Add support for Gradle Kotlin DSL (#140744)

This PR resolves #140548. It's based on my work in #118067.

examples/hello_world/android/settings.gradle

-// Copyright 2014 The Flutter Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style license that can be
-// found in the LICENSE file.
-
-// This file is auto generated.
-// To update all the settings.gradle files in the Flutter repo,
-// See dev/tools/bin/generate_gradle_lockfiles.dart.
-
-pluginManagement {
-    def flutterSdkPath = {
-        def properties = new Properties()
-        file("local.properties").withInputStream { properties.load(it) }
-        def flutterSdkPath = properties.getProperty("flutter.sdk")
-        assert flutterSdkPath != null, "flutter.sdk not set in local.properties"
-        return flutterSdkPath
-    }
-    settings.ext.flutterSdkPath = flutterSdkPath()
-
-    // Flutter Gradle Plugin ships together with the Flutter SDK
-    includeBuild("${settings.ext.flutterSdkPath}/packages/flutter_tools/gradle")
-
-    repositories {
-        google()
-        mavenCentral()
-        gradlePluginPortal()
-    }
-}
-
-plugins {
-    id "dev.flutter.flutter-plugin-loader" version "1.0.0"
-    id "com.android.application" version "7.4.2" apply false
-}
-
-include ':app'

examples/hello_world/android/settings.gradle.kts

+// Copyright 2014 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// Contents of this file automatically generated by dev/tools/bin/generate_gradle_lockfiles.dart.
+// Do not merge changes to this file. See #140115.
+
+pluginManagement {
+    val flutterSdkPath = run {
+        val properties = java.util.Properties()
+        file("local.properties").inputStream().use { properties.load(it) }
+        val flutterSdkPath = properties.getProperty("flutter.sdk")
+        require(flutterSdkPath != null) { "flutter.sdk not set in local.properties" }
+        flutterSdkPath
+    }
+
+    includeBuild("$flutterSdkPath/packages/flutter_tools/gradle")
+
+    repositories {
+        google()
+        mavenCentral()
+        gradlePluginPortal()
+    }
+}
+
+plugins {
+    id("dev.flutter.flutter-plugin-loader") version "1.0.0"
+    id("com.android.application") version "7.4.2" apply false
+}
+
+include(":app")

packages/flutter_tools/gradle/src/main/groovy/flutter.groovy

@@ -29,7 +29,8 @@ import org.gradle.api.tasks.bundling.Jar
 import org.gradle.internal.os.OperatingSystem
 
 /**
- * For apps only. Provides the flutter extension used in app/build.gradle.
+ * For apps only. Provides the flutter extension used in the app-level Gradle
+ * build file (app/build.gradle or app/build.gradle.kts).
  *
  * The versions specified here should match the values in
  * packages/flutter_tools/lib/src/android/gradle_utils.dart, so when bumping,
@@ -62,7 +63,7 @@ class FlutterExtension {
 
     /**
      * Specifies the relative directory to the Flutter project directory.
-     * In an app project, this is ../.. since the app's build.gradle is under android/app.
+     * In an app project, this is ../.. since the app's Gradle build file is under android/app.
      */
     String source = "../.."
 
@@ -90,7 +91,8 @@ buildscript {
 
 /**
  * Some apps don't set default compile options.
- * Apps can change these values in android/app/build.gradle.
+ * Apps can change these values in the app-level Gradle build file
+ * (android/app/build.gradle or android/app/build.gradle.kts).
  * This just ensures that default values are set.
  */
 android {
@@ -423,13 +425,12 @@ class FlutterPlugin implements Plugin<Project> {
      * just using the `plugins.android` list.
      */
     private configureLegacyPluginEachProjects(Project project) {
-        File settingsGradle = new File(project.projectDir.parentFile, "settings.gradle")
         try {
-            if (!settingsGradle.text.contains("'.flutter-plugins'")) {
+            if (!settingsGradleFile(project).text.contains("'.flutter-plugins'")) {
                 return
             }
         } catch (FileNotFoundException ignored) {
-            throw new GradleException("settings.gradle does not exist: ${settingsGradle.absolutePath}")
+            throw new GradleException("settings.gradle/settings.gradle.kts does not exist: ${settingsGradleFile(project).absolutePath}")
         }
         List<Map<String, Object>> deps = getPluginDependencies(project)
         List<String> plugins = getPluginList(project).collect { it.name as String }
@@ -438,7 +439,7 @@ class FlutterPlugin implements Plugin<Project> {
             Project pluginProject = project.rootProject.findProject(":${it.name}")
             if (pluginProject == null) {
                 // Plugin was not included in `settings.gradle`, but is listed in `.flutter-plugins`.
-                project.logger.error("Plugin project :${it.name} listed, but not found. Please fix your settings.gradle.")
+                project.logger.error("Plugin project :${it.name} listed, but not found. Please fix your settings.gradle/settings.gradle.kts.")
             } else if (doesSupportAndroidPlatform(pluginProject.projectDir.parentFile.path as String)) {
                 // Plugin has a functioning `android` folder and is included successfully, although it's not supported.
                 // It must be configured nonetheless, to not throw an "Unresolved reference" exception.
@@ -451,11 +452,38 @@ class FlutterPlugin implements Plugin<Project> {
 
     // TODO(54566): Can remove this function and its call sites once resolved.
     /**
-     * Returns `true` if the given path contains an `android/build.gradle` file.
+     * Returns `true` if the given path contains an `android` directory
+     * containing a `build.gradle` or `build.gradle.kts` file.
+     */
+    private Boolean doesSupportAndroidPlatform(String path) {
+        File buildGradle = new File(path, 'android' + File.separator + 'build.gradle')
+        File buildGradleKts = new File(path, 'android' + File.separator + 'build.gradle.kts')
+        if (buildGradle.exists() && buildGradleKts.exists()) {
+            logger.error(
+                "Both build.gradle and build.gradle.kts exist, so " +
+                "build.gradle.kts is ignored. This is likely a mistake."
+            )
+        }
+
+        return buildGradle.exists() || buildGradleKts.exists()
+    }
+
+    /**
+     * Returns the Gradle settings script for the build. When both Groovy and
+     * Kotlin variants exist, then Groovy (settings.gradle) is preferred over
+     * Kotlin (settings.gradle.kts). This is the same behavior as Gradle 8.5.
      */
-    private static Boolean doesSupportAndroidPlatform(String path) {
-        File editableAndroidProject = new File(path, "android" + File.separator + "build.gradle")
-        return editableAndroidProject.exists()
+    private File settingsGradleFile(Project project) {
+        File settingsGradle = new File(project.projectDir.parentFile, "settings.gradle")
+        File settingsGradleKts = new File(project.projectDir.parentFile, "settings.gradle.kts")
+        if (settingsGradle.exists() && settingsGradleKts.exists()) {
+            logger.error(
+                "Both settings.gradle and settings.gradle.kts exist, so " +
+                "settings.gradle.kts is ignored. This is likely a mistake."
+            )
+        }
+
+        return settingsGradle.exists() ? settingsGradle : settingsGradleKts
     }
 
     /** Adds the plugin project dependency to the app project. */

packages/flutter_tools/lib/src/android/deferred_components_prebuild_validator.dart

@@ -225,9 +225,15 @@ class _DeferredComponentAndroidFiles {
   Directory get componentDir => androidDir.childDirectory(name);
 
   File get androidManifestFile => componentDir.childDirectory('src').childDirectory('main').childFile('AndroidManifest.xml');
-  File get buildGradleFile => componentDir.childFile('build.gradle');
+  File get buildGradleFile {
+    if (componentDir.childFile('build.gradle').existsSync()) {
+      return componentDir.childFile('build.gradle');
+    }
+    return componentDir.childFile('build.gradle.kts');
+  }
 
-  // True when AndroidManifest.xml and build.gradle exist for the android dynamic feature.
+  // True when AndroidManifest.xml and build.gradle/build.gradle.kts exist for
+  // the android dynamic feature.
   bool verifyFilesExist() {
     return androidManifestFile.existsSync() && buildGradleFile.existsSync();
   }

packages/flutter_tools/lib/src/android/gradle_utils.dart

@@ -204,20 +204,26 @@ distributionUrl=https\\://services.gradle.org/distributions/gradle-$gradleVersio
 /// The Android plugin version is specified in the [build.gradle] file within
 /// the project's Android directory.
 String getGradleVersionForAndroidPlugin(Directory directory, Logger logger) {
-  final File buildFile = directory.childFile('build.gradle');
+  const String buildFileName = 'build.gradle/build.gradle.kts';
+
+  File buildFile = directory.childFile('build.gradle');
+  if (!buildFile.existsSync()) {
+    buildFile = directory.childFile('build.gradle.kts');
+  }
+
   if (!buildFile.existsSync()) {
     logger.printTrace(
-        "$buildFile doesn't exist, assuming Gradle version: $templateDefaultGradleVersion");
+        "$buildFileName doesn't exist, assuming Gradle version: $templateDefaultGradleVersion");
     return templateDefaultGradleVersion;
   }
   final String buildFileContent = buildFile.readAsStringSync();
   final Iterable<Match> pluginMatches = _buildAndroidGradlePluginRegExp.allMatches(buildFileContent);
   if (pluginMatches.isEmpty) {
-    logger.printTrace("$buildFile doesn't provide an AGP version, assuming Gradle version: $templateDefaultGradleVersion");
+    logger.printTrace("$buildFileName doesn't provide an AGP version, assuming Gradle version: $templateDefaultGradleVersion");
     return templateDefaultGradleVersion;
   }
   final String? androidPluginVersion = pluginMatches.first.group(1);
-  logger.printTrace('$buildFile provides AGP version: $androidPluginVersion');
+  logger.printTrace('$buildFileName provides AGP version: $androidPluginVersion');
   return getGradleVersionFor(androidPluginVersion ?? 'unknown');
 }
 
@@ -325,9 +331,13 @@ OS:           Mac OS X 13.2.1 aarch64
 /// [settings.gradle] file within the project's
 /// Android directory ([androidDirectory]).
 String? getAgpVersion(Directory androidDirectory, Logger logger) {
-  final File buildFile = androidDirectory.childFile('build.gradle');
+  File buildFile = androidDirectory.childFile('build.gradle');
+  if (!buildFile.existsSync()) {
+    buildFile = androidDirectory.childFile('build.gradle.kts');
+  }
+
   if (!buildFile.existsSync()) {
-    logger.printTrace('Can not find build.gradle in $androidDirectory');
+    logger.printTrace('Can not find build.gradle/build.gradle.kts in $androidDirectory');
     return null;
   }
   final String buildFileContent = buildFile.readAsStringSync();

packages/flutter_tools/lib/src/project.dart

@@ -459,6 +459,10 @@ class AndroidProject extends FlutterProjectPlatform {
   static final RegExp _applicationIdPattern = RegExp('^\\s*applicationId\\s+[\'"](.*)[\'"]\\s*\$');
   static final RegExp _imperativeKotlinPluginPattern = RegExp('^\\s*apply plugin\\:\\s+[\'"]kotlin-android[\'"]\\s*\$');
   static final RegExp _declarativeKotlinPluginPattern = RegExp('^\\s*id\\s+[\'"]kotlin-android[\'"]\\s*\$');
+
+  /// Pattern used to find the assignment of the "group" property in Gradle.
+  /// Expected example: `group "dev.flutter.plugin"`
+  /// Regex is used in both Groovy and Kotlin Gradle files.
   static final RegExp _groupPattern = RegExp('^\\s*group\\s+[\'"](.*)[\'"]\\s*\$');
 
   /// The Gradle root directory of the Android host app. This is the directory
@@ -552,10 +556,54 @@ class AndroidProject extends FlutterProjectPlatform {
     return imperativeMatch || declarativeMatch;
   }
 
+  /// Gets top-level Gradle build file.
+  /// See https://developer.android.com/build#top-level.
+  ///
+  /// The file must exist and it must be written in either Groovy (build.gradle)
+  /// or Kotlin (build.gradle.kts).
+  File get hostAppGradleFile {
+    final File buildGroovy = hostAppGradleRoot.childFile('build.gradle');
+    final File buildKotlin = hostAppGradleRoot.childFile('build.gradle.kts');
+
+    if (buildGroovy.existsSync() && buildKotlin.existsSync()) {
+      // We mimic Gradle's behavior of preferring Groovy over Kotlin when both files exist.
+      return buildGroovy;
+    }
+
+    if (buildKotlin.existsSync()) {
+      return buildKotlin;
+    }
+
+    // TODO(bartekpacia): An exception should be thrown when neither
+    // build.gradle nor build.gradle.kts exist, instead of falling back to the
+    // Groovy file. See #141180.
+    return buildGroovy;
+  }
+
   /// Gets the module-level build.gradle file.
   /// See https://developer.android.com/build#module-level.
-  File get appGradleFile => hostAppGradleRoot.childDirectory('app')
-      .childFile('build.gradle');
+  ///
+  /// The file must exist and it must be written in either Groovy (build.gradle)
+  /// or Kotlin (build.gradle.kts).
+  File get appGradleFile {
+    final Directory appDir = hostAppGradleRoot.childDirectory('app');
+    final File buildGroovy = appDir.childFile('build.gradle');
+    final File buildKotlin = appDir.childFile('build.gradle.kts');
+
+    if (buildGroovy.existsSync() && buildKotlin.existsSync()) {
+      // We mimic Gradle's behavior of preferring Groovy over Kotlin when both files exist.
+      return buildGroovy;
+    }
+
+    if (buildKotlin.existsSync()) {
+      return buildKotlin;
+    }
+
+    // TODO(bartekpacia): An exception should be thrown when neither
+    // build.gradle nor build.gradle.kts exist, instead of falling back to the
+    // Groovy file. See #141180.
+    return buildGroovy;
+  }
 
   File get appManifestFile {
     if (isUsingGradle) {
@@ -652,7 +700,7 @@ $javaGradleCompatUrl
   }
 
   bool get isUsingGradle {
-    return hostAppGradleRoot.childFile('build.gradle').existsSync();
+    return hostAppGradleFile.existsSync();
   }
 
   String? get applicationId {
@@ -671,8 +719,7 @@ $javaGradleCompatUrl
   }
 
   String? get group {
-    final File gradleFile = hostAppGradleRoot.childFile('build.gradle');
-    return firstMatchInFile(gradleFile, _groupPattern)?.group(1);
+    return firstMatchInFile(hostAppGradleFile, _groupPattern)?.group(1);
   }
 
   /// The build directory where the Android artifacts are placed.

packages/flutter_tools/test/general.shard/android/gradle_utils_test.dart

@@ -10,13 +10,14 @@ import 'package:flutter_tools/src/base/platform.dart';
 import 'package:flutter_tools/src/base/version_range.dart';
 import 'package:flutter_tools/src/cache.dart';
 import 'package:flutter_tools/src/project.dart';
+
 import '../../src/common.dart';
 import '../../src/fake_process_manager.dart';
 import '../../src/fakes.dart';
 
 void main() {
   group('injectGradleWrapperIfNeeded', () {
-    late MemoryFileSystem fileSystem;
+    late FileSystem fileSystem;
     late Directory gradleWrapperDirectory;
     late GradleUtils gradleUtils;
 
@@ -386,7 +387,7 @@ OS:           Mac OS X 13.2.1 aarch64
       );
     });
 
-    testWithoutContext('returns the AGP version when set', () async {
+    testWithoutContext('returns the AGP version when set in Groovy', () async {
       const String expectedVersion = '7.3.0';
       final Directory androidDirectory = fileSystem.directory('/android')
         ..createSync();
@@ -415,6 +416,90 @@ allprojects {
         expectedVersion,
       );
     });
+
+    testWithoutContext('returns the AGP version when set in Kotlin', () async {
+      const String expectedVersion = '7.3.0';
+      final Directory androidDirectory = fileSystem.directory('/android')
+        ..createSync();
+      androidDirectory.childFile('build.gradle.kts').writeAsStringSync('''
+buildscript {
+    repositories {
+        google()
+        mavenCentral()
+    }
+
+    dependencies {
+        classpath("com.android.tools.build:gradle:$expectedVersion")
+    }
+}
+
+allprojects {
+    repositories {
+        google()
+        mavenCentral()
+    }
+}
+''');
+
+      expect(
+        getAgpVersion(androidDirectory, BufferLogger.test()),
+        expectedVersion,
+      );
+    });
+
+    testWithoutContext('prefers the AGP version when set in Groovy, ignores Kotlin', () async {
+      const String versionInGroovy = '7.3.0';
+      const String versionInKotlin = '7.4.2';
+      final Directory androidDirectory = fileSystem.directory('/android')
+        ..createSync();
+
+      androidDirectory.childFile('build.gradle').writeAsStringSync('''
+buildscript {
+    repositories {
+        google()
+        mavenCentral()
+    }
+
+    dependencies {
+        classpath 'com.android.tools.build:gradle:$versionInGroovy'
+    }
+}
+
+allprojects {
+    repositories {
+        google()
+        mavenCentral()
+    }
+}
+''');
+
+      androidDirectory.childFile('build.gradle.kts').writeAsStringSync('''
+buildscript {
+    repositories {
+        google()
+        mavenCentral()
+    }
+
+    dependencies {
+        classpath("com.android.tools.build:gradle:$versionInKotlin")
+    }
+}
+
+allprojects {
+    repositories {
+        google()
+        mavenCentral()
+    }
+}
+''');
+
+      expect(
+        getAgpVersion(androidDirectory, BufferLogger.test()),
+        versionInGroovy,
+      );
+    });
+
+
     testWithoutContext('returns null when AGP version not set', () async {
       final Directory androidDirectory = fileSystem.directory('/android')
         ..createSync();
@@ -703,6 +788,68 @@ include ":app"
     });
   });
 
+  group('getGradleVersionForAndroidPlugin', () {
+    late FileSystem fileSystem;
+    late Logger testLogger;
+
+    setUp(() {
+      fileSystem = MemoryFileSystem.test();
+      testLogger = BufferLogger.test();
+    });
+
+    testWithoutContext('prefers build.gradle over build.gradle.kts', () async {
+      const String versionInGroovy = '4.0.0';
+      const String versionInKotlin = '7.4.2';
+      final Directory androidDirectory = fileSystem.directory('/android')..createSync();
+
+      androidDirectory.childFile('build.gradle').writeAsStringSync('''
+buildscript {
+    repositories {
+        google()
+        mavenCentral()
+    }
+
+    dependencies {
+        classpath 'com.android.tools.build:gradle:$versionInGroovy'
+    }
+}
+
+allprojects {
+    repositories {
+        google()
+        mavenCentral()
+    }
+}
+''');
+
+      androidDirectory.childFile('build.gradle.kts').writeAsStringSync('''
+buildscript {
+    repositories {
+        google()
+        mavenCentral()
+    }
+
+    dependencies {
+        classpath("com.android.tools.build:gradle:$versionInKotlin")
+    }
+}
+
+allprojects {
+    repositories {
+        google()
+        mavenCentral()
+    }
+}
+''');
+
+
+      expect(
+        getGradleVersionForAndroidPlugin(androidDirectory, testLogger),
+        '6.7', // as per compatibility matrix in gradle_utils.dart
+      );
+    });
+  });
+
   group('validates java/AGP versions', () {
     final List<JavaAgpTestData> testData = <JavaAgpTestData>[
       // Strictly too old Java versions for known AGP versions.

packages/flutter_tools/test/general.shard/project_test.dart

@@ -738,6 +738,60 @@ apply plugin: 'kotlin-android'
         XcodeProjectInterpreter: () => xcodeProjectInterpreter,
         FlutterProjectFactory: () => flutterProjectFactory,
       });
+
+    testUsingContext('kotlin host app language with Gradle Kotlin DSL', () async {
+      final FlutterProject project = await someProject();
+
+        addAndroidGradleFile(project.directory,
+          kotlinDsl: true,
+          gradleFileContent: () {
+            return '''
+plugins {
+    id "com.android.application"
+    id "kotlin-android"
+    id "dev.flutter.flutter-gradle-plugin"
+}
+''';
+        });
+        expect(project.android.isKotlin, isTrue);
+      }, overrides: <Type, Generator>{
+        FileSystem: () => fs,
+        ProcessManager: () => FakeProcessManager.any(),
+        XcodeProjectInterpreter: () => xcodeProjectInterpreter,
+        FlutterProjectFactory: () => flutterProjectFactory,
+      });
+
+    testUsingContext('Gradle Groovy files are preferred to Gradle Kotlin files', () async {
+      final FlutterProject project = await someProject();
+
+        addAndroidGradleFile(project.directory,
+          gradleFileContent: () {
+            return '''
+plugins {
+    id "com.android.application"
+    id "dev.flutter.flutter-gradle-plugin"
+}
+''';
+        });
+        addAndroidGradleFile(project.directory,
+          kotlinDsl: true,
+          gradleFileContent: () {
+            return '''
+plugins {
+    id("com.android.application")
+    id("kotlin-android")
+    id("dev.flutter.flutter-gradle-plugin")
+}
+''';
+        });
+
+        expect(project.android.isKotlin, isFalse);
+      }, overrides: <Type, Generator>{
+        FileSystem: () => fs,
+        ProcessManager: () => FakeProcessManager.any(),
+        XcodeProjectInterpreter: () => xcodeProjectInterpreter,
+        FlutterProjectFactory: () => flutterProjectFactory,
+      });
     });
 
     group('With mocked context', () {
@@ -1568,11 +1622,18 @@ void addIosProjectFile(Directory directory, {required String Function() projectF
     ..writeAsStringSync(projectFileContent());
 }
 
-void addAndroidGradleFile(Directory directory, { required String Function() gradleFileContent }) {
+/// Adds app-level Gradle Groovy build file (build.gradle) to [directory].
+///
+/// If [kotlinDsl] is true, then build.gradle.kts is created instead of
+/// build.gradle. It's the caller's responsibility to make sure that
+/// [gradleFileContent] is consistent with the value of the [kotlinDsl] flag.
+void addAndroidGradleFile(Directory directory, {
+  required String Function() gradleFileContent, bool kotlinDsl = false,
+}) {
   directory
       .childDirectory('android')
       .childDirectory('app')
-      .childFile('build.gradle')
+      .childFile(kotlinDsl ? 'build.gradle.kts' : 'build.gradle')
     ..createSync(recursive: true)
     ..writeAsStringSync(gradleFileContent());
 }
@@ -1608,8 +1669,8 @@ FileSystem getFileSystemForPlatform() {
   );
 }
 
-void addAndroidWithGroup(Directory directory, String id) {
-  directory.childDirectory('android').childFile('build.gradle')
+void addAndroidWithGroup(Directory directory, String id, {bool kotlinDsl = false}) {
+  directory.childDirectory('android').childFile(kotlinDsl ? 'build.gradle.kts' : 'build.gradle')
     ..createSync(recursive: true)
     ..writeAsStringSync(gradleFileWithGroupId(id));
 }

packages/flutter_tools/test/integration.shard/android_plugin_skip_unsupported_test.dart

@@ -147,7 +147,7 @@ void main() {
     final ProcessResult buildApkResult = await testUnsupportedPlugin(
         project: project, createAndroidPluginFolder: true);
     expect(buildApkResult.stderr.toString(),
-        isNot(contains('Please fix your settings.gradle.')));
+        isNot(contains('Please fix your settings.gradle')));
     expect(buildApkResult, const ProcessResultMatcher());
   });
 
@@ -167,7 +167,7 @@ void main() {
     expect(
       buildApkResult,
       const ProcessResultMatcher(
-          stderrPattern: 'Please fix your settings.gradle.'),
+          stderrPattern: 'Please fix your settings.gradle/settings.gradle.kts'),
     );
   });
 }