User-friendly error message when the plugin is applied the first time…

… and api folder does not yet exist

Fixes #21

src/functionalTest/kotlin/kotlinx/validation/test/DefaultConfigTests.kt

@@ -6,19 +6,9 @@
 package kotlinx.validation.test
 
 import kotlinx.validation.api.*
-import kotlinx.validation.api.BaseKotlinGradleTest
-import kotlinx.validation.api.assertTaskFailure
-import kotlinx.validation.api.assertTaskSuccess
-import kotlinx.validation.api.buildGradleKts
-import kotlinx.validation.api.kotlin
-import kotlinx.validation.api.readFileList
-import kotlinx.validation.api.resolve
-import kotlinx.validation.api.runner
-import kotlinx.validation.api.test
-import org.assertj.core.api.Assertions
+import org.assertj.core.api.*
 import org.junit.Test
-import kotlin.test.assertFalse
-import kotlin.test.assertTrue
+import kotlin.test.*
 
 internal class DefaultConfigTests : BaseKotlinGradleTest() {
 
@@ -28,13 +18,14 @@ internal class DefaultConfigTests : BaseKotlinGradleTest() {
             buildGradleKts {
                 resolve("examples/gradle/base/withPlugin.gradle.kts")
             }
-
             runner {
                 arguments.add(":apiCheck")
             }
         }
 
         runner.buildAndFail().apply {
+            println(output)
+            assertTrue { output.contains("Please ensure that ':apiDump' was executed") }
             assertTaskFailure(":apiCheck")
         }
     }

src/main/kotlin/ApiCompareCompareTask.kt

@@ -12,9 +12,20 @@ import org.gradle.api.tasks.*
 import java.io.*
 
 open class ApiCompareCompareTask : DefaultTask() {
+
+    /*
+     * Nullability and optionality is a workaround for
+     * https://github.com/gradle/gradle/issues/2016
+     *
+     * Unfortunately, there is no way to skip validation apart from setting 'null'
+     */
+    @Optional
     @InputDirectory
     @PathSensitive(PathSensitivity.RELATIVE)
-    lateinit var projectApiDir: File
+    var projectApiDir: File? = null
+
+    // Used for diagnostic error message when projectApiDir doesn't exist
+    var nonExistingProjectApiDir: String? = null
 
     @InputDirectory
     @PathSensitive(PathSensitivity.RELATIVE)
@@ -26,10 +37,11 @@ open class ApiCompareCompareTask : DefaultTask() {
 
     @TaskAction
     fun verify() {
-        if (!projectApiDir.exists())
-            error("Expected folder with API declarations '$projectApiDir' does not exist")
-        if (!apiBuildDir.exists())
-            error("Folder with built API declarations '$apiBuildDir' does not exist")
+        val projectApiDir = projectApiDir
+        if (projectApiDir == null) {
+            error("Expected folder with API declarations '$nonExistingProjectApiDir' does not exist.\n" +
+                    "Please ensure that ':apiDump' was executed in order to get API dump to compare the build against")
+        }
 
         val subject = project.name
         val apiBuildDirFiles = mutableSetOf<RelativePath>()
@@ -55,8 +67,7 @@ open class ApiCompareCompareTask : DefaultTask() {
         val expectedFile = expectedApiDeclaration.getFile(projectApiDir)
         val actualFile = expectedApiDeclaration.getFile(apiBuildDir)
         val diff = compareFiles(expectedFile, actualFile)
-        if (diff != null)
-            diffSet.add(diff)
+        if (diff != null) diffSet.add(diff)
         if (diffSet.isNotEmpty()) {
             val diffText = diffSet.joinToString("\n\n")
             error("API check failed for project $subject.\n$diffText\n\n You can run :$subject:apiDump task to overwrite API declarations")

src/main/kotlin/BinaryCompatibilityValidatorPlugin.kt

@@ -134,7 +134,12 @@ private fun Project.configureCheckTasks(
         isEnabled = apiCheckEnabled(extension) && apiBuild.map { it.enabled }.getOrElse(true)
         group = "verification"
         description = "Checks signatures of public API against the golden value in API folder for $projectName"
-        projectApiDir = apiCheckDir
+        projectApiDir = if (apiCheckDir.exists()) {
+            apiCheckDir
+        } else {
+            nonExistingProjectApiDir = apiCheckDir.toString()
+            null
+        }
         this.apiBuildDir = apiBuildDir
         dependsOn(apiBuild)
     }