Implement Audio Focus Manager for desktop.

content/browser/media/session/audio_focus_manager.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_AUDIO_FOCUS_MANAGER_H_
+#define CONTENT_BROWSER_MEDIA_SESSION_AUDIO_FOCUS_MANAGER_H_
+
+#include <unordered_map>
+
+#include "base/memory/singleton.h"
+#include "content/common/content_export.h"
+#include "content/public/browser/web_contents_observer.h"
+
+namespace content {
+
+class MediaSession;
+class WebContents;
+
+class CONTENT_EXPORT AudioFocusManager {
+ public:
+  enum class AudioFocusType {

[Zhiqiang Zhang (Slow), 2016/07/07 16:04:25]

Maybe we can merge this with MediaSession::Type? I can do it in my CL
(https://codereview.chromium.org/1996043002/), depending on which lands first.

[mlamouri (slow - plz ping), 2016/07/08 13:20:47]

I don't think it would make sense for now. In general, I would prefer to have
AudioFocusManager to be fairly independent from MediaSession. Maybe MediaSession
will not try to get AudioFocus in some cases for example.

+    Gain,
+    GainTransientMayDuck,
+  };
+
+  // Returns Chromium's internal AudioFocusManager.
+  static AudioFocusManager* GetInstance();
+
+  void RequestAudioFocus(MediaSession* media_session, AudioFocusType type);
+
+  void AbandonAudioFocus(MediaSession* media_session);
+
+ private:
+  friend struct base::DefaultSingletonTraits<AudioFocusManager>;
+  friend class AudioFocusManagerTest;
+
+  // TODO(mlamouri): in order to allow multiple MediaSession per WebContents, we
+  // will have to keep track of MediaSession's. Though, we can easily keep track
+  // of WebContents' life time right now but not MediaSession's.
+  class AudioFocusEntry : public WebContentsObserver {

[Zhiqiang Zhang (Slow), 2016/07/07 16:04:25]

As I use WebContentsObserver in the Pepper CL, one of the reviewers suggest me
to avoid inheriting from WebContentsObserver if possible, since it will add
overhead to all WebContents events.

[mlamouri (slow - plz ping), 2016/07/08 13:20:47]

This is on-demand so it's probably not that bad. Though, I'm open to alternative
ideas. Were you thinking of something in particular?

+   public:
+    AudioFocusEntry(WebContents* web_contents,
+                    AudioFocusManager* audio_focus_manager,
+                    AudioFocusType type);
+
+    AudioFocusType type() const;
+
+   private:
+    // WebContentsObserver implementation.
+    void WebContentsDestroyed() override;
+
+    AudioFocusManager* audio_focus_manager_;  // Owns |this|.
+    AudioFocusType type_;
+
+    DISALLOW_COPY_AND_ASSIGN(AudioFocusEntry);
+  };
+
+  AudioFocusManager();
+  ~AudioFocusManager();
+
+  void OnWebContentsDestroyed(WebContents* web_contents);
+
+  // This method is meant to be called when a new session is of type
+  // GainTransientMayDuck. If it is the first one, other clients will be asked
+  // to duck.
+  void MaybeStartDucking() const;
+
+  // This method is meant to be called when a session is no longer of type
+  // GainTransientMayDuck. If it was the last one, other clients will be asked
+  // to no longer duck.
+  void MaybeStopDucking() const;
+
+  // Returns how many sessions require current audio focused session to duck.
+  int TransientMayDuckEntriesCount() const;
+
+  // Internal method to request audio focus of type AudioFocusType::Gain.
+  void RequestAudioFocusGain(WebContents* web_contents);
+
+  // Removes the entry associated with |web_contents| from the
+  // |transient_entries_| if there is one.
+  void MaybeRemoveTransientEntry(WebContents* web_contents);
+
+  // Removes the focused session if it is associated with |web_contents|.
+  void MaybeRemoveFocusEntry(WebContents* web_contents);
+
+  std::unordered_map<WebContents*, std::unique_ptr<AudioFocusEntry>>
+      transient_entries_;
+  std::unique_ptr<AudioFocusEntry> focus_entry_;

[Zhiqiang Zhang (Slow), 2016/07/07 16:04:25]

Maybe we should keep all MediaSession entries? Pepper instances in a suspended
MediaSession needs a signal to unduck.

[mlamouri (slow - plz ping), 2016/07/08 13:20:47]

Let's deal with Flash in a follow-up :) We need to find out how to implement the
different possible solutions too :)

+};
+
+}  // namespace content
+
+#endif  // CONTENT_BROWSER_MEDIA_SESSION_AUDIO_FOCUS_MANAGER_H_