Allow Android SDK detection from environment variable as well

Author: mannodermaus

android-junit5/src/test/groovy/de/mannodermaus/gradle/plugins/android_junit5/util/TestEnvironment.groovy

@@ -1,5 +1,7 @@
 package de.mannodermaus.gradle.plugins.android_junit5.util
 
+import com.android.annotations.Nullable
+
 /*
  * Encapsulates environment properties related to running
  * Unit Tests that interface with an Android SDK installation.
@@ -12,6 +14,7 @@ class TestEnvironment {
 
   private static final ANDROID_SDK_FILE_NAME = "local.properties"
   private static final ANDROID_SDK_PROP_NAME = "sdk.dir"
+  private static final ANDROID_HOME_ENVVAR_NAME = "ANDROID_HOME"
 
   private static
   final ENVIRONMENT_RESOURCE_NAME = "/de/mannodermaus/gradle/plugins/android_junit5/testenv.properties"
@@ -51,18 +54,44 @@ class TestEnvironment {
   }
 
   private File loadAndroidSdkFolder() {
+    // Try local project first, fall back to Environment Variable, throw if nothing works
+    File folder = loadAndroidSdkFromProject()
+
+    if (folder == null) {
+      folder = loadAndroidSdkFromEnvVar()
+    }
+
+    if (folder == null) {
+      throw new AssertionError(
+          "Android SDK couldn't be found. Either local.properties file in project root is missing, " +
+              "it doesn't include the required 'sdk.dir' statement, " +
+              "or there is no ANDROID_HOME environment variable!")
+    }
+  }
+
+  @Nullable
+  private File loadAndroidSdkFromProject() {
     File rootFile = new File(System.getProperty("user.dir")).parentFile
     File localPropsFile = new File(rootFile, ANDROID_SDK_FILE_NAME)
-    if (!localPropsFile.exists()) {
-      throw new AssertionError(
-          "'sdk.dir' couldn't be found. Either local.properties file in folder '${rootFile.absolutePath}' is missing, " +
-              "or it doesn't include the required 'sdk.dir' statement!")
+    if (localPropsFile.exists()) {
+      def sdkFolderProp = localPropsFile.readLines()
+          .find { it.startsWith(ANDROID_SDK_PROP_NAME) }
+      sdkFolderProp = sdkFolderProp.substring(sdkFolderProp.indexOf('=') + 1).trim()
+
+      return new File(sdkFolderProp)
+    }
+
+    return null
+  }
+
+  @Nullable
+  private File loadAndroidSdkFromEnvVar() {
+    String envvar = System.getenv(ANDROID_HOME_ENVVAR_NAME)
+    if (envvar) {
+      return new File(envvar)
     }
-    def sdkFolderProp = localPropsFile.readLines()
-        .find { it.startsWith(ANDROID_SDK_PROP_NAME) }
-    sdkFolderProp = sdkFolderProp.substring(sdkFolderProp.indexOf('=') + 1).trim()
 
-    return new File(sdkFolderProp)
+    return null
   }
 
   private Properties loadAndroidEnvironment() {