Link MediaSessionTabHelper with native MediaSession [CL is going to be split]

content/browser/media/session/media_session_delegate_android.h

+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef CONTENT_BROWSER_MEDIA_SESSION_MEDIA_SESSION_DELEGATE_ANDROID_H_
+#define CONTENT_BROWSER_MEDIA_SESSION_MEDIA_SESSION_DELEGATE_ANDROID_H_
+
+#include <jni.h>
+
+#include "base/android/scoped_java_ref.h"
+#include "content/browser/media/session/media_session_observer.h"
+
+namespace content {
+
+class MediaSessionDelegateAndroid : public MediaSessionObserver {
+ public:
+
+  static bool Register(JNIEnv* env);
+
+  static void CreateAndRegister(
+      JNIEnv* env,
+      MediaSession* media_session,
+      const base::android::JavaParamRef<jobject>& j_delegate);
+
+  ~MediaSessionDelegateAndroid() override;
+
+  // MediaSessionObserver implementations
+  void MediaSessionDisconnected() override;
+  void MediaSessionStateChanged(bool is_controllable, bool is_suspended) override;
+  void MediaSessionMetadataChanged(const base::Optional<MediaMetadata>& metadata) override;
+
+  void ResumeMediaSession(JNIEnv* env, const base::android::JavaParamRef<jobject>& obj);
+  void SuspendMediaSession(JNIEnv* env, const base::android::JavaParamRef<jobject>& obj);
+  void StopMediaSession(JNIEnv* env, const base::android::JavaParamRef<jobject>& obj);
+  void UnlinkMediaSession(JNIEnv* env, const base::android::JavaParamRef<jobject>& obj);
+
+ private:
+  MediaSessionDelegateAndroid(
+      MediaSession* media_session,
+      const base::android::JavaParamRef<jobject>& j_delegate);
+
+  base::android::ScopedJavaGlobalRef<jobject> j_delegate_;

[boliu, 2016/10/19 20:39:40]

this is content, so need to be safe for android webview as well, where strong
references from native to java *may* lead to memory leaks because webview relies
on java gc to destroy native side. I know this doesn't run on webview same
argument goes in case someone does start using this in webview in the future.

so in this case, native "owns" the java object here. is there a guarantee when
exactly the java side is released, from inside content code? if not, maybe we
should use a weak ref and add null checks instead

[Zhiqiang Zhang (Slow), 2016/10/20 16:22:32]

You are right. For our case, we don't need strong reference from native to Java.
If the the reference count in Java is 0, it should be GCed.

Using JavaObjectWeakGlobalRef now.

[whywhat, 2016/10/21 18:31:02]

Would a guarantee like "when WebContents is destroyed, the GlobalRef and all
Java references to the Java object will be removed so the object will be GCed"
work?

[boliu, 2016/10/21 19:56:02]

No, because WebContents is destroyed by gc, so if any of these java references
accidentally prevents WebContents (or that whole chain of objects) from gc-ed,
then things will leak.

The argument that works for content is that the references has to be dropped
when onDetachedFromWindow is called.

+
+  DISALLOW_COPY_AND_ASSIGN(MediaSessionDelegateAndroid);
+};
+
+}  // namespace content
+
+#endif  // CONTENT_BROWSER_MEDIA_SESSION_MEDIA_SESSION_DELEGATE_ANDROID_H_