Reland "Letting Flash join MediaSession (stack implementaion)"

docs/audio_focus.md

+# Audio Focus Handling
+
+A MediaSession collects all audio-producing objects in one tab. It is usually
+unpleasant when multiple MediaSessions play sound at the same time. Audio focus
+handling manages the MediaSessions and mixes them in proper ways. This is part
+of the default media session on desktop project.
+
+[TOC]
+
+## Processing model
+
+### Audio focus types
+
+There are "persistent" and "transient" audio focus types.
+
+* Persistent audios are used for long media playback, and they should not mix
+  with each other. When they start to play, they should pause all other
+  playbacks.
+* Transient audios are used for short media playback such as a ping for incoming
+  message. When they start to play, they should play on top of other playbacks
+  and the other playbacks should duck (have reduced volume).
+
+### `MediaSession`
+
+Audio-producing objects should join `MediaSession` when they want to produce
+sound. `MediaSession` has the following states:
+
+* ACTIVE: the `MediaSession` has audio focus and its audio-producing objects can
+  play.
+* SUSPENDED: the MediaSession does not have audio focus. All audio-producing
+  objects are paused and can be resumed when the session gains audio focus.
+* INACTIVE: the MediaSession does not have audio focus, and there is no
+  audio-producing objects in this `MediaSession`.
+
+Besides, `MediaSession` has a `DUCKING` flag, which means its managed
+audio-producing objects has lowered volume. The flag is orthogonal with
+`MediaSession` state.
+
+### `AudioFocusManager`
+
+`AudioFocusManager` is a global instance which manages the state of
+`MediaSession`s. It is used for platforms (e.g. Android) that do not have a
+system audio focus.
+
+When an audio-producing object wants to play audio, it should join `MediaSession`
+and tell which kind of audio focus type it requires. `MediaSession` will then
+request audio focus from `AudioFocusManager`, and will allow the object to play
+sound if successful. `AudioFocusManager` will notify other `MediaSession`s if
+their states are changed.
+
+When an audio-producing object stops playing audio, it should be removed from
+its `MediaSession`, and `MediaSession` should abandon its audio focus if its
+audio-producing objects is empty. `AudioFocusManager` will notify other
+`MediaSession`s of state change if necessary.
+
+## The algorithm for handling audio focus
+
+`AudioFocusManager` uses a stack implementation. It keeps track of all
+ACTIVE/SUSPENDED `MediaSession`s. When a `MediaSession` requests audio focus, it
+will be put at the top of the stack, and will be removed from the stack when it
+abandons audio focus.
+
+The algorithm is as follows:
+
+* When a `MediaSession` requests audio focus:
+
+  * Remove it from the audio focus stack if it's already there, and place it at
+    the top of audio focus stack, grant focus to the session and let it play.
+  * If the session is persistent, suspend all the other sessions on the stack.
+  * If the session is transient, we should duck any active persistent audio
+    focus entry if present:
+
+    * If the next top entry is transient, do nothing, since if there is any
+      persistent session that is active, it is already ducking.
+    * If the next top entry is persistent, let the next top entry start ducking,
+      since it is the only active persisten session.
+
+* When a `MediaSession` abandons audio focus:
+
+  * If the session is not on the top, just remove it from the stack.
+  * If the session is on the top, remove it from the stack.
+
+    * If the stack becomes empty, do nothing.
+    * If the next top session is transient, do nothing.
+    * If the next top session is persistent, stop ducking it.
+
+### Handling Pepper
+
+Pepper is different from media elements since it has a different model. Pepper
+cannot be paused, but its volume can be changed. When considering Pepper, the
+above algorithm must be modified.
+
+When Pepper joins `MediaSession`, it should request persistent focus type. When
+AudioFocusManager wants to suspend a `MediaSession`, it must check whether the
+session has Pepper instance, and if yes, it should duck the session instead.
+
+Also, whenever a session abandons focus, and the next top session is INACTIVE,
+`AudioFocusManager` should find the next session having Pepper and unduck it.