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android10 AudioService之音频输出通道切换_android 音频输出

作者：你好赵伟 | 2024-05-20 10:50:45

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android 音频输出

前言：android系统中，音频输出的方式有很多种，外放即扬声器（Speaker）、听筒（Telephone Receiver）、有线耳机（WiredHeadset）、蓝牙音箱（Bluetooth A2DP）等，

android 系统默认有自己的音频输出优先级，那我们可以按照需求定制自己的音频切换方式么？答案是可以的。即可以在应用层修改，也可以在Framework修改，今天讲的就是Framework层的修改，也就是修改AudioService。接下来我们先了解下基本的使用

一音频输出通道


//base/media/java/android/media/AudioSystem.java下
    @UnsupportedAppUsage
public static final int FORCE_NONE = 0;//默认通道
public static final int FORCE_SPEAKER = 1;//扬声器通道
public static final int FORCE_HEADPHONES = 2;//耳机通道
//下面两个是蓝牙耳机通道
public static final int FORCE_BT_SCO = 3;//是一种双向的音频数据的传输链路,只能用于普通语音的传输,不能用于播放音乐
public static final int FORCE_BT_A2DP = 4;//是一种单向的高品质音频数据传输链路，通常用于播放立体声音乐
public static final int FORCE_WIRED_ACCESSORY = 5;//有线设备通道，如有线耳机
 
    @UnsupportedAppUsage
    public static final int FORCE_BT_CAR_DOCK = 6;
    @UnsupportedAppUsage
    public static final int FORCE_BT_DESK_DOCK = 7;
    @UnsupportedAppUsage
    public static final int FORCE_ANALOG_DOCK = 8;
    @UnsupportedAppUsage
    public static final int FORCE_DIGITAL_DOCK = 9;
    public static final int FORCE_NO_BT_A2DP = 10;
    public static final int FORCE_SYSTEM_ENFORCED = 11;
    public static final int FORCE_HDMI_SYSTEM_AUDIO_ENFORCED = 12;
    public static final int FORCE_ENCODED_SURROUND_NEVER = 13;
    public static final int FORCE_ENCODED_SURROUND_ALWAYS = 14;
    public static final int FORCE_ENCODED_SURROUND_MANUAL = 15;
    public static final int NUM_FORCE_CONFIG = 16;
    public static final int FORCE_DEFAULT = FORCE_NONE;

常用的也就是：扬声器，有线耳机，听筒，蓝牙耳机等；

二音频模式的理解和使用

音频模式设置的使用：


 public static void setSpeakerNormal(Context context, int mode) {
        Log.e(TAG, "setSpeakerNormal: " + on);
        AudioManager am=(AudioManager)context.getSystemService(Context.AUDIO_SERVICE);    
        am.setMode(mode);
    }

音频模式的选项：
在使用音频输出通道时，需要指定播放模式，设置音频模式的方法audioManager.setMode()方法的值有如下几种：


//base/media/java/android/media/AudioManager.java下
 
 
    /* modes for setMode/getMode/setRoute/getRoute */
    /**
     * Audio harware modes.
     */
    /**
     * Invalid audio mode.
     */
    public static final int MODE_INVALID            = AudioSystem.MODE_INVALID;
    /**
     * Current audio mode. Used to apply audio routing to current mode.
     */
    public static final int MODE_CURRENT            = AudioSystem.MODE_CURRENT;
    /**
     * Normal audio mode: not ringing and no call established.
     */
    public static final int MODE_NORMAL             = AudioSystem.MODE_NORMAL;
    /**
     * Ringing audio mode. An incoming is being signaled.
     */
    public static final int MODE_RINGTONE           = AudioSystem.MODE_RINGTONE;
    /**
     * In call audio mode. A telephony call is established.
     */
    public static final int MODE_IN_CALL            = AudioSystem.MODE_IN_CALL;
    /**
     * In communication audio mode. An audio/video chat or VoIP call is established.
     */
    public static final int MODE_IN_COMMUNICATION   = AudioSystem.MODE_IN_COMMUNICATION;

可用的模式有：MODE_NORMAL，//默认（平时）状态
                         MODE_RINGTONE，//响玲模式
                         MODE_IN_CALL，//通话模式
                         MODE_IN_COMMUNICATION//（非通话）切换至听筒模式

其引用的AudioSystem，该类定义如下


AudioSystem.java
/* modes for setPhoneState, must match AudioSystem.h audio_mode */
public static final int MODE_INVALID            = -2;
public static final int MODE_CURRENT            = -1;
public static final int MODE_NORMAL             = 0;//待机模式，既不是铃声模式也不是通话模式,如music
public static final int MODE_RINGTONE           = 1;//铃声模式
public static final int MODE_IN_CALL            = 2;//音频通话模式
public static final int MODE_IN_COMMUNICATION   = 3;//通信模式，包括音/视频,VoIP通话.(3.0加入的，与通话模式类似)
public static final int NUM_MODES               = 4;

设备默认模式：MODE_NORMAL：


public static void setSpeakerNormal(Context context) {
        AudioManager am = (AudioManager) context.getSystemService(Context.AUDIO_SERVICE);  
am.setMode(AudioManager.MODE_NORMAL); 
 }

设置听筒模式（非通话）：MODE_IN_COMMUNICATION


public static void setTingtong(Context context) {
   Log.e(TAG, "setTingtong start ");
   AudioManager am = (AudioManager)context.
               getSystemService(Context.AUDIO_SERVICE);
   am.setSpeakerphoneOn(false);
   am.setMode(AudioManager.MODE_IN_COMMUNICATION);
 
}

设置通话模式


      public static void setInCall(Context context) {
        Log.e(TAG, "setTingtong start ");
        AudioManager am = (AudioManager) context.getSystemService(Context.AUDIO_SERVICE);
        am.setSpeakerphoneOn(false);
        am.setMode(AudioManager.MODE_IN_CALL);
 
    }

三流类型

设置播放模式的时候，需要考虑流类型，常用的流类型有：


//base/media/java/android/media/AudioSystem.java下
 
/* These values must be kept in sync with system/audio.h */
    /*
     * If these are modified, please also update Settings.System.VOLUME_SETTINGS
     * and attrs.xml and AudioManager.java.
     */
    /** Used to identify the default audio stream volume */
    public static final int STREAM_DEFAULT = -1;
    /** Used to identify the volume of audio streams for phone calls */
    public static final int STREAM_VOICE_CALL = 0;//用于电话通话的音频流。
    /** Used to identify the volume of audio streams for system sounds */
    public static final int STREAM_SYSTEM = 1;//用于系统声音的音频流
    /** Used to identify the volume of audio streams for the phone ring and message alerts */
    public static final int STREAM_RING = 2;//用于电话铃声的音频流
    /** Used to identify the volume of audio streams for music playback */
    public static final int STREAM_MUSIC = 3;//用于音乐播放的音频流
    /** Used to identify the volume of audio streams for alarms */
    public static final int STREAM_ALARM = 4;//用于警报的音频流
    /** Used to identify the volume of audio streams for notifications */
    public static final int STREAM_NOTIFICATION = 5;//用于通知的音频流
    /** Used to identify the volume of audio streams for phone calls when connected on bluetooth */
    public static final int STREAM_BLUETOOTH_SCO = 6;//用于连接到蓝牙电话的手机音频流
    /** Used to identify the volume of audio streams for enforced system sounds in certain
     * countries (e.g camera in Japan) */
    @UnsupportedAppUsage
    public static final int STREAM_SYSTEM_ENFORCED = 7;//在某些国家实施的系统声音的音频流
    /** Used to identify the volume of audio streams for DTMF tones */
    public static final int STREAM_DTMF = 8;//DTMF音调的音频流。
    /** Used to identify the volume of audio streams exclusively transmitted through the
     *  speaker (TTS) of the device */
    public static final int STREAM_TTS = 9;//文本到语音转换（TTS）的音频流。	
    /** Used to identify the volume of audio streams for accessibility prompts */
    public static final int STREAM_ACCESSIBILITY = 10;//辅助功能提示音频流

四音频输出通道，播放模式和流类型的关系

音频通道是与播放模式一起用的，而播放模式与音频流类型有关系；
(1)音频通道是指声音从哪里出来，这个容易理解；
(2)播放模式，也叫音频状态，手机有4种音频状态：待机状态，音视频通话状态，视频/VoIP通话状态与响铃状态。这4种状态对底层的音频输出设备的选择影响很大，相应的情景下就得使用相应的模式，如视频情景的播放模式就是MODE_IN_COMMUNICATION，或者，播放音乐情景的播放模式就是MODE_NORMAL，什么样的情形就得用什么样的播放模式，不能搞混，比如MODE_IN_CALL，就只能由通话时才能使用；
(3)音频流类型，我们操作手机的音频时需要指定操作的是哪一个流，虽然手机的中音频流类型有很多，但是一旦进入到属性里，android就会将其整理成几种类型，这才是实际的类型，与上面的播放模式对应；

五 audioManager.setMode(int mode)的源码实现

在AudioManager类中的实现


/**
     * Sets the audio mode.
     * <p>
     * The audio mode encompasses audio routing AND the behavior of
     * the telephony layer. Therefore this method should only be used by applications that
     * replace the platform-wide management of audio settings or the main telephony application.
     * In particular, the {@link #MODE_IN_CALL} mode should only be used by the telephony
     * application when it places a phone call, as it will cause signals from the radio layer
     * to feed the platform mixer.
     *
     * @param mode  the requested audio mode ({@link #MODE_NORMAL}, {@link #MODE_RINGTONE},
     *              {@link #MODE_IN_CALL} or {@link #MODE_IN_COMMUNICATION}).
     *              Informs the HAL about the current audio state so that
     *              it can route the audio appropriately.
     */
    public void setMode(int mode) {
        final IAudioService service = getService();
        try {
            service.setMode(mode, mICallBack, mApplicationContext.getOpPackageName());
        } catch (RemoteException e) {
            throw e.rethrowFromSystemServer();
        }
    }

接下来是AudioService中方法的实现


  /** @see AudioManager#setMode(int) */
    public void setMode(int mode, IBinder cb, String callingPackage) {
        if (DEBUG_MODE) { Log.v(TAG, "setMode(mode=" + mode + ", callingPackage=" + callingPackage + ")"); }
       //检测权限不合法，return 
       if (!checkAudioSettingsPermission("setMode()")) {
            return;
        }
        //通话模式下，无权限return
        if ( (mode == AudioSystem.MODE_IN_CALL) &&
                (mContext.checkCallingOrSelfPermission(
                        android.Manifest.permission.MODIFY_PHONE_STATE)
                            != PackageManager.PERMISSION_GRANTED)) {
            Log.w(TAG, "MODIFY_PHONE_STATE Permission Denial: setMode(MODE_IN_CALL) from pid="
                    + Binder.getCallingPid() + ", uid=" + Binder.getCallingUid());
            return;
        }
        //新的mode值不合法return
        if (mode < AudioSystem.MODE_CURRENT || mode >= AudioSystem.NUM_MODES) {
            return;
        }
 
        int oldModeOwnerPid = 0;
        int newModeOwnerPid = 0;
        synchronized (mDeviceBroker.mSetModeLock) {
            if (!mSetModeDeathHandlers.isEmpty()) {
                oldModeOwnerPid = mSetModeDeathHandlers.get(0).getPid();
            }
            if (mode == AudioSystem.MODE_CURRENT) {
                mode = mMode;
            }
            //设置新的模式值，如果此次设置的音频播放模式和上一次的不同,返回这次使用新音频播放模式的进程的pid
            newModeOwnerPid = setModeInt(mode, cb, Binder.getCallingPid(), callingPackage);
        }
        // when entering RINGTONE, IN_CALL or IN_COMMUNICATION mode, clear all
        // SCO connections not started by the application changing the mode when 
pid changes
        ///如果进入了RINGTONE, IN_CALL 或者IN_COMMUNICATION模式,清除掉当前更改音频模式的应用进程的蓝牙SCO连接
        if ((newModeOwnerPid != oldModeOwnerPid) && (newModeOwnerPid != 0)) {
            mDeviceBroker.postDisconnectBluetoothSco(newModeOwnerPid);
        }
    }

//设置新的模式值
newModeOwnerPid = setModeInt(mode, cb, Binder.getCallingPid(), callingPackage);

3. setModeInt的实现


// setModeInt() returns a valid PID if the audio mode was successfully set to
    // any mode other than NORMAL.
    @GuardedBy("mDeviceBroker.mSetModeLock")
    private int setModeInt(int mode, IBinder cb, int pid, String caller) {
        if (DEBUG_MODE) { Log.v(TAG, "setModeInt(mode=" + mode + ", pid=" + pid + ", caller="
                + caller + ")"); }
        int newModeOwnerPid = 0;
        if (cb == null) {
            Log.e(TAG, "setModeInt() called with null binder");
            return newModeOwnerPid;
        }
 
        if(caller != null && caller.equals("com.android.soundrecorder") && mode == AudioSystem.MODE_IN_CALL) {
            mIsSoundRecorderPlayInEARPIECE = true;
        }else{
            mIsSoundRecorderPlayInEARPIECE = false;
        }
 
        SetModeDeathHandler hdlr = null;
        Iterator iter = mSetModeDeathHandlers.iterator();
        //循环遍历mSetModeDeathHandlers,找到与传递进来的相同pid的SetModeDeathHandler,并赋值给hdlr,
    //相同的pid也就是相同的应用,即找到相同的应用
        while (iter.hasNext()) {
            SetModeDeathHandler h = (SetModeDeathHandler)iter.next();
            if (h.getPid() == pid) {
                hdlr = h;
                // Remove from client list so that it is re-inserted at top of list
                iter.remove();
                hdlr.getBinder().unlinkToDeath(hdlr, 0);
                break;
            }
        }
        final int oldMode = mMode;
        int status = AudioSystem.AUDIO_STATUS_OK;
        int actualMode;
        do {
            //将传递进来的mode赋值初始化实际的播放模式
            actualMode = mode;
            //如果设置的模式是正常的播放模式,那就从mSetModeDeathHandlers列表的顶端获取一个模式给actualMode,
        //最近一次设置非正常音频模式的应用都会被放在mSetModeDeathHandlers的顶端
            if (mode == AudioSystem.MODE_NORMAL) {
                // get new mode from client at top the list if any
                if (!mSetModeDeathHandlers.isEmpty()) {
                    hdlr = mSetModeDeathHandlers.get(0);
                    cb = hdlr.getBinder();
                    actualMode = hdlr.getMode();
                    if (DEBUG_MODE) {
                        Log.w(TAG, " using mode=" + mode + " instead due to death hdlr at pid="
                                + hdlr.mPid);
                    }
                }
            } else {
                if (hdlr == null) {
                    hdlr = new SetModeDeathHandler(cb, pid);
                }
                // Register for client death notification
                try {
                    cb.linkToDeath(hdlr, 0);
                } catch (RemoteException e) {
                    // Client has died!
                    Log.w(TAG, "setMode() could not link to "+cb+" binder death");
                }
 
                // Last client to call setMode() is always at top of client list
                // as required by SetModeDeathHandler.binderDied()
                //将hdlr加到mSetModeDeathHandlers中,并放到首位,也就是最后一个调用setMode()的进程位于列表的顶部
                mSetModeDeathHandlers.add(0, hdlr);
            //设置当前进程的音频播放模式,hdlr.setMode()会将mode设置给mMode,这个要注意,要不然很容易跟下面的"actualMode != mMode"混淆
                hdlr.setMode(mode);
            }
 
            if (actualMode != mMode) {
                final long identity = Binder.clearCallingIdentity();
            //通过AudioSystem将当前的音频模式设置到底层去,status返回设置的结果
                status = AudioSystem.setPhoneState(actualMode);
                Binder.restoreCallingIdentity(identity);
                if (status == AudioSystem.AUDIO_STATUS_OK) {
                    if (DEBUG_MODE) { Log.v(TAG, " mode successfully set to " + actualMode); }
                //如果设置成功,保存当前的音频播放模式
                    mMode = actualMode;
                } else {
                    if (hdlr != null) {
                //如果设置不成功,从mSetModeDeathHandlers中删除该应用
                        mSetModeDeathHandlers.remove(hdlr);
                        cb.unlinkToDeath(hdlr, 0);
                    }
                    // force reading new top of mSetModeDeathHandlers stack
                    if (DEBUG_MODE) { Log.w(TAG, " mode set to MODE_NORMAL after phoneState pb"); }
                    mode = AudioSystem.MODE_NORMAL;
                }
            } else {
                status = AudioSystem.AUDIO_STATUS_OK;
            }
        } while (status != AudioSystem.AUDIO_STATUS_OK && !mSetModeDeathHandlers.isEmpty());
 
        if (status == AudioSystem.AUDIO_STATUS_OK) {
            if (actualMode != AudioSystem.MODE_NORMAL) {
                if (mSetModeDeathHandlers.isEmpty()) {
                    Log.e(TAG, "setMode() different from MODE_NORMAL with empty mode client stack");
                } else {
                //如果这个进程设置的音频模式为非正常模式,那就返回这个进程的pid
                    newModeOwnerPid = mSetModeDeathHandlers.get(0).getPid();
                }
            }
            // Note: newModeOwnerPid is always 0 when actualMode is MODE_NORMAL
            mModeLogger.log(
                    new PhoneStateEvent(caller, pid, mode, newModeOwnerPid, actualMode));
        //下面的代码用来设置当前音频流类型的音量
            int streamType = getActiveStreamType(AudioManager.USE_DEFAULT_STREAM_TYPE);
            int device = getDeviceForStream(streamType);
            int index = mStreamStates[mStreamVolumeAlias[streamType]].getIndex(device);
            setStreamVolumeInt(mStreamVolumeAlias[streamType], index, device, true, caller);
 
            updateStreamVolumeAlias(true /*updateVolumes*/, caller);
 
            // change of mode may require volume to be re-applied on some devices
            updateAbsVolumeMultiModeDevices(oldMode, actualMode);
        }
        return newModeOwnerPid;
    }

AudioService用mMode来保存当前的音频播放模式。

六设置音频输出管道

设置音频输出管道的方法有两个分别是setSpeakerphoneOn()和setBluetoothScoOn()，我们来就看下最常用的设置扬声器播放；


 /**
     * Sets the speakerphone on or off.
     * <p>
     * This method should only be used by applications that replace the platform-wide
     * management of audio settings or the main telephony application.
     *
     * @param on set <var>true</var> to turn on speakerphone;
     *           <var>false</var> to turn it off
     *true 是开启扬声器，false关闭扬声器
     */
    public void setSpeakerphoneOn(boolean on){
        final IAudioService service = getService();
        try {
            service.setSpeakerphoneOn(on);
        } catch (RemoteException e) {
            throw e.rethrowFromSystemServer();
        }
    }

以上方法是调用AudioService中的setSpeakerphoneOn（boolean on）

看下AudioSevice中该方法的实现


/** @see AudioManager#setSpeakerphoneOn(boolean) */
    public void setSpeakerphoneOn(boolean on){
        //检查android.Manifest.permission.MODIFY_AUDIO_SETTINGS是否允许
        if (!checkAudioSettingsPermission("setSpeakerphoneOn()")) {
            return;
        }
        //通话模式，检查权限是否允许
        if (mContext.checkCallingOrSelfPermission(
                android.Manifest.permission.MODIFY_PHONE_STATE)
                != PackageManager.PERMISSION_GRANTED) {
            synchronized (mSetModeDeathHandlers) {
                for (SetModeDeathHandler h : mSetModeDeathHandlers) {
                    if (h.getMode() == AudioSystem.MODE_IN_CALL) {
                        Log.w(TAG, "getMode is call, Permission Denial: setSpeakerphoneOn from pid="
                                + Binder.getCallingPid() + ", uid=" + Binder.getCallingUid());
                        return;
                    }
                }
            }
        }
 
        // for logging only
        final String eventSource = new StringBuilder("setSpeakerphoneOn(").append(on)
                .append(") from u/pid:").append(Binder.getCallingUid()).append("/")
                .append(Binder.getCallingPid()).toString();
        //设置扬声器开/关
        final boolean stateChanged = mDeviceBroker.setSpeakerphoneOn(on, eventSource);
        if (stateChanged) {
            //设置成功
            final long ident = Binder.clearCallingIdentity();
            try {
                //发送成功后，发送扬声器状态改变的广播
                mContext.sendBroadcastAsUser(
                        new Intent(AudioManager.ACTION_SPEAKERPHONE_STATE_CHANGED)
                                .setFlags(Intent.FLAG_RECEIVER_REGISTERED_ONLY), UserHandle.ALL);
            } finally {
                Binder.restoreCallingIdentity(ident);
            }
        }
    }

以上方法体中使用 final boolean stateChanged = mDeviceBroker.setSpeakerphoneOn(on, eventSource); 修改扬声器开关。返回true，则修改成功。该方法在AudioDeviceBroker中实现


/**
     * Turns speakerphone on/off
     * @param on
     * @param eventSource for logging purposes
     * @return true if speakerphone state changed
     */
    /*package*/ boolean setSpeakerphoneOn(boolean on, String eventSource) {
        synchronized (mDeviceStateLock) {
            final boolean wasOn = isSpeakerphoneOn();
            if (on) {
        //开启扬声器
                if (mForcedUseForComm == AudioSystem.FORCE_BT_SCO) {
                    setForceUse_Async(AudioSystem.FOR_RECORD, AudioSystem.FORCE_NONE, eventSource);
                }
        //进入扬声器播放的标志AudioSystem.FORCE_SPEAKER
                mForcedUseForComm = AudioSystem.FORCE_SPEAKER;
            } else if (mForcedUseForComm == AudioSystem.FORCE_SPEAKER) {
                //取消扬声器
                mForcedUseForComm = AudioSystem.FORCE_NONE;
            }
 
            mForcedUseForCommExt = mForcedUseForComm;
    //此时是语音模式AudioSystem.FOR_COMMUNICATION,mForcedUseForComm表示当前是哪种音频通道
            setForceUse_Async(AudioSystem.FOR_COMMUNICATION, mForcedUseForComm, eventSource);
            return (wasOn != isSpeakerphoneOn());
        }
    }

以上方法中可以看到AudioService用mForcedUseForComm和mForcedUseForCommExt保存了当前的音频通道。以上方法根据状态设置对应的模式使用方法 setForceUse_Async(AudioSystem.FOR_COMMUNICATION, mForcedUseForComm, eventSource);


/*package*/ void setForceUse_Async(int useCase, int config, String eventSource) {
         //传递进来的参数封装,交给mAudioHandler处理；
        //arg1是AudioSystem.FOR_COMMUNICATION, arg2是mForcedUseForComm, 
        //obj是eventSource,eventSource就是调用扬声器的进程的信息
            sendIILMsgNoDelay(MSG_IIL_SET_FORCE_USE, SENDMSG_QUEUE,
                useCase, config, eventSource);
    }

我们看下sendIILMsgNoDelay方法的实现


private void sendIILMsgNoDelay(int msg, int existingMsgPolicy, int arg1, int arg2, Object obj) {
        sendIILMsg(msg, existingMsgPolicy, arg1, arg2, obj, 0);
    }
 
    private void sendIILMsg(int msg, int existingMsgPolicy, int arg1, int arg2, Object obj,
                            int delay) {
        if (existingMsgPolicy == SENDMSG_REPLACE) {
            mBrokerHandler.removeMessages(msg);
        } else if (existingMsgPolicy == SENDMSG_NOOP && mBrokerHandler.hasMessages(msg)) {
            return;
        }
 
        if (isMessageHandledUnderWakelock(msg)) {
            final long identity = Binder.clearCallingIdentity();
            try {
                mBrokerEventWakeLock.acquire(BROKER_WAKELOCK_TIMEOUT_MS);
            } catch (Exception e) {
                Log.e(TAG, "Exception acquiring wakelock", e);
            }
            Binder.restoreCallingIdentity(identity);
        }
 
        synchronized (sLastDeviceConnectionMsgTimeLock) {
            long time = SystemClock.uptimeMillis() + delay;
 
            switch (msg) {
                case MSG_IL_SET_A2DP_SOURCE_CONNECTION_STATE:
                case MSG_IL_SET_A2DP_SINK_CONNECTION_STATE:
                case MSG_IL_SET_HEARING_AID_CONNECTION_STATE:
                case MSG_L_SET_WIRED_DEVICE_CONNECTION_STATE:
                case MSG_IL_BTA2DP_DOCK_TIMEOUT:
                case MSG_L_A2DP_DEVICE_CONFIG_CHANGE:
                case MSG_L_A2DP_ACTIVE_DEVICE_CHANGE:
                    if (sLastDeviceConnectMsgTime >= time) {
                        // add a little delay to make sure messages are ordered as expected
                        time = sLastDeviceConnectMsgTime + 30;
                    }
                    sLastDeviceConnectMsgTime = time;
                    break;
                default:
                    break;
            }
private void sendIILMsgNoDelay(int msg, int existingMsgPolicy, int arg1, int arg2, Object obj) {
        sendIILMsg(msg, existingMsgPolicy, arg1, arg2, obj, 0);
    }
 
    private void sendIILMsg(int msg, int existingMsgPolicy, int arg1, int arg2, Object obj,
                            int delay) {
        if (existingMsgPolicy == SENDMSG_REPLACE) {
            mBrokerHandler.removeMessages(msg);
        } else if (existingMsgPolicy == SENDMSG_NOOP && mBrokerHandler.hasMessages(msg)) {
            return;
        }
 
        if (isMessageHandledUnderWakelock(msg)) {
            final long identity = Binder.clearCallingIdentity();
            try {
                mBrokerEventWakeLock.acquire(BROKER_WAKELOCK_TIMEOUT_MS);
            } catch (Exception e) {
                Log.e(TAG, "Exception acquiring wakelock", e);
            }
            Binder.restoreCallingIdentity(identity);
        }
 
        synchronized (sLastDeviceConnectionMsgTimeLock) {
            long time = SystemClock.uptimeMillis() + delay;
 
            switch (msg) {
                case MSG_IL_SET_A2DP_SOURCE_CONNECTION_STATE:
                case MSG_IL_SET_A2DP_SINK_CONNECTION_STATE:
                case MSG_IL_SET_HEARING_AID_CONNECTION_STATE:
                case MSG_L_SET_WIRED_DEVICE_CONNECTION_STATE:
                case MSG_IL_BTA2DP_DOCK_TIMEOUT:
                case MSG_L_A2DP_DEVICE_CONFIG_CHANGE:
                case MSG_L_A2DP_ACTIVE_DEVICE_CHANGE:
                    if (sLastDeviceConnectMsgTime >= time) {
                        // add a little delay to make sure messages are ordered as expected
                        time = sLastDeviceConnectMsgTime + 30;
                    }
                    sLastDeviceConnectMsgTime = time;
                    break;
                default:
                    break;
            }
         //传递进来的参数封装,交给mAudioHandler处理；
        //arg1是AudioSystem.FOR_COMMUNICATION, arg2是mForcedUseForComm, 
        //obj是eventSource,eventSource就是调用扬声器的进程的信息
        mBrokerHandler.sendMessageAtTime(mBrokerHandler.obtainMessage(msg, arg1, arg2, obj),
                    time);
        }
    }
            mBrokerHandler.sendMessageAtTime(mBrokerHandler.obtainMessage(msg, arg1, arg2, obj),
                    time);
        }
    }

根据代码逻辑看Message的参数arg1,arg2,obj 三个参数的意义

arg1 = AudioSystem.FOR_COMMUNICATION
arg2 = mForcedUseForComm,
obj = eventSource//eventSource就是调用扬声器的进程的信息

以上方法发送消息消息接收


 case MSG_IIL_SET_FORCE_USE: // intended fall-through
                case MSG_IIL_SET_FORCE_BT_A2DP_USE:
                    onSetForceUse(msg.arg1, msg.arg2, (String) msg.obj);

该方法的实现


//---------------------------------------------------------------------
    // Internal handling of messages
    // These methods are ALL synchronous, in response to message handling in BrokerHandler
    // Blocking in any of those will block the message queue
 
    private void onSetForceUse(int useCase, int config, String eventSource) {
        Log.d(TAG, "onSetForceUse  usage= "+useCase+"  config= "+config+"  eventSource="+eventSource);
        if (useCase == AudioSystem.FOR_MEDIA) {
            postReportNewRoutes();
        }
        AudioService.sForceUseLogger.log(
                new AudioServiceEvents.ForceUseEvent(useCase, config, eventSource));
        //将语音模式AudioSystem.FOR_COMMUNICATION,音频通道mForcedUseForComm交给AudioSystem,
        //AudioSystem会将其设置到HAL底层        
        AudioSystem.setForceUse(useCase, config);
    }

AudioService在调用AudioSystem.setForceUse(usage, config)方法时，会将相应的音频播放模式和音频通道设置到底层，从这里可以就看出，为什么在调用setSpeakerphoneOn()时要结合setMode()一起使用了；

我们看到调用了AudioSystem的setForceUse方法，该在AudioSystem中的实现是一个native方法。


//base/media/java/android/media/AudioSystem.java
@UnsupportedAppUsage
public static native int setForceUse(int usage, int config);

总结：

疑惑：

前面介绍中就说了android手机中有很多的音频输出通道，为啥AudioService只提供了setSpeakerphoneOn()和setBluetoothScoOn这两个手动切换音频输出通道的方法呢？
之所以AudioService只提供这两个方法，是因为有些切换是系统自动完成的，比如有线耳机，蓝牙耳机的插入和拔出等，这些音频外设的切换在应用层是无法处理的；

听筒，扬声器，有线耳机这三个输出设备的切换

1)听筒通道

听筒模式一般只会在通话或者语音过程中才会用到，所以，要使用听筒模式，必须得指定播放模式为MODE_IN_CALL或者是MODE_IN_COMMUNICATION；MODE_IN_CALL只有在通话时才可以用到。非通话时使用MODE_IN_COMMUNICATION。

2)扬声器通道

在不插入音频外设如耳机的情况下，手机中的输出设备只有听筒和扬声器，要想在听筒和扬声器中切换是比较容易的，无非就是setSpeakerphoneOn(boolean)方法调用以及设置播放模式为MODE_IN_CALL或者MODE_IN_COMMUNICATION；

3)有线耳机

耳机是音频外设，此时手机中的音频输出设备有3个，除了耳机还有听筒和扬声器；那底层是怎样选择一个设备进行音频输出的呢？这就和音频系统中的音频路由策略有关，底层在播放音频时会选择一个设备，这个逻辑跟设备的优先级有关，代码在AudoPolicyServcie中，有时间在剖析这个具体原理；

所以，当手机中的音频输出设备有耳机，听筒和扬声器时，会根据设备的优先级来进行选择；

从测试的结果来看，3个当中，耳机的优先级最高，其次是听筒；

参考文章：android音频系统(6)：AudioService之音频输出通道切换

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android10 AudioService之音频输出通道切换_android 音频输出

一音频输出通道

二 音频模式的理解和使用

三 流类型

四 音频输出通道，播放模式和流类型的关系

五 audioManager.setMode(int mode)的源码实现

六 设置音频输出管道

总结：

二音频模式的理解和使用

三流类型

四音频输出通道，播放模式和流类型的关系

六设置音频输出管道