Merge branch 'kotlin-1.4-compat' into 'dev'

Make objectbox-kotlin compilable by Kotlin 1.4

See merge request objectbox/objectbox-java!126

Author: greenrobot-team

build.gradle.kts

@@ -38,9 +38,12 @@ buildscript {
     val essentialsVersion by extra("3.1.0")
     val junitVersion by extra("4.13.2")
     val mockitoVersion by extra("3.8.0")
-    val kotlinVersion by extra("1.7.20")
-    val coroutinesVersion by extra("1.6.4")
-    val dokkaVersion by extra("1.7.20")
+    // The versions of Kotlin, Kotlin Coroutines and Dokka must work together.
+    // Check https://github.com/Kotlin/kotlinx.coroutines#readme
+    // and https://github.com/Kotlin/dokka/releases
+    val kotlinVersion by extra("1.8.20")
+    val coroutinesVersion by extra("1.7.3")
+    val dokkaVersion by extra("1.8.20")
 
     println("version=$obxJavaVersion")
     println("objectboxNativeDependency=$obxJniLibVersion")

objectbox-kotlin/build.gradle

@@ -14,10 +14,13 @@ tasks.withType(JavaCompile).configureEach {
     options.release.set(8)
 }
 
-// Produce Java 8 byte code, would default to Java 6.
 tasks.withType(org.jetbrains.kotlin.gradle.tasks.KotlinCompile).configureEach {
     kotlinOptions {
+        // Produce Java 8 byte code, would default to Java 6.
         jvmTarget = "1.8"
+        // Try to use APIs at most one version newer than lowest supported (notably by Gradle plugin).
+        // Note: Kotlin is able to compile with binaries up to one later version.
+        apiVersion = "1.5"
     }
 }
 

objectbox-rxjava3/src/main/java/io/objectbox/rx3/Query.kt

@@ -9,7 +9,7 @@ import io.reactivex.rxjava3.core.Single
 /**
  * Shortcut for [`RxQuery.flowableOneByOne(query, strategy)`][RxQuery.flowableOneByOne].
  */
-fun <T> Query<T>.flowableOneByOne(strategy: BackpressureStrategy = BackpressureStrategy.BUFFER): Flowable<T> {
+fun <T : Any> Query<T>.flowableOneByOne(strategy: BackpressureStrategy = BackpressureStrategy.BUFFER): Flowable<T> {
     return RxQuery.flowableOneByOne(this, strategy)
 }