Add a test to make sure the prism train is running.

selfie-runner-junit5/src/test/kotlin/com/diffplug/selfie/junit5/Harness.kt

@@ -30,7 +30,7 @@ import org.opentest4j.AssertionFailedError
 import org.w3c.dom.NodeList
 import org.xml.sax.InputSource
 
-open class Harness(subproject: String) {
+open class Harness(subproject: String, val onlyRunThisTest: Boolean = false) {
   val subprojectFolder: Path
   var settings = ""
 
@@ -196,22 +196,37 @@ open class Harness(subproject: String) {
         .connect()
         .use { connection ->
           try {
-            val buildLauncher =
-                connection
-                    .newBuild()
-                    .setStandardError(System.err)
-                    .setStandardOutput(System.out)
-                    .forTasks(":${subprojectFolder.name}:$task")
-                    .withArguments(
-                        buildList<String> {
-                          addAll(args)
-                          add("--configuration-cache") // enabled vs disabled is 11s vs 24s
-                          add("--stacktrace")
-                        })
-            buildLauncher.run()
-            return null
+            if (onlyRunThisTest) {
+              connection
+                  .newTestLauncher()
+                  .setStandardError(System.err)
+                  .setStandardOutput(System.out)
+                  .withTaskAndTestClasses(
+                      ":${subprojectFolder.name}:$task", listOf("UT_${javaClass.simpleName}"))
+                  .withArguments(
+                      buildList<String> {
+                        addAll(args)
+                        add("--configuration-cache") // enabled vs disabled is 11s vs 24s
+                        add("--stacktrace")
+                      })
+                  .run()
+            } else {
+              connection
+                  .newBuild()
+                  .setStandardError(System.err)
+                  .setStandardOutput(System.out)
+                  .forTasks(":${subprojectFolder.name}:$task")
+                  .withArguments(
+                      buildList<String> {
+                        addAll(args)
+                        add("--configuration-cache") // enabled vs disabled is 11s vs 24s
+                        add("--stacktrace")
+                      })
+                  .run()
+            }
+            null
           } catch (e: BuildException) {
-            return parseBuildException(task, e)
+            parseBuildException(task, e)
           }
         }
   }

selfie-runner-junit5/src/test/kotlin/com/diffplug/selfie/junit5/PrismTrainTest.kt

@@ -0,0 +1,68 @@
+/*
+ * Copyright (C) 2023 DiffPlug
+ *
+ * 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
+ *
+ *     https://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 com.diffplug.selfie.junit5
+
+import kotlin.test.Test
+import org.junit.jupiter.api.MethodOrderer
+import org.junit.jupiter.api.Order
+import org.junit.jupiter.api.TestMethodOrder
+import org.junitpioneer.jupiter.DisableIfTestFails
+
+/** Simplest test for verifying read/write of a snapshot. */
+@TestMethodOrder(MethodOrderer.OrderAnnotation::class)
+@DisableIfTestFails
+class PrismTrainTest : Harness("undertest-junit5", onlyRunThisTest = true) {
+  @Test @Order(1)
+  fun noSelfie() {
+    ut_snapshot().deleteIfExists()
+    ut_snapshot().assertDoesNotExist()
+  }
+
+  @Test @Order(2)
+  fun noTrain() {
+    gradleWriteSS()
+    ut_snapshot()
+        .assertContent(
+            """
+            ╔═ selfie ═╗
+            apple
+            ╔═ [end of file] ═╗
+            
+        """
+                .trimIndent())
+    gradleReadSS()
+  }
+
+  @Test @Order(3)
+  fun withTrain() {
+    settings = "undertest.junit5.SettingsLensCount"
+    gradleReadSSFail()
+    // now let's write it
+    gradleWriteSS()
+    ut_snapshot()
+        .assertContent(
+            """
+            ╔═ selfie ═╗
+            apple
+            ╔═ selfie[count] ═╗
+            5
+            ╔═ [end of file] ═╗
+            
+        """
+                .trimIndent())
+    gradleReadSS()
+  }
+}

undertest-junit5/src/test/kotlin/undertest/junit5/SettingsLensCount.kt

@@ -0,0 +1,21 @@
+package undertest.junit5
+
+import com.diffplug.selfie.CompoundPrism
+import com.diffplug.selfie.Snapshot
+import com.diffplug.selfie.SnapshotLens
+import com.diffplug.selfie.SnapshotValue
+import com.diffplug.selfie.junit5.StandardSelfieSettings
+
+class SettingsLensCount : StandardSelfieSettings() {
+  override fun setupPrismTrain(prismTrain: CompoundPrism) {
+    prismTrain
+        .forEverySnapshot()
+        .addLens(
+            object : SnapshotLens {
+              override val defaultLensName = "count"
+
+              override fun transform(testClass: String, key: String, snapshot: Snapshot) =
+                  SnapshotValue.of(snapshot.value.valueString().count().toString())
+            })
+  }
+}

undertest-junit5/src/test/kotlin/undertest/junit5/UT_PrismTrainTest.kt

@@ -0,0 +1,10 @@
+package undertest.junit5
+
+import com.diffplug.selfie.Selfie.expectSelfie
+import kotlin.test.Test
+
+class UT_PrismTrainTest {
+  @Test fun selfie() {
+    expectSelfie("apple").toMatchDisk()
+  }
+}

undertest-junit5/src/test/kotlin/undertest/junit5/__snapshots__/UT_PrismTrainTest.ss

@@ -0,0 +1,3 @@
+╔═ selfie ═╗
+apple
+╔═ [end of file] ═╗