Implement Audio Focus Manager for desktop.

content/browser/media/session/audio_focus_manager.cc

+// 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.
+
+#include "content/browser/media/session/audio_focus_manager.h"
+
+#include "base/optional.h"
+#include "content/browser/media/session/media_session.h"
+#include "content/public/browser/web_contents.h"
+
+namespace content {
+
+namespace {
+
+const double kDuckingVolumeMultiplier = 0.2;
+const double kDefaultVolumeMultiplier = 1.0;
+
+}  // anonymous namespace
+
+AudioFocusManager::AudioFocusEntry::AudioFocusEntry(WebContents* web_contents, AudioFocusManager* audio_focus_manager, AudioFocusType type)
+    : WebContentsObserver(web_contents),
+      audio_focus_manager_(audio_focus_manager) {

[Zhiqiang Zhang (Slow), 2016/05/11 15:57:32]

You forgot type_(type) here :)

+}
+
+AudioFocusManager::AudioFocusType AudioFocusManager::AudioFocusEntry::type() const {
+  return type_;
+}
+
+void AudioFocusManager::AudioFocusEntry::set_type(AudioFocusType type) {
+  type_ = type;
+}
+
+void AudioFocusManager::AudioFocusEntry::WebContentsDestroyed() {
+  audio_focus_manager_->OnWebContentsDestroyed(web_contents());
+  // |this| will be destroyed now.
+}
+
+// static
+AudioFocusManager* AudioFocusManager::GetInstance() {
+  return base::Singleton<AudioFocusManager>::get();
+}
+
+void AudioFocusManager::RequestAudioFocus(
+    MediaSession* media_session, AudioFocusType type) {
+  WebContents* web_contents = media_session->web_contents();
+  base::Optional<AudioFocusType> previous_type;
+
+  auto it = entries_.find(web_contents);
+  if (it != entries_.end()) {
+    previous_type = it->second->type();
+    it->second->set_type(type);
+  } else {
+    entries_[web_contents] = std::unique_ptr<AudioFocusEntry>(
+        new AudioFocusEntry(web_contents, this, type));
+  }
+
+  if (previous_type && previous_type.value() != type) {
+    switch (previous_type.value()) {
+      case AudioFocusType::GainTransientMayDuck:
+        MaybeStopDucking();
+        break;
+      case AudioFocusType::Gain:
+        RemoveFromFocusStack(web_contents);
+        break;
+    }
+  }
+
+  if (type == AudioFocusType::GainTransientMayDuck) {
+    MaybeStartDucking();
+    return;
+  }
+
+  DCHECK(type == AudioFocusType::Gain);
+
+  if (!focus_stack_.empty()) {
+    // Already on top, it's a no-op.
+    if (*focus_stack_.begin() == web_contents)
+      return;
+
+    MediaSession* other_session = MediaSession::Get(*focus_stack_.begin());
+    if (other_session->IsActive())
+      other_session->Suspend(MediaSession::SuspendType::SYSTEM);
+
+    RemoveFromFocusStack(web_contents);
+  }
+
+  focus_stack_.push_front(web_contents);
+}
+
+void AudioFocusManager::AbandonAudioFocus(MediaSession* media_session) {
+  WebContents* web_contents = media_session->web_contents();
+
+  // We do not expect to receive an AbandonAudioFocus call for a session that
+  // didn't ask for audio focus at some point.
+  auto it = entries_.find(web_contents);
+  DCHECK(it != entries_.end());
+
+  AudioFocusType type = it->second->type();
+  entries_.erase(web_contents);
+
+  if (type == AudioFocusType::GainTransientMayDuck) {
+    MaybeStopDucking();
+    return;
+  }
+
+  DCHECK(type == AudioFocusType::Gain);
+
+  if (focus_stack_.empty())
+    return;
+
+  // Something that doesn't have focus is closing.
+  if (*focus_stack_.begin() != web_contents) {
+    RemoveFromFocusStack(web_contents);
+    return;
+  }
+
+  focus_stack_.erase(focus_stack_.begin());
+  if (!focus_stack_.empty())
+    MediaSession::Get(*focus_stack_.begin())->Resume(MediaSession::SuspendType::SYSTEM);

[Zhiqiang Zhang (Slow), 2016/05/11 15:57:32]

On Android, we don't resume other sessions when abandoning audio focus of type
GAIN. Actually, I think the stack should be cleared for requesting focus of GAIN
type.

[Zhiqiang Zhang (Slow), 2016/05/20 14:14:43]

Sorry, no need to clear the stack here.
Our current behavior is that MediaSession abandons the audio focus when losing
focus.

+}
+
+AudioFocusManager::AudioFocusManager() = default;
+
+AudioFocusManager::~AudioFocusManager() = default;
+
+void AudioFocusManager::OnWebContentsDestroyed(WebContents* web_contents) {
+  entries_.erase(web_contents);
+  RemoveFromFocusStack(web_contents);
+}
+
+void AudioFocusManager::MaybeStartDucking() const {
+  if (TransientMayDuckEntriesCount() != 1)
+    return;
+
+  // TODO(mlamouri): add StartDuck to MediaSession.
+  for (const auto& it : focus_stack_)
+    MediaSession::Get(it)->SetVolumeMultiplier(kDuckingVolumeMultiplier);
+}
+
+void AudioFocusManager::MaybeStopDucking() const {
+  if (TransientMayDuckEntriesCount() != 0)
+    return;
+
+  // TODO(mlamouri): add StopDuck to MediaSession.
+  for (const auto& it : focus_stack_)
+    MediaSession::Get(it)->SetVolumeMultiplier(kDefaultVolumeMultiplier);
+}
+
+int AudioFocusManager::TransientMayDuckEntriesCount() const {
+  int count = 0;
+  for (const auto& it : entries_) {
+    if (it.second->type() != AudioFocusType::GainTransientMayDuck)
+      continue;
+    ++count;
+  }
+  return count;
+}
+
+void AudioFocusManager::RemoveFromFocusStack(WebContents* web_contents) {
+  focus_stack_.remove_if([web_contents](WebContents* wc) -> bool { return wc == web_contents; });
+}
+
+}  // namespace content