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1 stream_type—流类型

enum stream_type {
        DEFAULT          	=-1,
        VOICE_CALL       	= 0,	//电话
        SYSTEM           	= 1,	//系统声音
        RING             	= 2,	//铃声
        MUSIC            	= 3,	//音乐声
        ALARM            	= 4,	//警告
        NOTIFICATION     	= 5,	//通知
        BLUETOOTH_SCO    	= 6,	//蓝牙耳机接通
        ENFORCED_AUDIBLE 	= 7, 	//不能被静音必须路由到喇叭播放的声音
        DTMF             	= 8,	//双音多频
        TTS              	= 9,	//从文本到语音的合成技术
        NUM_STREAM_TYPES
};

2 routing_strategy—策略

enum routing_strategy {
            STRATEGY_MEDIA,
            STRATEGY_PHONE,
            STRATEGY_SONIFICATION,
            STRATEGY_DTMF,
            NUM_STRATEGIES
        };

3 audio_devices--输入输出设备定义

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_FM_HEADPHONE = 0x800,
	DEVICE_OUT_FM_SPEAKER = 0x1000,
	DEVICE_OUT_TTY = 0x2000,
        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_FM_HEADPHONE | DEVICE_OUT_FM_SPEAKER | DEVICE_OUT_TTY | 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)
};

4 stream_type、device、strategy三者间的关系

首先要搞清楚stream_type，device，strategy三者之间的关系：

·AudioSystem::stream_type 音频流的类型，一共有10种类型，在应用层上，就会根据不同场景把stream_type定义好，然后再传到JAVA层或者JNI层。

·AudioSystem::audio_devices 音频输入输出设备

·AudioPolicyManager::routing_strategy 音频路由策略，可以有4种策略

三者的关系如图所示：

5 getStrategy--流类型与策略之间的映射关系函数

getStrategy(stream_type)根据stream type，返回对应的routing strategy值，getDeviceForStrategy()则是根据routing strategy，返回可用的device。Android把10种stream type归纳为4种路由策略，然后根据路由策略决定具体的输出设备。
AudioPolicyManagerBase::routing_strategy AudioPolicyManagerBase::getStrategy(
        AudioSystem::stream_type stream) {
    switch (stream) {
    case AudioSystem::VOICE_CALL: 		// 电话来了
    case AudioSystem::BLUETOOTH_SCO: 	// 蓝牙耳机接通了
        return STRATEGY_PHONE;
    case AudioSystem::RING: 				// 铃声响了
    case AudioSystem::NOTIFICATION: 		// 通知，例如界面中最上面一栏中有消息了
    case AudioSystem::ALARM: 			// 警告，电池没电时的警告？
    case AudioSystem::ENFORCED_AUDIBLE:
        return STRATEGY_SONIFICATION;
    case AudioSystem::DTMF: 	
        return STRATEGY_DTMF;
    default:
        LOGE("unknown stream type");
    case AudioSystem::SYSTEM: 			// 系统声音,播放音乐的时候按键音，
/*mute 掉音乐，并切换 output 的话，将导致 很差的用户体验。
其中可以得到以下信息：
1、按键声音属于 system stream type 。
2、策略的改变将会导致 output 的切换
3、优先级高的策略 start 时会 mute 掉优先级低的策略*/
    case AudioSystem::TTS: 				// TTS 就是 Text To Speech
    case AudioSystem::MUSIC:
        return STRATEGY_MEDIA;
    }
}

6 audio_mode—模式

 enum audio_mode {
        MODE_INVALID = -2,
        MODE_CURRENT = -1,
        MODE_NORMAL = 0,
        MODE_RINGTONE,
        MODE_IN_CALL,
        MODE_IN_COMMUNICATION,
        NUM_MODES  // not a valid entry, denotes end-of-list
    };

音频流分析 五 策略与流类型