Allow users to add custom Toree magic handlers

modules/scala/scala-interpreter/src/test/scala/almond/ScalaKernelTests.scala

@@ -1108,6 +1108,63 @@ object ScalaKernelTests extends TestSuite {
             .mkString("Vector(" + "\n", "", "...")
       )
     }
+
+    test("toree custom cell magic") {
+
+      val predef =
+        """almond.toree.CellMagicHook.addHandler("test") { (_, content) =>
+          |  import almond.api.JupyterApi
+          |  import almond.interpreter.api.DisplayData
+          |
+          |  Left(JupyterApi.ExecuteHookResult.Success(DisplayData.text(content)))
+          |}
+          |
+          |almond.toree.CellMagicHook.addHandler("thing") { (_, content) =>
+          |  import almond.api.JupyterApi
+          |  import almond.interpreter.api.DisplayData
+          |
+          |  val nl = System.lineSeparator()
+          |  Right(s"val thing = {" + nl + content + nl + "}" + nl)
+          |}
+          |""".stripMargin
+
+      val interpreter = new ScalaInterpreter(
+        params = ScalaInterpreterParams(
+          initialColors = Colors.BlackWhite,
+          predefCode = predef,
+          toreeMagics = true
+        ),
+        logCtx = logCtx
+      )
+
+      val kernel = Kernel.create(interpreter, interpreterEc, threads, logCtx)
+        .unsafeRunTimedOrThrow()
+
+      implicit val sessionId: SessionId = SessionId()
+
+      kernel.execute(
+        """%%test
+          |foo
+          |a
+          |""".stripMargin,
+        """foo
+          |a
+          |""".stripMargin
+      )
+
+      val nl = System.lineSeparator()
+
+      kernel.execute(
+        """%%thing
+          |println("Hello")
+          |2
+          |""".stripMargin,
+        "thing: Int = 2",
+        stdout =
+          "Hello" + nl +
+            "thing: Int = 2"
+      )
+    }
   }
 
 }

modules/scala/toree-hooks/src/main/scala/almond/toree/CellMagicHandler.scala

@@ -2,6 +2,7 @@ package almond.toree
 
 import almond.api.JupyterApi
 
+@FunctionalInterface
 trait CellMagicHandler {
   def handle(name: String, content: String): Either[JupyterApi.ExecuteHookResult, String]
 }

modules/scala/toree-hooks/src/main/scala/almond/toree/CellMagicHook.scala

@@ -5,8 +5,18 @@ import almond.interpreter.api.OutputHandler
 
 import java.util.Locale
 
+import scala.collection.mutable
+
 object CellMagicHook {
 
+  private var userHandlers = new mutable.HashMap[String, CellMagicHandler]
+
+  def addHandler(name: String)(handler: CellMagicHandler): Unit =
+    userHandlers += name -> handler
+
+  def clearHandlers(): Unit =
+    userHandlers.clear()
+
   def hook(publish: OutputHandler): JupyterApi.ExecuteHook = {
     val handlers = CellMagicHandlers.handlers(publish)
     code =>
@@ -16,7 +26,8 @@ object CellMagicHook {
       }
       nameOpt match {
         case Some(name) =>
-          handlers.get(name.toLowerCase(Locale.ROOT)) match {
+          val name0 = name.toLowerCase(Locale.ROOT)
+          userHandlers.get(name0).orElse(handlers.get(name0)) match {
             case Some(handler) =>
               val content = code.linesWithSeparators.drop(1).mkString
               handler.handle(name, content)

modules/scala/toree-hooks/src/main/scala/almond/toree/LineMagicHandler.scala

@@ -2,6 +2,7 @@ package almond.toree
 
 import almond.api.JupyterApi
 
+@FunctionalInterface
 trait LineMagicHandler {
   def handle(name: String, values: Seq[String]): Either[JupyterApi.ExecuteHookResult, String]
 }

modules/scala/toree-hooks/src/main/scala/almond/toree/LineMagicHook.scala

@@ -10,6 +10,14 @@ import scala.collection.mutable
 
 object LineMagicHook {
 
+  private var userHandlers = new mutable.HashMap[String, LineMagicHandler]
+
+  def addHandler(name: String)(handler: LineMagicHandler): Unit =
+    userHandlers += name -> handler
+
+  def clearHandlers(): Unit =
+    userHandlers.clear()
+
   private val sep = Pattern.compile("\\s+")
 
   def inspect(code: String): Iterator[Either[(Seq[String], String, String), String]] = {
@@ -47,7 +55,9 @@ object LineMagicHook {
 
             assert(name.startsWith("%"))
 
-            handlers.get(name.toLowerCase(Locale.ROOT).stripPrefix("%")) match {
+            val name0 = name.toLowerCase(Locale.ROOT).stripPrefix("%")
+
+            userHandlers.get(name0).orElse(handlers.get(name0)) match {
               case None =>
                 System.err.println(s"Warning: ignoring unrecognized Toree line magic $name")
               case Some(handler) =>