docs/audio_focus.md - Issue 2274873003: Letting Flash join MediaSession (stack implementaion)

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Issue 2274873003: Letting Flash join MediaSession (stack implementaion) (Closed) Base URL: https://chromium.googlesource.com/chromium/src.git@media_session_type

Patch Set: added doc Created 4 years, 2 months ago

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	1 # Audio Focus Handling

	2

	3 Audio focus handling is part of the default media session on desktop, which

	4 manages the mixing of MediaSessions.
	whywhat 2016/09/27 17:54:17 Expand on what MediaSession is unless we have a do Expand on what MediaSession is unless we have a doc about that? E.g. all audio-producing media elements, WebAudio AudioContexts (?) and audible Pepper plugins from the same page form one MediaSession. Each page can only have one session until the MediaSession API allows them to form custom sets of audible objects? Zhiqiang Zhang (Slow) 2016/09/28 16:12:18 Done. Show quoted text On 2016/09/27 17:54:17, whywhat wrote: > Expand on what MediaSession is unless we have a doc about that? E.g. all > audio-producing media elements, WebAudio AudioContexts (?) and audible Pepper > plugins from the same page form one MediaSession. Each page can only have one > session until the MediaSession API allows them to form custom sets of audible > objects? Done.
	5

	6 [TOC]

	7

	8 ## Processing model

	9

	10 ### Audio focus types

	11

	12 There are "persistent" and "transient" audio focus types. Persistent audios

	13 should not mix with each other and transient audio should play on top of
	whywhat 2016/09/27 17:54:16 Audio playback with persistent audio focus stops o Audio playback with persistent audio focus stops other playbacks with persistent audio focus and ducks playbacks with transient audio focus. Playback with transient audio focus ducks all other playing media no matter what audio focus they have. MediaSession's with Pepper plugins (and/or WebAudio?) behave like they have transient audio focus since Pepper plugins are unstoppable! Zhiqiang Zhang (Slow) 2016/09/28 16:12:18 Elaborated a bit here. But we treat Pepper as Pers Show quoted text On 2016/09/27 17:54:16, whywhat wrote: > Audio playback with persistent audio focus stops other playbacks with persistent > audio focus and ducks playbacks with transient audio focus. Playback with > transient audio focus ducks all other playing media no matter what audio focus > they have. MediaSession's with Pepper plugins (and/or WebAudio?) behave like > they have transient audio focus since Pepper plugins are unstoppable! Elaborated a bit here. But we treat Pepper as Persistent. I have a section Pepper in the very end. WebAudio is currently not managed by MediaSession I think?
	14 persistent audio.

	15

	16 ### `MediaSession`

	17

	18 Audio-producing objects should join `MediaSession` when they want to produce

	19 sound. `MediaSession` has the following states:

	20

	21 * ACTIVE: the `MediaSession` has audio focus and should be play normally.
	whywhat 2016/09/27 17:54:16 s/be play/be played What does "normally" mean? Ma s/be play/be played What does "normally" mean? Maybe "has audio focus and is playing"? Zhiqiang Zhang (Slow) 2016/09/28 16:12:18 Done. Show quoted text On 2016/09/27 17:54:16, whywhat wrote: > s/be play/be played > > What does "normally" mean? Maybe "has audio focus and is playing"? Done.
	22 * SUSPENDED: the MediaSession does not has audio focus. All audio-producing
	whywhat 2016/09/27 17:54:16 s/has/have s/has/have Zhiqiang Zhang (Slow) 2016/09/28 16:12:18 Done. Show quoted text On 2016/09/27 17:54:16, whywhat wrote: > s/has/have Done.
	23 objects are paused and can be resumed when the session resumes.
	whywhat 2016/09/27 17:54:16 s/resumes/gains audio focus or is resumed by user? s/resumes/gains audio focus or is resumed by user? Zhiqiang Zhang (Slow) 2016/09/28 16:12:18 Done. Show quoted text On 2016/09/27 17:54:16, whywhat wrote: > s/resumes/gains audio focus or is resumed by user? Done. Zhiqiang Zhang (Slow) 2016/09/28 16:12:18 Done. Show quoted text On 2016/09/27 17:54:16, whywhat wrote: > s/resumes/gains audio focus or is resumed by user? Done.
	24 * INACTIVE: the MediaSession does not has audio focus, and there is no
	whywhat 2016/09/27 17:54:16 s/has/have s/has/have Zhiqiang Zhang (Slow) 2016/09/28 16:12:18 Done. Show quoted text On 2016/09/27 17:54:16, whywhat wrote: > s/has/have Done.
	25 audio-producing objects in this `MediaSession`.

	26

	27 Besides, `MediaSession` has a `DUCKING` flag, which means its managed

	28 audio-producing objects has lowered volume. The flag is orthogonal with

	29 `MediaSession` state.

	30

	31 ### `AudioFocusManager`

	32

	33 `AudioFocusManager` is a global instance which manages the state of

	34 `MediaSession`s.

	35

	36 When an audio-producing object wants to play audio, it should join `MediaSession `

	37 and tell which kind of audio focus type it requires. `MediaSession` will then

	38 request audio focus from `AudioFocusManager`, and will allow the object to play

	39 sound if successful. `AudioFocusManager` will notify other `MediaSession`s if

	40 their states are changed.

	41

	42 When an audio-producing object stops playing audio, it should be removed from

	43 its `MediaSession`, and `MediaSession` should abandon its audio focus if there

	44 are no audio-producing object is managed by it. `AudioFocusManager` will notify
	whywhat 2016/09/27 17:54:16 s/is// or maybe just say if it's empty? s/is// or maybe just say if it's empty? Zhiqiang Zhang (Slow) 2016/09/28 16:12:17 Done. Show quoted text On 2016/09/27 17:54:16, whywhat wrote: > s/is// or maybe just say if it's empty? Done.
	45 other `MediaSession`s of state change if necessary.

	46

	47 ## The algorithm for handling audio focus

	48

	49 `AudioFocusManager` uses a stack implementation. It keeps track of all

	50 ACTIVE/SUSPENDED `MediaSession`s. When a `MediaSession` requests audio focus, it

	51 will be put at the top of the stack, and will be removed from the stack when it

	52 abandons audio focus.

	53

	54 The algorithm is as follows:

	55

	56 * When a `MediaSession` requests audio focus:

	57

	58 * Remove it from the audio focus stack, and place it at the top of audio
	whywhat 2016/09/27 17:54:16 "Remove it from the audio focus stack if it's alre "Remove it from the audio focus stack if it's already there" otherwise it's unclear why you're removing MS from the stack before putting it in it. Zhiqiang Zhang (Slow) 2016/09/28 16:12:18 Done. Show quoted text On 2016/09/27 17:54:16, whywhat wrote: > "Remove it from the audio focus stack if it's already there" otherwise it's > unclear why you're removing MS from the stack before putting it in it. Done.
	59 focus stack, grant focus to the session and let it play.

	60 * If the session is persistent, suspend all sessions on the stack
	whywhat 2016/09/27 17:54:16 s/all/all the other s/all/all the other Zhiqiang Zhang (Slow) 2016/09/28 16:12:17 Done. Show quoted text On 2016/09/27 17:54:16, whywhat wrote: > s/all/all the other Done.
	61 * If the session is transient:

	62

	63 * If the next top entry is transient, do nothing.

	64 * If the next top entry is persistent, let the next top entry start ducking.
	whywhat 2016/09/27 17:54:16 Maybe clarify we took care of the remaining sessio Maybe clarify we took care of the remaining session when putting the previous one on top? Zhiqiang Zhang (Slow) 2016/09/28 16:12:18 Sorry, I'm not very sure of what you mean here. Ca Show quoted text On 2016/09/27 17:54:16, whywhat wrote: > Maybe clarify we took care of the remaining session when putting the previous > one on top? Sorry, I'm not very sure of what you mean here. Can you explain a bit more? whywhat 2016/09/29 22:40:47 I meant, clarify why we only duck the top session, Show quoted text On 2016/09/28 at 16:12:18, Zhiqiang Zhang wrote: > On 2016/09/27 17:54:16, whywhat wrote: > > Maybe clarify we took care of the remaining session when putting the previous > > one on top? > > Sorry, I'm not very sure of what you mean here. Can you explain a bit more? I meant, clarify why we only duck the top session, not all sessions (like we should ideally, right?) The clarification would be that we would duck the remaining sessions before when adding the "next top session", so only the top session needs to be ducked at this point.
	65

	66 * When a `MediaSession` abandons audio focus:

	67

	68 * If the session is not on the top, just remove it from the stack.

	69 * If the session is on the top, remove it from the stack.

	70

	71 * If the stack becomes empty, do nothing.

	72 * If the next top session is transient, do nothing.

	73 * If the next top session is persistent, stop ducking it.

	74

	75 ### Handling Pepper

	76

	77 Pepper is different from media elements since it has a different model. Pepper

	78 cannot be paused, but its volume can be changed. When considering Pepper, the

	79 above algorithm must be modified.

	80

	81 When Pepper joins `MediaSession`, it should request persistent focus type. When

	82 AudioFocusManager wants to suspend a `MediaSession`, it must check whether the

	83 session has Pepper instance, and if yes, it should duck the session instead.

	84

	85 Also, whenever a session abandons focus, and the next top session is INACTIVE,

	86 `AudioFocusManager` should find the next session having Pepper and unduck it.

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