Implement Audio Focus Manager for desktop.

content/browser/media/session/audio_focus_manager.h

Index: content/browser/media/session/audio_focus_manager.h
diff --git a/content/browser/media/session/audio_focus_manager.h b/content/browser/media/session/audio_focus_manager.h
new file mode 100644
index 0000000000000000000000000000000000000000..1fa9c307381507a908fba630ce327770881eadd9
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+++ b/content/browser/media/session/audio_focus_manager.h
@@ -0,0 +1,82 @@
+// 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 "base/memory/singleton.h"
+#include "content/public/browser/web_contents_observer.h"
+
+#include <list>
+#include <unordered_map>
+
+namespace content {
+
+class MediaSession;
+class WebContents;
+
+class AudioFocusManager {
+ public:
+  enum class AudioFocusType {
+    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>;
+
+  // 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 {
+   public:
+    AudioFocusEntry(WebContents* web_contents, AudioFocusManager* audio_focus_manager, AudioFocusType type);
+
+    AudioFocusType type() const;
+    void set_type(AudioFocusType type);
+
+   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 ducking.
+  int TransientMayDuckEntriesCount() const;
+
+  void RemoveFromFocusStack(WebContents* web_contents);
+
+  std::unordered_map<WebContents*, std::unique_ptr<AudioFocusEntry>> entries_;
+  std::list<WebContents*> focus_stack_;
+};
+
+}  // namespace content
+
+#endif  // CONTENT_BROWSER_MEDIA_SESSION_AUDIO_FOCUS_MANAGER_H_