对 AudioManager.STREAM_VOICE_CALL 音频流类型进行分析

hardware/libhardware_legacy/audio/AudioPolicyManagerBase.cpp
frameworks/base/media/java/android/media/AudioManager.java
frameworks/base/media/java/android/media/AudioService.java
frameworks/base/media/java/android/media/AudioSystem.java

通过音频流的类型获取对应的音频策略

AudioPolicyManagerBase.cpp:

AudioPolicyManagerBase::routing_strategy AudioPolicyManagerBase::getStrategy(AudioSystem::stream_type stream) {
    // stream to strategy mapping
    switch (stream) {
    case AudioSystem::VOICE_CALL:
    case AudioSystem::BLUETOOTH_SCO:
        return STRATEGY_PHONE;
    ...
    }
}

可以， voice call的音频策略是 phone

通过音频策略获取到对应的输出设备

AudioPolicyManagerBase.cpp:

audio_devices_t AudioPolicyManagerBase::getDeviceForStrategy(routing_strategy strategy, bool fromCache) {
    uint32_t device = AUDIO_DEVICE_NONE;
    ... 
    switch (strategy) {
    ... 
    case STRATEGY_PHONE:
        // for phone strategy, we first consider the forced use and then the available devices by order
        // of priority
        switch (mForceUse[AudioSystem::FOR_COMMUNICATION]) { // 判断是否有设置在通讯过程中, 强制使用某种输出设备
        case AudioSystem::FORCE_BT_SCO: // 强制使用蓝牙, 作为输出设备
            if (!isInCall() || strategy != STRATEGY_DTMF) {
                device = mAvailableOutputDevices & AUDIO_DEVICE_OUT_BLUETOOTH_SCO_CARKIT;
                if (device) break;
            }
            device = mAvailableOutputDevices & AUDIO_DEVICE_OUT_BLUETOOTH_SCO_HEADSET;
            if (device) break;
            device = mAvailableOutputDevices & AUDIO_DEVICE_OUT_BLUETOOTH_SCO;
            if (device) break;
            // if SCO device is requested but no SCO device is available, fall back to default case
            // FALL THROUGH

        default:    // FORCE_NONE 没有设置通讯过程中, 强制使用某种输出设备
            // when not in a phone call, phone strategy should route STREAM_VOICE_CALL to A2DP
            if (mHasA2dp && !isInCall() &&
                    (mForceUse[AudioSystem::FOR_MEDIA] != AudioSystem::FORCE_NO_BT_A2DP) &&
                    (getA2dpOutput() != 0) && !mA2dpSuspended) {
                device = mAvailableOutputDevices & AUDIO_DEVICE_OUT_BLUETOOTH_A2DP;
                if (device) break;
                device = mAvailableOutputDevices & AUDIO_DEVICE_OUT_BLUETOOTH_A2DP_HEADPHONES;
                if (device) break;
            }
            device = mAvailableOutputDevices & AUDIO_DEVICE_OUT_WIRED_HEADPHONE;
            if (device) break;
            device = mAvailableOutputDevices & AUDIO_DEVICE_OUT_WIRED_HEADSET;
            if (device) break;
            if (mPhoneState != AudioSystem::MODE_IN_CALL) {
                device = mAvailableOutputDevices & AUDIO_DEVICE_OUT_USB_ACCESSORY;
                if (device) break;
                device = mAvailableOutputDevices & AUDIO_DEVICE_OUT_USB_DEVICE;
                if (device) break;
                device = mAvailableOutputDevices & AUDIO_DEVICE_OUT_DGTL_DOCK_HEADSET;
                if (device) break;
                device = mAvailableOutputDevices & AUDIO_DEVICE_OUT_AUX_DIGITAL;
                if (device) break;
                device = mAvailableOutputDevices & AUDIO_DEVICE_OUT_ANLG_DOCK_HEADSET;
                if (device) break;
            }
			// 没有设置通讯过程中, 音频的输出设备, 则默认使用听筒作为输出设备
            device = mAvailableOutputDevices & AUDIO_DEVICE_OUT_EARPIECE;
            if (device) break;
            device = mDefaultOutputDevice;
            if (device == AUDIO_DEVICE_NONE) {
                ALOGE("getDeviceForStrategy() no device found for STRATEGY_PHONE");
            }
            break;

        case AudioSystem::FORCE_SPEAKER: // 强制使用扬声器, 作为输出设备
            // when not in a phone call, phone strategy should route STREAM_VOICE_CALL to
            // A2DP speaker when forcing to speaker output
            if (mHasA2dp && !isInCall() &&
                    (mForceUse[AudioSystem::FOR_MEDIA] != AudioSystem::FORCE_NO_BT_A2DP) &&
                    (getA2dpOutput() != 0) && !mA2dpSuspended) {
                device = mAvailableOutputDevices & AUDIO_DEVICE_OUT_BLUETOOTH_A2DP_SPEAKER;
                if (device) break;
            }
            if (mPhoneState != AudioSystem::MODE_IN_CALL) {
                device = mAvailableOutputDevices & AUDIO_DEVICE_OUT_USB_ACCESSORY;
                if (device) break;
                device = mAvailableOutputDevices & AUDIO_DEVICE_OUT_USB_DEVICE;
                if (device) break;
                device = mAvailableOutputDevices & AUDIO_DEVICE_OUT_DGTL_DOCK_HEADSET;
                if (device) break;
                device = mAvailableOutputDevices & AUDIO_DEVICE_OUT_AUX_DIGITAL;
                if (device) break;
                device = mAvailableOutputDevices & AUDIO_DEVICE_OUT_ANLG_DOCK_HEADSET;
                if (device) break;
            }
            device = mAvailableOutputDevices & AUDIO_DEVICE_OUT_SPEAKER;
            if (device) break;
            device = mDefaultOutputDevice;
            if (device == AUDIO_DEVICE_NONE) {
                ALOGE("getDeviceForStrategy() no device found for STRATEGY_PHONE, FORCE_SPEAKER");
            }
            break;
        }
    break;
	// 关于上面default位置的问题, 如果每个case选择(包括default)最终都有break语句, 则default的位置不影响执行顺序
	// 当每个case选择(包括default)不一定有break语句时, 如果输入不满足其他选择, 最终执行default. 程序会从default处从上向下执行, 直到遇到break语句停止
	// 此处的default位置并不影响执行顺序
	... // 省略
    }
    return device;
}