Add MediaSessionObserver to allow clients observe MediaSession directly

content/public/browser/media_session.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_PUBLIC_BROWSER_MEDIA_SESSION_H_
+#define CONTENT_PUBLIC_BROWSER_MEDIA_SESSION_H_
+
+#include "base/macros.h"
+#include "content/common/content_export.h"
+
+namespace content {
+
+class WebContents;
+
+// MediaSession manages the media session and audio focus for a given
+// WebContents. There is only one MediaSession per WebContents. The underlying
+// implementation will request audio focus when players want to play audio, and

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

nit: Don't think sharing the implementation details for an interface is
necessary.

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

Removed.

+// will respond to audio focus changes (such as another tab or app start to
+// produce audio).
+//
+// MediaSession allows clients to observe its changes via MediaSessionObserver,
+// and allow clients to resume/suspend/stop the managed players. MediaSession is

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

nit: s/allow/allows

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

Done.

+// also the interface for communicating with blink::MediaSession.

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

nit: same w/r/t sharing the implementation details

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

Done.

+class MediaSession {
+ public:
+  enum class SuspendType {
+    // Suspended by the system because a transient sound needs to be played.
+    SYSTEM,
+    // Suspended by the UI.
+    UI,
+    // Suspended by the page via script or user interaction.
+    CONTENT,
+  };
+
+  // Returns the MediaSession associated to this WebContents. Creates one if
+  // none is currently available.
+  CONTENT_EXPORT static MediaSession* Get(WebContents* contents);
+
+  virtual ~MediaSession() = default;
+
+  // Resume the media session.
+  // |type| represents the origin of the request.
+  virtual void Resume(SuspendType suspend_type) = 0;
+
+  // Resume the media session.
+  // |type| represents the origin of the request.
+  virtual void Suspend(SuspendType suspend_type) = 0;
+
+  // Resume the media session.
+  // |type| represents the origin of the request.
+  virtual void Stop(SuspendType suspend_type) = 0;
+
+ private:
+  // This interface should only be implemented inside content.
+  friend class MediaSessionImpl;
+  MediaSession() = default;
+};
+
+}  // namespace content
+
+#endif  // CONTENT_PUBLIC_BROWSER_MEDIA_SESSION_H_