Add MediaSessionObserver to allow clients observe MediaSession directly

content/public/browser/media_session_observer.h

Index: content/public/browser/media_session_observer.h
diff --git a/content/public/browser/media_session_observer.h b/content/public/browser/media_session_observer.h
new file mode 100644
index 0000000000000000000000000000000000000000..be266754320256ff3251c8093be83a9f1c5ab4df
--- /dev/null
+++ b/content/public/browser/media_session_observer.h
@@ -0,0 +1,52 @@
+// 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_PUBLIC_BROWSER_MEDIA_SESSION_OBSERVER_H_
+#define CONTENT_PUBLIC_BROWSER_MEDIA_SESSION_OBSERVER_H_
+
+#include "base/macros.h"
+#include "base/optional.h"
+#include "content/common/content_export.h"
+#include "content/public/common/media_metadata.h"
+
+namespace content {
+
+class MediaSession;
+class MediaSessionImpl;
+
+// The observer for observing MediaSession events.
+class CONTENT_EXPORT MediaSessionObserver {
+ public:
+  // Gets the observed MediaSession.
+  MediaSession* media_session() const;
+
+  // Called when the observed MediaSession is destroyed.
+  virtual void MediaSessionDestroyed() {}

[whywhat, 2016/10/24 20:29:39]

This should update session_.
IMHO, it'd be more convenient for subclasses if MSO set session_ to nullptr and
then called a virtual method that subclasses would override vs subclasses
remembering to call the base class method, but I'll leave it up to you.

[Zhiqiang Zhang (Slow), 2016/10/25 19:51:43]

Maybe we should do reset after the method is called. WebContentsObserver also
has an empty body, but WebContents will do the cleanup after notifying all
observers about WebContentsDestroyed(). Will do it later.

+
+  // Called when the observed MediaSession has changed its state.
+  virtual void MediaSessionStateChanged(bool is_controllable,
+                                        bool is_suspended) {}
+
+  // Called when the observed MediaSession has changed metadata.
+  virtual void MediaSessionMetadataChanged(
+      const base::Optional<MediaMetadata>& metadata) {}
+
+ protected:
+  // Create a MediaSessionObserver and start observing |session|.
+  explicit MediaSessionObserver(MediaSession* session);
+
+  // Destructor of MediaSessionObserver. It will remove itself from |session| if
+  // the observer is still observing a MediaSession.
+  virtual ~MediaSessionObserver();
+
+ private:
+  // Weak pointer to MediaSession.
+  MediaSessionImpl* session_;
+
+  DISALLOW_COPY_AND_ASSIGN(MediaSessionObserver);
+};
+
+}  // namespace content
+
+#endif  // CONTENT_PUBLIC_BROWSER_MEDIA_SESSION_OBSERVER_H_