Add runtime kotlin version detection for platform info purposes.

firebase-common/firebase-common.gradle

@@ -64,6 +64,9 @@ dependencies {
     api 'com.google.auto.value:auto-value-annotations:1.6.5'
     compileOnly 'com.google.code.findbugs:jsr305:3.0.2'
 
+    // needed for Kotlin detection to compile, but not necessarily present at runtime.
+    compileOnly "org.jetbrains.kotlin:kotlin-stdlib-jdk7:$kotlinVersion"
+
     testImplementation 'androidx.test:runner:1.2.0'
     testImplementation 'androidx.test.ext:junit:1.1.1'
     testImplementation "org.robolectric:robolectric:$robolectricVersion"

firebase-common/ktx/ktx.gradle

@@ -27,7 +27,6 @@ android {
         minSdkVersion project.minSdkVersion
         targetSdkVersion project.targetSdkVersion
         versionName version
-        buildConfigField('String', 'KOTLIN_VERSION', "\"$kotlinVersion\"")
     }
     sourceSets {
         main.java.srcDirs += 'src/main/kotlin'

firebase-common/ktx/src/main/kotlin/com/google/firebase/ktx/Firebase.kt

@@ -57,7 +57,6 @@ internal const val LIBRARY_NAME: String = "fire-core-ktx"
 class FirebaseCommonKtxRegistrar : ComponentRegistrar {
     override fun getComponents(): List<Component<*>> {
         return listOf(
-                LibraryVersionComponent.create(LIBRARY_NAME, BuildConfig.VERSION_NAME),
-                LibraryVersionComponent.create("kotlin", BuildConfig.KOTLIN_VERSION))
+                LibraryVersionComponent.create(LIBRARY_NAME, BuildConfig.VERSION_NAME))
     }
 }

firebase-common/proguard.txt

@@ -1,3 +1,4 @@
 -dontwarn com.google.firebase.components.Component$Instantiation
 -dontwarn com.google.firebase.components.Component$ComponentType
+-dontwarn com.google.firebase.platforminfo.KotlinDetector
 -keep class * implements com.google.firebase.components.ComponentRegistrar

firebase-common/src/main/java/com/google/firebase/FirebaseApp.java

@@ -46,6 +46,7 @@
 import com.google.firebase.events.Publisher;
 import com.google.firebase.internal.DataCollectionConfigStorage;
 import com.google.firebase.platforminfo.DefaultUserAgentPublisher;
+import com.google.firebase.platforminfo.KotlinDetector;
 import com.google.firebase.platforminfo.LibraryVersionComponent;
 import java.nio.charset.Charset;
 import java.util.ArrayList;
@@ -100,6 +101,7 @@ public class FirebaseApp {
 
   private static final String FIREBASE_ANDROID = "fire-android";
   private static final String FIREBASE_COMMON = "fire-core";
+  private static final String KOTLIN = "kotlin";
 
   private final Context applicationContext;
   private final String name;
@@ -397,6 +399,8 @@ protected FirebaseApp(Context applicationContext, String name, FirebaseOptions o
 
     List<ComponentRegistrar> registrars =
         ComponentDiscovery.forContext(applicationContext).discover();
+
+    String kotlinVersion = KotlinDetector.detectVersion();
     componentRuntime =
         new ComponentRuntime(
             UI_EXECUTOR,
@@ -406,7 +410,9 @@ protected FirebaseApp(Context applicationContext, String name, FirebaseOptions o
             Component.of(options, FirebaseOptions.class),
             LibraryVersionComponent.create(FIREBASE_ANDROID, ""),
             LibraryVersionComponent.create(FIREBASE_COMMON, BuildConfig.VERSION_NAME),
+            kotlinVersion != null ? LibraryVersionComponent.create(KOTLIN, kotlinVersion) : null,
             DefaultUserAgentPublisher.component());
+
     dataCollectionConfigStorage =
         new Lazy<>(
             () ->

firebase-common/src/main/java/com/google/firebase/components/ComponentRuntime.java

@@ -55,7 +55,11 @@ public ComponentRuntime(
     for (ComponentRegistrar registrar : registrars) {
       componentsToAdd.addAll(registrar.getComponents());
     }
-    Collections.addAll(componentsToAdd, additionalComponents);
+    for (Component<?> additionalComponent : additionalComponents) {
+      if (additionalComponent != null) {
+        componentsToAdd.add(additionalComponent);
+      }
+    }
 
     CycleDetector.detect(componentsToAdd);
 

firebase-common/src/main/java/com/google/firebase/platforminfo/KotlinDetector.java

@@ -0,0 +1,36 @@
+// Copyright 2019 Google LLC
+//
+// 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 com.google.firebase.platforminfo;
+
+import androidx.annotation.Nullable;
+
+/**
+ * Detects presence of Kotlin stdlib on the classpath.
+ *
+ * <p>If it is present, it is inferred that the application or its subset is written in Kotlin.
+ */
+public final class KotlinDetector {
+  private KotlinDetector() {}
+
+  /** Returns the version of Kotlin stdlib if found, {@code null} otherwise. */
+  @Nullable
+  public static String detectVersion() {
+    try {
+      return kotlin.KotlinVersion.CURRENT.toString();
+    } catch (NoClassDefFoundError ex) {
+      return null;
+    }
+  }
+}