Android Audio 分析

一、架构

二、MediaServer初始化

所有的media服务都在进程mediaserver里。其代码在framework/base/media/mediaserver/main_mediaserver.cpp中。初始化过程如下。

1. 先初始化AudioFlinger

其初始化通过AudioFlinger的父类BindService创建唯一的AudioFlinger实例。

2. 然后初始化MediaPlayerService和CameraService

3. 最后初始化AudioPolicyService

三、 AudioFlinger分析

AudioFlinger负责管理每个音轨AudioTrack及RecordTrack，主音量控制，每种声音流的属性设置，设备控制，音效控制。

四、 AudioPolicyService分析

AudioPolicyService用于声音策略管理

客户端代码在framework/base/media/libmedia/AudioSystem.cpp中，其实该客户端只是监测AudioPolicyService是否死亡（binderDied)。真正的客户端为BpAudioPolicyService类，在framework/base/media/libmedia/IAudioPolicyService.cpp中。

接口

 adjustVolume
 adjustSuggestedStreamVolume
 adjustStreamVolume
 setStreamVolume
 setStreamSolo
 setStreamMute
 getStreamVolume
 getStreamMaxVolume
 setRingerMode
 getRingerMode
 setVibrateSetting
 getVibrateSetting
 shouldVibrate
 setMode
 getMode
 playSoundEffect
 playSoundEffectVolume
 loadSoundEffects
 unloadSoundEffects
 reloadAudioSettings
 setSpeakerphoneOn
 isSpeakerphoneOn
 setBluetoothScoOn
 isBluetoothScoOn
 requestAudioFocus
 abandonAudioFocus
 unregisterAudioFocusClient
 registerMediaButtonEventReceiver
 unregisterMediaButtonEventReceiver
 startBluetoothSco
 stopBluetoothSco

服务端代码在framework/base/services/audioflinger/AudioPolicyService.cpp中，实现了AudioPolicyService本地代码及服务内部使用的AudioPolicyClientInterface接口，该接口被AudioPolicyManager调用。

 openOutput
 openDuplicateOutput
 closeOutput
 suspendOutput
 restoreOutput
 openInput
 loseInput
 setStreamVolume
 setStreamOutput
 setParameters
 getParameters
 startTone
 stopTone
 setVoiceVolume
 moveEffects

AudioPolicyService会启动两个AudioCommandThread类的线程mTonePlaybackThread和mAudioCommandThread，然后构造AudioPolicyManager的实例mpPolicyManager。基本上所有的服务接口都会在检查权限后调用mpPolicyManager的相应实现。

mTonePlaybackThread用户播放通话中的通知音

mAudioCommandThread用于设置AudioFlinger的一些设置(为什么使用AudioCommandThread，然后调用AudioSystem中接口控制AudioFlinger?)

五、 AudioManager分析

1. 音频焦点转

 requestAudioFocus         请求焦点
 abandonAudioFocus         放弃焦点

requestAudioFocus的第一参数是Listener,可以是下面枚举的后三个值

第二个参数是当前流类型

最后一个参数可以是以下的前三个值

 AUDIOFOCUS_GAIN
 AUDIOFOCUS_GAIN_TRANSIENT
 AUDIOFOCUS_GAIN_TRANSIENT_MAY_DUCK = ;
 AUDIOFOCUS_LOSS = - * AUDIOFOCUS_GAIN;
 AUDIOFOCUS_LOSS_TRANSIENT = - * AUDIOFOCUS_GAIN_TRANSIENT;
 AUDIOFOCUS_LOSS_TRANSIENT_CAN_DUCK =
  - * AUDIOFOCUS_GAIN_TRANSIENT_MAY_DUCK;

六、 AudioPolicyManager分析

流类型stream_type(在 AudioSystem中定义)

TYPE                          VALUE                                STRATEGY
DEFAULT(->MUSIC)                -                             STRATEGY_MEDIA()
VOICE_CALL                                                    STRATEGY_PHONE()
SYSTEM                                                        STRATEGY_MEDIA()
RING                                                          STRATEGY_SONIFICATION()
MUSIC                                                         STRATEGY_MEDIA()
ALARM                                                         STRATEGY_SONIFICATION()
NOTIFICATION                                                  STRATEGY_SONIFICATION()
BLUETOOTH_SCO                                                 STRATEGY_PHONE()
// Sounds that cannot be muted by user and must be routed to speaker
ENFORCED_AUDIBLE                                              STRATEGY_SONIFICATION()
DTMF                                                          STRATEGY_DTMF()
TTS                                                           STRATEGY_MEDIA()
HDMI                               MV的自定义类型              STRATEGY_MEDIA()
FM                                 MV的自定义类型              STRATEGY_MEDIA()

音频状态audio_mode(在 AudioSystem中定义)

1  enum audio_mode {
  MODE_INVALID = -,
  MODE_CURRENT = -,
  MODE_NORMAL = ,
  MODE_RINGTONE,
  MODE_IN_CALL,
  MODE_IN_COMMUNICATION,
  NUM_MODES  // not a valid entry, denotes end-of-list
  };

策略类型routing_strategy:

STRATEGY_MEDIA
STRATEGY_PHONE
STRATEGY_SONIFICATION
STRATEGY_DTMF              

设备类型audio_devices：

 enum audio_devices {
  // output devices
  DEVICE_OUT_EARPIECE = 0x1,
  DEVICE_OUT_SPEAKER = 0x2,
  DEVICE_OUT_WIRED_HEADSET = 0x4,
  DEVICE_OUT_WIRED_HEADPHONE = 0x8,
  DEVICE_OUT_BLUETOOTH_SCO = 0x10,
  DEVICE_OUT_BLUETOOTH_SCO_HEADSET = 0x20,
  DEVICE_OUT_BLUETOOTH_SCO_CARKIT = 0x40,
  DEVICE_OUT_BLUETOOTH_A2DP = 0x80,
  DEVICE_OUT_BLUETOOTH_A2DP_HEADPHONES = 0x100,
  DEVICE_OUT_BLUETOOTH_A2DP_SPEAKER = 0x200,
  DEVICE_OUT_AUX_DIGITAL = 0x400,
  DEVICE_OUT_DEFAULT = 0x8000,
  DEVICE_OUT_ALL = (DEVICE_OUT_EARPIECE | DEVICE_OUT_SPEAKER | DEVICE_OUT_WIRED_HEADSET |
  DEVICE_OUT_WIRED_HEADPHONE | DEVICE_OUT_BLUETOOTH_SCO | DEVICE_OUT_BLUETOOTH_SCO_HEADSET |
  DEVICE_OUT_BLUETOOTH_SCO_CARKIT | DEVICE_OUT_BLUETOOTH_A2DP | DEVICE_OUT_BLUETOOTH_A2DP_HEADPHONES |
  DEVICE_OUT_BLUETOOTH_A2DP_SPEAKER | DEVICE_OUT_AUX_DIGITAL | DEVICE_OUT_DEFAULT),
  DEVICE_OUT_ALL_A2DP = (DEVICE_OUT_BLUETOOTH_A2DP | DEVICE_OUT_BLUETOOTH_A2DP_HEADPHONES |
  DEVICE_OUT_BLUETOOTH_A2DP_SPEAKER),

  // input devices
  DEVICE_IN_COMMUNICATION = 0x10000,
  DEVICE_IN_AMBIENT = 0x20000,
  DEVICE_IN_BUILTIN_MIC = 0x40000,
  DEVICE_IN_BLUETOOTH_SCO_HEADSET = 0x80000,
  DEVICE_IN_WIRED_HEADSET = 0x100000,
  DEVICE_IN_AUX_DIGITAL = 0x200000,
  DEVICE_IN_VOICE_CALL = 0x400000,
  DEVICE_IN_BACK_MIC = 0x800000,
  DEVICE_IN_DEFAULT = 0x80000000,

  DEVICE_IN_ALL = (DEVICE_IN_COMMUNICATION | DEVICE_IN_AMBIENT | DEVICE_IN_BUILTIN_MIC |
  DEVICE_IN_BLUETOOTH_SCO_HEADSET | DEVICE_IN_WIRED_HEADSET | DEVICE_IN_AUX_DIGITAL |
  DEVICE_IN_VOICE_CALL | DEVICE_IN_BACK_MIC | DEVICE_IN_DEFAULT)
  };

流程分析：

1.音量控制

initStreamVolume
setStreamVolumeIndex
setPhoneState

2.播放控制

入口函数

 startOutput

 getStrategy

 getDeviceForStrategy

 setOutputDevice

 applyStreamVolume

 checkAndSetVolume