Provide convenience closureOf<T>(T.() -> Unit)

For interoperability with Groovy plugins and/or missing Action<T> APIs.

Resolves #103

src/main/kotlin/org/gradle/script/lang/kotlin/GroovyInteroperability.kt

@@ -0,0 +1,51 @@
+/*
+ * Copyright 2016 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.gradle.script.lang.kotlin
+
+import groovy.lang.Closure
+
+/**
+ * Adapts a Kotlin function to a single argument Groovy [Closure].
+ *
+ * @param T the expected type of the single argument to the closure.
+ * @param function the function to be adapted.
+ *
+ * @see KotlinClosure
+ */
+fun <T : Any> closureOf(function: T.() -> Unit): Closure<Any?> =
+    KotlinClosure(function)
+
+/**
+ * Adapts a Kotlin function to a single argument Groovy [Closure].
+ *
+ * @param T the type of the single argument to the closure.
+ * @param V the return type.
+ * @param function the function to be adapted.
+ * @param owner optional owner of the Closure.
+ * @param thisObject optional _this Object_ of the Closure.
+ *
+ * @see Closure
+ */
+class KotlinClosure<in T : Any, V : Any>(
+    val function: T.() -> V?,
+    owner: Any? = null,
+    thisObject: Any? = null) : Closure<V?>(owner, thisObject) {
+
+    @Suppress("unused") // to be called dynamically by Groovy
+    fun doCall(it: T): V? =
+        it.function()
+}

src/test/kotlin/org/gradle/script/lang/kotlin/GroovyInteroperabilityTest.kt

@@ -0,0 +1,24 @@
+package org.gradle.script.lang.kotlin
+
+import org.junit.Test
+import kotlin.test.assertEquals
+
+class GroovyInteroperabilityTest {
+
+    @Test
+    fun `can use closure with single argument call`() {
+        val list = arrayListOf<Int>()
+        closureOf<MutableList<Int>> { add(42) }.call(list)
+        assertEquals(42, list.first())
+    }
+
+    @Test
+    fun `can use KotlinClosure to control return type`() {
+        fun stringClosure(function: String.() -> String) =
+            KotlinClosure(function)
+
+        assertEquals(
+            "GROOVY",
+            stringClosure { toUpperCase() }.call("groovy"))
+    }
+}

src/test/kotlin/org/gradle/script/lang/kotlin/integration/GradleScriptKotlinIntegrationTest.kt

@@ -230,6 +230,19 @@ class GradleScriptKotlinIntegrationTest {
             kotlinBuildScriptModel().classPath.isNotEmpty())
     }
 
+    @Test
+    fun `can use Closure only APIs`() {
+
+        withBuildScript("""
+            gradle.buildFinished(closureOf<org.gradle.BuildResult> {
+                println("*" + action + "*") // <- BuildResult.getAction()
+            })
+        """)
+
+        assert(
+            build("build").output.contains("*Build*"))
+    }
+
     private fun buildSrcOutput(): File =
         existing("buildSrc/build/classes/main")
 
@@ -283,7 +296,7 @@ class GradleScriptKotlinIntegrationTest {
     private fun gradleRunnerFor(projectDir: File): GradleRunner =
         GradleRunner
             .create()
-            .withDebug(false)
+            .withDebug(true)
             .withGradleInstallation(customInstallation())
             .withProjectDir(projectDir)