[jni] Fix the context when using multiple flutter engines

Author: HosseinYousefi

pkgs/jni/CHANGELOG.md

@@ -1,5 +1,9 @@
-## 0.14.3-wip
+## 0.15.0-wip
 
+- **Breaking Change**: Removed `Jni.getApplicationClassLoader()`,
+  `Jni.getCurrentActivity()`, and `Jni.getCachedApplicationContext()`. Instead
+  use `Jni.applicationContext(engineId)` to access the application context and
+  listen to `Jni.activityStream(engineId)` to acccess the activity.
 - Update to the latest lints.
 
 ## 0.14.2
@@ -10,12 +14,13 @@
 
 ## 0.14.1
 
-- Updated `bin/setup.dart` to use Gradle instead of Maven for building Java sources. Added gradle executables 
-  and bootstrap jars [#2003](https://github.com/dart-lang/native/issues/2003)
-- Added `JObject.isInstanceOf` which checks whether a `JObject` is an instance 
+- Updated `bin/setup.dart` to use Gradle instead of Maven for building Java
+  sources. Added gradle executables and bootstrap jars
+  [#2003](https://github.com/dart-lang/native/issues/2003)
+- Added `JObject.isInstanceOf` which checks whether a `JObject` is an instance
   of a java class.
-- Fixed a [bug](https://github.com/dart-lang/native/issues/1908) where
-  Java interfaces implemented in on the main thread in Dart could deadlock when
+- Fixed a [bug](https://github.com/dart-lang/native/issues/1908) where Java
+  interfaces implemented in on the main thread in Dart could deadlock when
   invoked from the main thread outside the context of a Dart isolate.
 
 ## 0.14.0

pkgs/jni/android/src/main/java/com/github/dart_lang/jni/JniPlugin.java

@@ -10,45 +10,88 @@
 import io.flutter.embedding.engine.plugins.FlutterPlugin;
 import io.flutter.embedding.engine.plugins.activity.ActivityAware;
 import io.flutter.embedding.engine.plugins.activity.ActivityPluginBinding;
+import java.util.ArrayList;
+import java.util.List;
+import java.util.Objects;
+import java.util.concurrent.ConcurrentHashMap;
 
 public class JniPlugin implements FlutterPlugin, ActivityAware {
+  private static final ConcurrentHashMap<Long, JniPlugin> pluginMap = new ConcurrentHashMap<>();
+
+  private long engineId;
+  private Context context;
+  private Activity activity;
+  private final List<ActivityListener> activityListeners = new ArrayList<>();
+
+  public static @NonNull Context getApplicationContext(long engineId) {
+    return Objects.requireNonNull(pluginMap.get(engineId)).context;
+  }
+
+  public interface ActivityListener {
+    void onActivityChanged(Activity activity);
+  }
+
+  public static void addActivityListener(long engineId, @NonNull ActivityListener listener) {
+    var plugin = Objects.requireNonNull(pluginMap.get(engineId));
+    plugin.activityListeners.add(listener);
+    listener.onActivityChanged(plugin.activity);
+  }
+
+  public static void removeActivityListener(long engineId, @NonNull ActivityListener listener) {
+    var plugin = Objects.requireNonNull(pluginMap.get(engineId));
+    plugin.activityListeners.remove(listener);
+  }
 
   @Override
   public void onAttachedToEngine(@NonNull FlutterPluginBinding binding) {
-    setup(binding.getApplicationContext());
+    //noinspection deprecation
+    engineId = binding.getFlutterEngine().getEngineId();
+    context = binding.getApplicationContext();
+    pluginMap.put(engineId, this);
   }
 
-  private void setup(Context context) {
-    initializeJni(context, getClass().getClassLoader());
+  @Override
+  public void onDetachedFromEngine(@NonNull FlutterPluginBinding binding) {
+    context = null;
+    activity = null;
+    pluginMap.remove(engineId);
   }
 
-  @Override
-  public void onDetachedFromEngine(@NonNull FlutterPluginBinding binding) {}
+  private void notifyActivityListeners() {
+    for (var listener : activityListeners) {
+      listener.onActivityChanged(activity);
+    }
+  }
+
+  private void setActivity(Activity newActivity) {
+    activity = newActivity;
+    notifyActivityListeners();
+  }
 
-  // Activity handling methods
   @Override
   public void onAttachedToActivity(@NonNull ActivityPluginBinding binding) {
-    Activity activity = binding.getActivity();
-    setJniActivity(activity, activity.getApplicationContext());
+    setActivity(binding.getActivity());
   }
 
   @Override
-  public void onDetachedFromActivityForConfigChanges() {}
+  public void onDetachedFromActivityForConfigChanges() {
+    setActivity(null);
+  }
 
   @Override
   public void onReattachedToActivityForConfigChanges(@NonNull ActivityPluginBinding binding) {
-    Activity activity = binding.getActivity();
-    setJniActivity(activity, activity.getApplicationContext());
+    setActivity(binding.getActivity());
   }
 
   @Override
-  public void onDetachedFromActivity() {}
-
-  native void initializeJni(Context context, ClassLoader classLoader);
+  public void onDetachedFromActivity() {
+    setActivity(null);
+  }
 
-  native void setJniActivity(Activity activity, Context context);
+  static native void setClassLoader(ClassLoader classLoader);
 
   static {
     System.loadLibrary("dartjni");
+    setClassLoader(JniPlugin.class.getClassLoader());
   }
 }

pkgs/jni/example/lib/main.dart

@@ -6,6 +6,7 @@
 
 import 'dart:ffi';
 import 'dart:io';
+import 'dart:ui';
 
 import 'package:ffi/ffi.dart';
 import 'package:flutter/material.dart';
@@ -73,12 +74,13 @@ String backAndForth() {
   return dartString;
 }
 
-void quit() {
-  JObject.fromReference(Jni.getCurrentActivity()).use((ac) =>
-      ac.jClass.instanceMethodId("finish", "()V").call(ac, jvoid.type, []));
+void quit(JObject activity) {
+  activity.jClass
+      .instanceMethodId("finish", "()V")
+      .call(activity, jvoid.type, []);
 }
 
-void showToast(String text) {
+void showToast(String text, JObject activity) {
   // This is example for calling your app's custom java code.
   // Place the Toaster class in the app's android/ source Folder, with a Keep
   // annotation or appropriate proguard rules to retain classes in release mode.
@@ -93,9 +95,9 @@ void showToast(String text) {
       '(Landroid/app/Activity;Landroid/content/Context;'
           'Ljava/lang/CharSequence;I)'
           'Lcom/github/dart_lang/jni_example/Toaster;');
-  final toaster = makeText.call(toasterClass, JObject.type, [
-    Jni.getCurrentActivity(),
-    Jni.getCachedApplicationContext(),
+  final toaster = makeText(toasterClass, JObject.type, [
+    activity,
+    Jni.androidApplicationContext(PlatformDispatcher.instance.engineId!),
     '😀'.toJString(),
     0,
   ]);
@@ -104,19 +106,20 @@ void showToast(String text) {
 }
 
 void main() {
+  WidgetsFlutterBinding.ensureInitialized();
   if (!Platform.isAndroid) {
     Jni.spawn();
   }
   final examples = [
-    Example("String.valueOf(1332)", () => toJavaStringUsingEnv(1332)),
-    Example("Generate random number", () => randomUsingEnv(180),
+    Example("String.valueOf(1332)", (_) => toJavaStringUsingEnv(1332)),
+    Example("Generate random number", (_) => randomUsingEnv(180),
         runInitially: false),
-    Example("Math.random()", () => randomDouble(), runInitially: false),
+    Example("Math.random()", (_) => randomDouble(), runInitially: false),
     if (Platform.isAndroid) ...[
       Example("Minutes of usage since reboot",
-          () => (uptime() / (60 * 1000)).floor()),
-      Example("Back and forth string conversion", () => backAndForth()),
-      Example("Device name", () {
+          (_) => (uptime() / (60 * 1000)).floor()),
+      Example("Back and forth string conversion", (_) => backAndForth()),
+      Example("Device name", (_) {
         final buildClass = JClass.forName("android/os/Build");
         return buildClass
             .staticFieldId("DEVICE", JString.type.signature)
@@ -125,13 +128,15 @@ void main() {
       }),
       Example(
         "Package name",
-        () => JObject.fromReference(Jni.getCurrentActivity()).use((activity) =>
-            activity.jClass
-                .instanceMethodId("getPackageName", "()Ljava/lang/String;")
-                .call(activity, JString.type, [])),
+        (activity) => activity.jClass
+            .instanceMethodId("getPackageName", "()Ljava/lang/String;")
+            .call(activity, JString.type, []),
+      ),
+      Example(
+        "Show toast",
+        (activity) => showToast("Hello from JNI!", activity),
+        runInitially: false,
       ),
-      Example("Show toast", () => showToast("Hello from JNI!"),
-          runInitially: false),
       Example(
         "Quit",
         quit,
@@ -144,7 +149,7 @@ void main() {
 
 class Example {
   String title;
-  dynamic Function() callback;
+  dynamic Function(JObject activity) callback;
   bool runInitially;
   Example(this.title, this.callback, {this.runInitially = true});
 }
@@ -158,8 +163,12 @@ class MyApp extends StatefulWidget {
 }
 
 class _MyAppState extends State<MyApp> {
+  late Stream<JObject?> activityStream;
+
   @override
   void initState() {
+    activityStream =
+        Jni.androidActivities(PlatformDispatcher.instance.engineId!);
     super.initState();
   }
 
@@ -170,20 +179,29 @@ class _MyAppState extends State<MyApp> {
         appBar: AppBar(
           title: const Text('JNI Examples'),
         ),
-        body: ListView.builder(
-            itemCount: widget.examples.length,
-            itemBuilder: (context, i) {
-              final eg = widget.examples[i];
-              return ExampleCard(eg);
+        body: StreamBuilder(
+            stream: activityStream,
+            builder: (_, snapshot) {
+              if (!snapshot.hasData || snapshot.data == null) {
+                return Container();
+              }
+              return ListView.builder(
+                itemCount: widget.examples.length,
+                itemBuilder: (context, i) {
+                  final eg = widget.examples[i];
+                  return ExampleCard(eg, snapshot.data!);
+                },
+              );
             }),
       ),
     );
   }
 }
 
 class ExampleCard extends StatefulWidget {
-  const ExampleCard(this.example, {super.key});
+  const ExampleCard(this.example, this.activity, {super.key});
   final Example example;
+  final JObject activity;
 
   @override
   _ExampleCardState createState() => _ExampleCardState();
@@ -210,7 +228,7 @@ class _ExampleCardState extends State<ExampleCard> {
     var hasError = false;
     if (_run) {
       try {
-        result = eg.callback().toString();
+        result = eg.callback(widget.activity).toString();
       } on Exception catch (e) {
         hasError = true;
         result = e.toString();

pkgs/jni/lib/src/jni.dart

@@ -2,6 +2,7 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+import 'dart:async';
 import 'dart:ffi';
 import 'dart:io';
 import 'dart:isolate';
@@ -14,6 +15,7 @@ import 'accessors.dart';
 import 'errors.dart';
 import 'jobject.dart';
 import 'jreference.dart';
+import 'plugin/generated_plugin.dart';
 import 'third_party/generated_bindings.dart';
 import 'types.dart';
 
@@ -211,22 +213,37 @@ abstract final class Jni {
   /// any thread, and always returns global object references.
   static final env = GlobalJniEnv(_fetchGlobalEnv());
 
-  /// Returns current application context on Android.
-  static JReference getCachedApplicationContext() {
-    return JGlobalReference(_bindings.GetApplicationContext());
+  /// Android application context.
+  ///
+  /// Pass `PlatformDispatcher.instance.engineId` to the [engineId] field.
+  static JObject androidApplicationContext(int engineId) {
+    return JniPlugin.getApplicationContext(engineId);
   }
 
-  /// Returns current activity.
-  static JReference getCurrentActivity() =>
-      JGlobalReference(_bindings.GetCurrentActivity());
-
-  /// Get the initial classLoader of the application.
+  /// A stream of Android activities.
   ///
-  /// This is especially useful on Android, where
-  /// JNI threads cannot access application classes using
-  /// the usual `JniEnv.FindClass` method.
-  static JReference getApplicationClassLoader() =>
-      JGlobalReference(_bindings.GetClassLoader());
+  /// Pass `PlatformDispatcher.instance.engineId` to the [engineId] field.
+  static Stream<JObject?> androidActivities(int engineId) {
+    late final StreamController<JObject?> androidActivitiesController;
+    final activityListener = JniPlugin$ActivityListener.implement(
+      $JniPlugin$ActivityListener(
+        onActivityChanged: (activity) {
+          androidActivitiesController.add(activity);
+        },
+      ),
+    );
+    androidActivitiesController = StreamController(
+      sync: true,
+      onListen: () {
+        JniPlugin.addActivityListener(engineId, activityListener);
+      },
+      onCancel: () {
+        JniPlugin.removeActivityListener(engineId, activityListener);
+        activityListener.release();
+      },
+    );
+    return androidActivitiesController.stream;
+  }
 }
 
 /// Extensions for use by `jnigen` generated code.