-shouldmatchers" % "3.2.0" % Test,
+      "org.scalatest" %% "scalatest-flatspec"       % scalaTestVersion % Test,
+      "org.scalatest" %% "scalatest-shouldmatchers" % scalaTestVersion % Test,
     ),
     // Adds a `src/main/scala-2.13+` source directory for Scala 2.13 and newer
     // and  a `src/main/scala-2.13-` source directory for Scala version older than 2.13
     Compile / unmanagedSourceDirectories += {
       val sourceDir = (Compile / sourceDirectory).value
       CrossVersion.partialVersion(scalaVersion.value) match {
         case Some((2, n)) if n >= 13 => sourceDir / "scala-2.13+"
+        case Some((0, _))            => sourceDir / "scala-2.13+" // Dotty
         case _                       => sourceDir / "scala-2.13-"
       }
     }
   )
 
 lazy val examples = project.in(file("examples"))
   .dependsOn(swing)
+  .settings(commonSettings)
   .settings(
     scalaVersion := (swing / scalaVersion).value,
     run / fork := true,
@@ -32,6 +42,7 @@ lazy val examples = project.in(file("examples"))
 
 lazy val uitest = project.in(file("uitest"))
   .dependsOn(swing)
+  .settings(commonSettings)
   .settings(
     scalaVersion := (swing / scalaVersion).value,
     run / fork := true,

examples/src/main/scala/scala/swing/examples/tutorials/components/ButtonHtmlDemo.scala

@@ -88,19 +88,19 @@ class ButtonHtmlDemo extends FlowPanel {
   listenTo(enable)
 
   reactions += {
-    case ButtonClicked(`enable`) => enableMiddle
-    case ButtonClicked(`disable`) => disableMiddle
+    case ButtonClicked(`enable`)  => enableMiddle()
+    case ButtonClicked(`disable`) => disableMiddle()
   }
 
   def enableMiddle(): Unit = {
-    enable.enabled = false
-    middle.enabled = true
-    disable.enabled = true
+    enable  .enabled = false
+    middle  .enabled = true
+    disable .enabled = true
   }
   def disableMiddle(): Unit = {
-    enable.enabled = true
-    middle.enabled = false
-    disable.enabled = false
+    enable  .enabled = true
+    middle  .enabled = false
+    disable .enabled = false
   }
   def createImageIcon(path: String): Option[javax.swing.ImageIcon] = {
     val imgURL: java.net.URL = getClass().getResource(path)

examples/src/main/scala/scala/swing/examples/tutorials/components/FrameDemo2.scala

@@ -241,9 +241,7 @@ class MyFrame extends Frame {
   preferredSize = new Dimension(150, 150)
   pack()
   visible = true
-  override def closeOperation() = {
-    close
-  }
+  override def closeOperation() = close()
 }
 
 class MyFrameUndecorated extends Frame with RichWindow.Undecorated {
@@ -270,9 +268,7 @@ class MyFrameUndecorated extends Frame with RichWindow.Undecorated {
   preferredSize = new Dimension(150, 150)
   pack()
   visible = true
-  override def closeOperation() = {
-    close
-  }
+  override def closeOperation() = close()
 }
 
 object FrameDemo2 extends SimpleSwingApplication {

examples/src/main/scala/scala/swing/examples/tutorials/components/IconDemoApp.scala

@@ -123,18 +123,16 @@ class IconDemoApp extends MainFrame {
     }.toList
   }
 
-  f.foreach {
-    thumbs:List[Option[ThumbnailAction]] =>
-      buttonBar.contents.dropRight(1)
-      thumbs.foreach{ thumbAction => {
-        thumbAction.foreach { ta =>
-          buttonBar.contents += new Button(ta)
-        }
-      }}
-      buttonBar.contents += Swing.Glue
+  f.foreach { thumbs =>
+    buttonBar.contents.dropRight(1)
+    thumbs.foreach { thumbAction =>
+      thumbAction.foreach { ta =>
+        buttonBar.contents += new Button(ta)
+      }
+    }
+    buttonBar.contents += Swing.Glue
   }
 
-
   /**
    * Resizes an image using a Graphics2D object backed by a BufferedImage.
    * @param srcImg - source image to scale

examples/src/main/scala/scala/swing/examples/tutorials/layout/BoxLayoutDemo2.scala

@@ -93,9 +93,10 @@ class BoxLayoutDemo2 extends BorderPanel {
   }
 
   def notifyBldComponents(): Unit = {
-    for (i <- 0 until NumComponents)
+    for (i <- 0 until NumComponents) {
       bldComponent(i).setSizeRestriction(restrictSize)
-    bldComponent(0).revalidate
+    }
+    bldComponent(0).revalidate()
   }
 }
 

examples/src/main/scala/scala/swing/examples/tutorials/layout/DiagonalLayout.scala

@@ -48,7 +48,7 @@ class DiagonalLayout(private var vgap: Int) extends LayoutManager {
 
   private var sizeUnknown: Boolean = true
 
-  def this() {
+  def this() = {
     this(5)
   }
 

project/plugins.sbt

@@ -1 +1,3 @@
-addSbtPlugin("org.scala-lang.modules" % "sbt-scala-module" % "2.2.0")
+addSbtPlugin("org.scala-lang.modules" % "sbt-scala-module" % "2.2.1")
+addSbtPlugin("ch.epfl.lamp"           % "sbt-dotty"        % "0.4.2")  // cross-compile for dotty
+

src/main/scala/scala/swing/ButtonGroup.scala

@@ -32,9 +32,9 @@ class ButtonGroup(initialButtons: AbstractButton*) {
     override def size: Int = peer.getButtonCount
 
     def iterator: Iterator[AbstractButton] = new Iterator[AbstractButton] {
-      private val enum = peer.getElements
-      def next(): AbstractButton = UIElement.cachedWrapper[AbstractButton](enum.nextElement())
-      def hasNext: Boolean = enum.hasMoreElements
+      private val elements = peer.getElements
+      def next(): AbstractButton = UIElement.cachedWrapper[AbstractButton](elements.nextElement())
+      def hasNext: Boolean = elements.hasMoreElements
     }
   }
   buttons ++= initialButtons

src/main/scala/scala/swing/MainFrame.scala

@@ -16,6 +16,6 @@ package scala.swing
  * A frame that can be used for main application windows. It shuts down the
  * framework and quits the application when closed.
  */
-class MainFrame(gc: java.awt.GraphicsConfiguration = null) extends Frame(gc) {
+class MainFrame(/*gc: java.awt.GraphicsConfiguration = null*/) extends Frame /*(gc)*/ {
   override def closeOperation(): Unit = sys.exit(0)
 }

src/main/scala/scala/swing/Publisher.scala

@@ -79,7 +79,7 @@ private[swing] trait LazyPublisher extends Publisher {
 
 import scala.ref._
 
-private[swing] trait SingleRefCollection[+A <: AnyRef] extends Iterable[A] { self =>
+private[swing] trait SingleRefCollection[/*+*/ A <: AnyRef] extends Iterable[A] { self =>
 
   trait Ref[+B <: AnyRef] extends Reference[B] {
     override def hashCode(): Int = get match {
@@ -119,7 +119,7 @@ private[swing] trait SingleRefCollection[+A <: AnyRef] extends Iterable[A] { sel
       while (!ahead && elems.hasNext) {
         // make sure we have a reference to the next element,
         // otherwise it might be garbage collected
-        val next = elems.next.get
+        val next = elems.next().get
         ahead = next.isDefined
         if (ahead) hd = next.get
       }