Bug 1627999 - part1 : manage the audio focus in `MediaPlaybackStatus` r=bryce

This patch will do :
- introduce a concept `audio focus` among different contexts within a tab
- determine the audio focus owner when the context becomes audible or the owner destroys

The advantage of doing so :
- the audio focus helps us to decide the active media session that would be implemented in the following part

More details:
When there are serveral contexts playing at the same time within a tab, we would like to determine an audible context from them to represent the tab, and that is the `audio focus` we mean in this bug.

Differential Revision: https://phabricator.services.mozilla.com/D72495

dom/media/mediacontrol/MediaPlaybackStatus.cpp

@@ -46,6 +46,11 @@ void MediaPlaybackStatus::DestroyContextInfo(uint64_t aContextId) {
   MOZ_ASSERT(NS_IsMainThread());
   LOG("Remove context %" PRIu64, aContextId);
   mContextInfoMap.Remove(aContextId);
+  // If the removed context is owning the audio focus, we would find another
+  // context to take the audio focus if it's possible.
+  if (IsContextOwningAudioFocus(aContextId)) {
+    ChooseNewContextToOwnAudioFocus();
+  }
 }
 
 void MediaPlaybackStatus::UpdateMediaAudibleState(uint64_t aContextId,
@@ -60,6 +65,11 @@ void MediaPlaybackStatus::UpdateMediaAudibleState(uint64_t aContextId,
     MOZ_ASSERT(aState == MediaAudibleState::eInaudible);
     info.DecreaseAudibleMediaNum();
   }
+  if (ShouldRequestAudioFocusForInfo(info)) {
+    SetOwningAudioFocusContextId(Some(aContextId));
+  } else if (ShouldAbandonAudioFocusForInfo(info)) {
+    ChooseNewContextToOwnAudioFocus();
+  }
 }
 
 bool MediaPlaybackStatus::IsPlaying() const {
@@ -101,5 +111,46 @@ MediaPlaybackStatus::GetNotNullContextInfo(uint64_t aContextId) {
   return *(mContextInfoMap.GetValue(aContextId)->get());
 }
 
+Maybe<uint64_t> MediaPlaybackStatus::GetAudioFocusOwnerContextId() const {
+  return mOwningAudioFocusContextId;
+}
+
+void MediaPlaybackStatus::ChooseNewContextToOwnAudioFocus() {
+  for (auto iter = mContextInfoMap.ConstIter(); !iter.Done(); iter.Next()) {
+    if (iter.Data()->IsAudible()) {
+      SetOwningAudioFocusContextId(Some(iter.Data()->Id()));
+      return;
+    }
+  }
+  // No context is audible, so no one should the own audio focus.
+  SetOwningAudioFocusContextId(Nothing());
+}
+
+void MediaPlaybackStatus::SetOwningAudioFocusContextId(
+    Maybe<uint64_t>&& aContextId) {
+  if (mOwningAudioFocusContextId == aContextId) {
+    return;
+  }
+  mOwningAudioFocusContextId = aContextId;
+}
+
+bool MediaPlaybackStatus::ShouldRequestAudioFocusForInfo(
+    const ContextMediaInfo& aInfo) const {
+  return aInfo.IsAudible() && !IsContextOwningAudioFocus(aInfo.Id());
+}
+
+bool MediaPlaybackStatus::ShouldAbandonAudioFocusForInfo(
+    const ContextMediaInfo& aInfo) const {
+  // The owner becomes inaudible and there is other context still playing, so we
+  // should switch the audio focus to the audible context.
+  return !aInfo.IsAudible() && IsContextOwningAudioFocus(aInfo.Id()) &&
+         IsAudible();
+}
+
+bool MediaPlaybackStatus::IsContextOwningAudioFocus(uint64_t aContextId) const {
+  return mOwningAudioFocusContextId ? *mOwningAudioFocusContextId == aContextId
+                                    : false;
+}
+
 }  // namespace dom
 }  // namespace mozilla

dom/media/mediacontrol/MediaPlaybackStatus.h

@@ -27,6 +27,13 @@ namespace dom {
  *
  * Use `UpdateMediaXXXState()` to update controlled media status, and use
  * `IsXXX()` methods to acquire the playback status of the tab.
+ *
+ * As we know each context's audible state, we can decide which context should
+ * owns the audio focus when multiple contexts are all playing audible media at
+ * the same time. In that cases, the latest context that plays media would own
+ * the audio focus. When the context owning the audio focus is destroyed, we
+ * would see if there is another other context still playing audible media, and
+ * switch the audio focus to another context.
  */
 class MediaPlaybackStatus final {
  public:
@@ -37,6 +44,8 @@ class MediaPlaybackStatus final {
   bool IsAudible() const;
   bool IsAnyMediaBeingControlled() const;
 
+  Maybe<uint64_t> GetAudioFocusOwnerContextId() const;
+
  private:
   /**
    * This internal class stores detailed media status of controlled media for
@@ -90,8 +99,15 @@ class MediaPlaybackStatus final {
   ContextMediaInfo& GetNotNullContextInfo(uint64_t aContextId);
   void DestroyContextInfo(uint64_t aContextId);
 
+  void ChooseNewContextToOwnAudioFocus();
+  void SetOwningAudioFocusContextId(Maybe<uint64_t>&& aContextId);
+  bool IsContextOwningAudioFocus(uint64_t aContextId) const;
+  bool ShouldRequestAudioFocusForInfo(const ContextMediaInfo& aInfo) const;
+  bool ShouldAbandonAudioFocusForInfo(const ContextMediaInfo& aInfo) const;
+
   // This contains all the media status of browsing contexts within a tab.
   nsDataHashtable<nsUint64HashKey, UniquePtr<ContextMediaInfo>> mContextInfoMap;
+  Maybe<uint64_t> mOwningAudioFocusContextId;
 };
 
 }  // namespace dom