在看AudioSessionId相關程式碼的時候瞭解到，共用一個AudioSessionId的AudioTrack和MediaPlayer會共用一個AudioEffect。
今天就來看看AudioEffect是個什麼東東。

看這個類的目的，主要是為了搞清楚AudioEffect是個什麼東東。
打算重點看看類的介紹及其建構函式上。

*****************************************原始碼*************************************************

public class AudioEffect {
    static {
        System.loadLibrary("audioeffect_jni");
        native_init();
    }
...
    public AudioEffect(UUID type, UUID uuid, int priority, int audioSession)
            throws IllegalArgumentException, UnsupportedOperationException,
            RuntimeException {
        int[] id = new int[1];
        Descriptor[] desc = new Descriptor[1];
        // native initialization
        int initResult = native_setup(new WeakReference<AudioEffect>(this),
                type.toString(), uuid.toString(), priority, audioSession, id,
                desc);
        if (initResult != SUCCESS && initResult != ALREADY_EXISTS) {
            Log.e(TAG, "Error code " + initResult
                    + " when initializing AudioEffect.");
            switch (initResult) {
            case ERROR_BAD_VALUE:
                throw (new IllegalArgumentException("Effect type: " + type
                        + " not supported."));
            case ERROR_INVALID_OPERATION:
                throw (new UnsupportedOperationException(
                        "Effect library not loaded"));
            default:
                throw (new RuntimeException(
                        "Cannot initialize effect engine for type: " + type
                                + "Error: " + initResult));
            }
        }
        mId = id[0];
        mDescriptor = desc[0];
        synchronized (mStateLock) {
            mState = STATE_INITIALIZED;
        }
    }
...
}
 

**********************************************************************************************
原始碼路徑：
frameworks\base\media\java\android\media\audiofx\AudioEffect.java

###########################################說明################################################
先看看整個類的註釋：

/**
 * AudioEffect is the base class for controlling audio effects provided by the android audio
 * framework.
 * <p>Applications should not use the AudioEffect class directly but one of its derived classes to
 * control specific effects:
 * <ul>
 *   <li> {@link android.media.audiofx.Equalizer}</li>
 *   <li> {@link android.media.audiofx.Virtualizer}</li>
 *   <li> {@link android.media.audiofx.BassBoost}</li>
 *   <li> {@link android.media.audiofx.PresetReverb}</li>
 *   <li> {@link android.media.audiofx.EnvironmentalReverb}</li>
 * </ul>
 * <p>If the audio effect is to be applied to a specific AudioTrack or MediaPlayer instance,
 * the application must specify the audio session ID of that instance when creating the AudioEffect.
 * (see {@link android.media.MediaPlayer#getAudioSessionId()} for details on audio sessions).
 * To apply an effect to the global audio output mix, session 0 must be specified when creating the
 * AudioEffect.
 * <p>Creating an effect on the output mix (audio session 0) requires permission
 * {@link android.Manifest.permission#MODIFY_AUDIO_SETTINGS}
 * <p>Creating an AudioEffect object will create the corresponding effect engine in the audio
 * framework if no instance of the same effect type exists in the specified audio session.
 * If one exists, this instance will be used.
 * <p>The application creating the AudioEffect object (or a derived class) will either receive
 * control of the effect engine or not depending on the priority parameter. If priority is higher
 * than the priority used by the current effect engine owner, the control will be transfered to the
 * new object. Otherwise control will remain with the previous object. In this case, the new
 * application will be notified of changes in effect engine state or control ownership by the
 * appropiate listener.
 */
 

 大致意思如下：
 AudioEffect是由android audio framework提供的控制audio effect的基類。
 應用程式不應該直接建立AudioEffect的物件，而應該建立由其派生的，控制特定effect的類的物件。
 可建立物件的派生類如下：
  *   <li> {@link android.media.audiofx.Equalizer}</li>
 *   <li> {@link android.media.audiofx.Virtualizer}</li>
 *   <li> {@link android.media.audiofx.BassBoost}</li>
 *   <li>

{@link android.media.audiofx.PresetReverb}</li>
 *   <li> {@link android.media.audiofx.EnvironmentalReverb}</li>
 * </ul>
 如果一個audio effect被指定給一個特定的AudioTrack或者MediaPlayer，建立AudioEffect物件的時候必須指定一個AudioSessionId。
 如果想建立一個對global audio output mix都起作用的audio effect，需要將AudioSessionId指定為0.
 想建立一個對global audio output mix都起作用的audio effect需要特定的許可：
  * {@link android.Manifest.permission#MODIFY_AUDIO_SETTINGS}
 建立AudioEffect物件時，如果指定的AudioSession中不存在該型別的AudioEffect，則會在audio framework中建立一個effect engine。
 否則，將使用已存在的AudioEffect物件。
 應用程式建立了一個AudioEffect物件後，能否取得effect enging的控制權，取決於優先順序。
 如果優先順序比正控制effect enging的應用程式的優先順序高，則會把控制權搶過來。
 否則只能眼睜睜的看著人家繼續控制。
 不過，如果effect enging的狀態或者所有權改變時，監聽器將會通知應用程式。
 
 再看看其建構函式及註釋

     // Constructor, Finalize
    // --------------------
    /**
     * Class constructor.
     *
     * @param type type of effect engine created. See {@link #EFFECT_TYPE_ENV_REVERB},
     *            {@link #EFFECT_TYPE_EQUALIZER} ... Types corresponding to
     *            built-in effects are defined by AudioEffect class. Other types
     *            can be specified provided they correspond an existing OpenSL
     *            ES interface ID and the corresponsing effect is available on
     *            the platform. If an unspecified effect type is requested, the
     *            constructor with throw the IllegalArgumentException. This
     *            parameter can be set to {@link #EFFECT_TYPE_NULL} in which
     *            case only the uuid will be used to select the effect.
     * @param uuid unique identifier of a particular effect implementation.
     *            Must be specified if the caller wants to use a particular
     *            implementation of an effect type. This parameter can be set to
     *            {@link #EFFECT_TYPE_NULL} in which case only the type will
     *            be used to select the effect.
     * @param priority the priority level requested by the application for
     *            controlling the effect engine. As the same effect engine can
     *            be shared by several applications, this parameter indicates
     *            how much the requesting application needs control of effect
     *            parameters. The normal priority is 0, above normal is a
     *            positive number, below normal a negative number.
     * @param audioSession system wide unique audio session identifier. If audioSession
     *            is not 0, the effect will be attached to the MediaPlayer or
     *            AudioTrack in the same audio session. Otherwise, the effect
     *            will apply to the output mix.
     *
     * @throws java.lang.IllegalArgumentException
     * @throws java.lang.UnsupportedOperationException
     * @throws java.lang.RuntimeException
     * @hide
     */

  註釋中詳細介紹了各個引數。
 
  引數type是要建立的effect的型別。標準的型別已經在AudioEffect類中定義了，可以直接使用。
  若要使用其他的型別，需要提供OpenSLES interface ID，並且在平臺中存在該型別。
  若指定的型別不存在，則會丟擲IllegalArgumentException異常。
  若type為EFFECT_TYPE_NULL，則只會根據uuid來選擇effect。
  標準的型別如下：

//++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
    /**
     * The following UUIDs define effect types corresponding to standard audio
     * effects whose implementation and interface conform to the OpenSL ES
     * specification. The definitions match the corresponding interface IDs in
     * OpenSLES_IID.h
     */
    /**
     * UUID for environmental reverb effect
     * @hide
     */
    public static final UUID EFFECT_TYPE_ENV_REVERB = UUID
            .fromString("c2e5d5f0-94bd-4763-9cac-4e234d06839e");
    /**
     * UUID for preset reverb effect
     * @hide
     */
    public static final UUID EFFECT_TYPE_PRESET_REVERB = UUID
            .fromString("47382d60-ddd8-11db-bf3a-0002a5d5c51b");
    /**
     * UUID for equalizer effect
     * @hide
     */
    public static final UUID EFFECT_TYPE_EQUALIZER = UUID
            .fromString("0bed4300-ddd6-11db-8f34-0002a5d5c51b");
    /**
     * UUID for bass boost effect
     * @hide
     */
    public static final UUID EFFECT_TYPE_BASS_BOOST = UUID
            .fromString("0634f220-ddd4-11db-a0fc-0002a5d5c51b");
    /**
     * UUID for virtualizer effect
     * @hide
     */
    public static final UUID EFFECT_TYPE_VIRTUALIZER = UUID
            .fromString("37cc2c00-dddd-11db-8577-0002a5d5c51b");
    /**
     * Null effect UUID. Used when the UUID for effect type of
     * @hide
     */
    public static final UUID EFFECT_TYPE_NULL = UUID
            .fromString("ec7178ec-e5e1-4432-a3f4-4657e6795210");
//----------------------------------------------------------------

  引數uuid用於指定一個特定effect的實現。
  若uuid為EFFECT_TYPE_NULL，則只會根據type來選擇effect。
  關於uuid的取值，搜了下程式碼，發現，如果type不為EFFECT_TYPE_NULL，uuid多被指定為EFFECT_TYPE_NULL。
  uuid不為EFFECT_TYPE_NULL的地方，type多被指定為EFFECT_TYPE_NULL。
  基本上是靠兩個中的一個來選擇型別的，為何要兩個引數？
  難道type用來指定標準的？uuid用來指定其他的？
  發現有如下的程式碼：

//++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
            // creating a volume controller on output mix ensures that ro.audio.silent mutes
            // audio after the effects and not before
            vc = new AudioEffect(
                    AudioEffect.EFFECT_TYPE_NULL,
                    UUID.fromString("119341a0-8469-11df-81f9-0002a5d5c51b"),
                      0,
                      0);
//----------------------------------------------------------------

 
  priority是優先順序，前文有說過，會根據優先順序來判斷effect engine的控制權。
 
  audioSession我們已經多次提到過。
  相同audioSession ID的AudioTrack和MediaPlayer共享Audio Effect。
  

    public AudioEffect(UUID type, UUID uuid, int priority, int audioSession)
            throws IllegalArgumentException, UnsupportedOperationException,
            RuntimeException {
        int[] id = new int[1];
// ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
// 類Descriptor的註釋：
    /**
     * The effect descriptor contains information on a particular effect implemented in the
     * audio framework:<br>
     * <ul>
     *  <li>type: UUID corresponding to the OpenSL ES interface implemented by this effect</li>
     *  <li>uuid: UUID for this particular implementation</li>
     *  <li>connectMode: {@link #EFFECT_INSERT} or {@link #EFFECT_AUXILIARY}</li>
     *  <li>name: human readable effect name</li>
     *  <li>implementor: human readable effect implementor name</li>
     * </ul>
     * The method {@link #queryEffects()} returns an array of Descriptors to facilitate effects
     * enumeration.
     */
// ----------------------------------------------------------------
        Descriptor[] desc = new Descriptor[1];
        // native initialization
  // native_setup在以前有說過。
  // 該函式肯定會調到native中的XXX_native_setup之類的函式中，
  // 並最終建立一個native側的AudioEffect物件，並通過函式函式SetIntField儲存到java側。
// ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
// 果然，在檔案frameworks\base\media\audioeffect\android_media_AudioEffect.cpp中找到了JNI函式對照表：
// Dalvik VM type signatures
static JNINativeMethod gMethods[] = {
    {"native_init",          "()V",      (void *)android_media_AudioEffect_native_init},
    {"native_setup",         "(Ljava/lang/Object;Ljava/lang/String;Ljava/lang/String;II[I[Ljava/lang/Object;)I",
                                         (void *)android_media_AudioEffect_native_setup},
    {"native_finalize",      "()V",      (void *)android_media_AudioEffect_native_finalize},
    {"native_release",       "()V",      (void *)android_media_AudioEffect_native_release},
    {"native_setEnabled",    "(Z)I",      (void *)android_media_AudioEffect_native_setEnabled},
    {"native_getEnabled",    "()Z",      (void *)android_media_AudioEffect_native_getEnabled},
    {"native_hasControl",    "()Z",      (void *)android_media_AudioEffect_native_hasControl},
    {"native_setParameter",  "(I[BI[B)I",  (void *)android_media_AudioEffect_native_setParameter},
    {"native_getParameter",  "(I[B[I[B)I",  (void *)android_media_AudioEffect_native_getParameter},
    {"native_command",       "(II[B[I[B)I", (void *)android_media_AudioEffect_native_command},
    {"native_query_effects", "()[Ljava/lang/Object;", (void *)android_media_AudioEffect_native_queryEffects},
};
// ----------------------------------------------------------------
// ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
// 函式android_media_AudioEffect_native_setup的實現也在檔案frameworks\base\media\audioeffect\android_media_AudioEffect.cpp中。
static jint
android_media_AudioEffect_native_setup(JNIEnv *env, jobject thiz, jobject weak_this,
        jstring type, jstring uuid, jint priority, jint sessionId, jintArray jId, jobjectArray javadesc)
{
    LOGV("android_media_AudioEffect_native_setup");
    AudioEffectJniStorage* lpJniStorage = NULL;
    int lStatus = AUDIOEFFECT_ERROR_NO_MEMORY;
    AudioEffect* lpAudioEffect = NULL;
    jint* nId = NULL;
    const char *typeStr = NULL;
    const char *uuidStr = NULL;
    effect_descriptor_t desc;
    jobject jdesc;
    char str[EFFECT_STRING_LEN_MAX];
    jstring jdescType;
    jstring jdescUuid;
    jstring jdescConnect;
    jstring jdescName;
    jstring jdescImplementor;
 // 下面的幾步都是引數的獲取與判斷
    if (type != NULL) {
        typeStr = env->GetStringUTFChars(type, NULL);
        if (typeStr == NULL) {  // Out of memory
            jniThrowException(env, "java/lang/RuntimeException", "Out of memory");
            goto setup_failure;
        }
    }
    if (uuid != NULL) {
        uuidStr = env->GetStringUTFChars(uuid, NULL);
        if (uuidStr == NULL) {  // Out of memory
            jniThrowException(env, "java/lang/RuntimeException", "Out of memory");
            goto setup_failure;
        }
    }
    if (typeStr == NULL && uuidStr == NULL) {
        lStatus = AUDIOEFFECT_ERROR_BAD_VALUE;
        goto setup_failure;
    }
 // 此處建立了一個AudioEffectJniStorage物件。
 // 在看AudioTrack程式碼的時候，見到過相似的類。
 // 是Android程序間共享記憶體用的。
    lpJniStorage = new AudioEffectJniStorage();
    if (lpJniStorage == NULL) {
        LOGE("setup: Error creating JNI Storage");
        goto setup_failure;
    }
    lpJniStorage->mCallbackData.audioEffect_class = (jclass)env->NewGlobalRef(fields.clazzEffect);
    // we use a weak reference so the AudioEffect object can be garbage collected.
    lpJniStorage->mCallbackData.audioEffect_ref = env->NewGlobalRef(weak_this);
    LOGV("setup: lpJniStorage: %p audioEffect_ref %p audioEffect_class %p, &mCallbackData %p",
            lpJniStorage,
            lpJniStorage->mCallbackData.audioEffect_ref,
            lpJniStorage->mCallbackData.audioEffect_class,
            &lpJniStorage->mCallbackData);
    if (jId == NULL) {
        LOGE("setup: NULL java array for id pointer");
        lStatus = AUDIOEFFECT_ERROR_BAD_VALUE;
        goto setup_failure;
    }
 // 如我們所料，此處建立了一個native側的AudioEffect（class AudioEffect : public RefBase）物件。
// ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
// 關於類AudioEffect的說明。與java側類的說明意思相同。
    /* Constructor.
     * AudioEffect is the base class for creating and controlling an effect engine from
     * the application process. Creating an AudioEffect object will create the effect engine
     * in the AudioFlinger if no engine of the specified type exists. If one exists, this engine
     * will be used. The application creating the AudioEffect object (or a derived class like
     * Reverb for instance) will either receive control of the effect engine or not, depending
     * on the priority parameter. If priority is higher than the priority used by the current
     * effect engine owner, the control will be transfered to the new application. Otherwise
     * control will remain to the previous application. In this case, the new application will be
     * notified of changes in effect engine state or control ownership by the effect callback.
     * After creating the AudioEffect, the application must call the initCheck() method and
     * check the creation status before trying to control the effect engine (see initCheck()).
     * If the effect is to be applied to an AudioTrack or MediaPlayer only the application
     * must specify the audio session ID corresponding to this player.
     */
// ----------------------------------------------------------------
// ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
// 建構函式中，各引數的說明。
    /* Constructor.
     *
     * Parameters:
     *
     * type:  type of effect created: can be null if uuid is specified. This corresponds to
     *        the OpenSL ES interface implemented by this effect.
     * uuid:  Uuid of effect created: can be null if type is specified. This uuid corresponds to
     *        a particular implementation of an effect type.
     * priority:    requested priority for effect control: the priority level corresponds to the
     *      value of priority parameter: negative values indicate lower priorities, positive values
     *      higher priorities, 0 being the normal priority.
     * cbf:         optional callback function (see effect_callback_t)
     * user:        pointer to context for use by the callback receiver.
     * sessionID:   audio session this effect is associated to. If 0, the effect will be global to
     *      the output mix. If not 0, the effect will be applied to all players
     *      (AudioTrack or MediaPLayer) within the same audio session.
     * output:  HAL audio output stream to which this effect must be attached. Leave at 0 for
     *      automatic output selection by AudioFlinger.
     */
// 這些引數以前都有遇到過。
----------------------------------------------------------------
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
// 類AudioEffect的建構函式程式碼：
//（可見其中沒做太多工作。
// 主要工作應該是在set函式中完成的。）
AudioEffect::AudioEffect(const char *typeStr,
                const char *uuidStr,
                int32_t priority,
                effect_callback_t cbf,
                void* user,
                int sessionId,
                audio_io_handle_t output
                )
    : mStatus(NO_INIT)
{
    effect_uuid_t type;
    effect_uuid_t *pType = NULL;
    effect_uuid_t uuid;
    effect_uuid_t *pUuid = NULL;
    LOGV("Constructor string\n - type: %s\n - uuid: %s", typeStr, uuidStr);
    if (typeStr != NULL) {
        if (stringToGuid(typeStr, &type) == NO_ERROR) {
            pType = &type;
        }
    }
    if (uuidStr != NULL) {
        if (stringToGuid(uuidStr, &uuid) == NO_ERROR) {
            pUuid = &uuid;
        }
    }
// ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
// set函式註釋：
    /* Initialize an uninitialized AudioEffect.
    * Returned status (from utils/Errors.h) can be:
    *  - NO_ERROR or ALREADY_EXISTS: successful initialization
    *  - INVALID_OPERATION: AudioEffect is already initialized
    *  - BAD_VALUE: invalid parameter
    *  - NO_INIT: audio flinger or audio hardware not initialized
    * */
 
// 程式碼：
status_t AudioEffect::set(const effect_uuid_t *type,
                const effect_uuid_t *uuid,
                int32_t priority,
                effect_callback_t cbf,
                void* user,
                int sessionId,
                audio_io_handle_t output)
{
    sp<IEffect> iEffect;
    sp<IMemory> cblk;
    int enabled;
    LOGV("set %p mUserData: %p", this, user);
    if (mIEffect != 0) {
        LOGW("Effect already in use");
        return INVALID_OPERATION;
    }
    const sp<IAudioFlinger>& audioFlinger = AudioSystem::get_audio_flinger();
    if (audioFlinger == 0) {
        LOGE("set(): Could not get audioflinger");
        return NO_INIT;
    }
    if (type == NULL && uuid == NULL) {
        LOGW("Must specify at least type or uuid");
        return BAD_VALUE;
    }
    mPriority = priority;
    mCbf = cbf;
    mUserData = user;
    mSessionId = sessionId;
    memset(&mDescriptor, 0, sizeof(effect_descriptor_t));
    memcpy(&mDescriptor.type, EFFECT_UUID_NULL, sizeof(effect_uuid_t));
    memcpy(&mDescriptor.uuid, EFFECT_UUID_NULL, sizeof(effect_uuid_t));
    if (type != NULL) {
        memcpy(&mDescriptor.type, type, sizeof(effect_uuid_t));
    }
    if (uuid != NULL) {
        memcpy(&mDescriptor.uuid, uuid, sizeof(effect_uuid_t));
    }
 // 這邊只是相當於客戶端，服務端在audioflinger側
 // Implements the IEffectClient interface
 // class EffectClient : public android::BnEffectClient,  public android::IBinder::DeathRecipient
    mIEffectClient = new EffectClient(this);
// ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
sp<IEffect> AudioFlinger::createEffect(pid_t pid,
        effect_descriptor_t *pDesc,
        const sp<IEffectClient>& effectClient,
        int32_t priority,
        int output,
        int sessionId,
        status_t *status,
        int *id,
        int *enabled)
{
    status_t lStatus = NO_ERROR;
    sp<EffectHandle> handle;
    effect_interface_t itfe;
    effect_descriptor_t desc;
    sp<Client> client;
    wp<Client> wclient;
    LOGV("createEffect pid %d, client %p, priority %d, sessionId %d, output %d",
            pid, effectClient.get(), priority, sessionId, output);
 // 引數檢查
    if (pDesc == NULL) {
        lStatus = BAD_VALUE;
        goto Exit;
    }
 // AudioSystem::SESSION_OUTPUT_MIX其實就是0.
 // 介紹AudioSessionId的時候，說過如果AudioSessionid為0，則AudioEffect對整個output mix都起作用。
 // 不過需要有特殊的許可。該許可通過函式settingsAllowed來判斷
// ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
// 函式settingsAllowed的實現：
static bool settingsAllowed() {
#ifndef HAVE_ANDROID_OS
    return true;
#endif
#if AUDIOFLINGER_SECURITY_ENABLED
    if (getpid() == IPCThreadState::self()->getCallingPid()) return true;
    bool ok = checkCallingPermission(String16("android.permission.MODIFY_AUDIO_SETTINGS"));
    if (!ok) LOGE("Request requires android.permission.MODIFY_AUDIO_SETTINGS");
    return ok;
#else
    if (!checkCallingPermission(String16("android.permission.MODIFY_AUDIO_SETTINGS")))
        LOGW("WARNING: Need to add android.permission.MODIFY_AUDIO_SETTINGS to manifest");
    return true;
#endif
}
// ----------------------------------------------------------------
    // check audio settings permission for global effects
    if (sessionId == AudioSystem::SESSION_OUTPUT_MIX && !settingsAllowed()) {
        lStatus = PERMISSION_DENIED;
        goto Exit;
    }
// ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
        SESSION_OUTPUT_STAGE = -1, // session for effects attached to a particular output stream
                                   // (value must be less than 0)
// ----------------------------------------------------------------
    // Session AudioSystem::SESSION_OUTPUT_STAGE is reserved for output stage effects
    // that can only be created by audio policy manager (running in same process)
    if (sessionId == AudioSystem::SESSION_OUTPUT_STAGE && getpid() != pid) {
        lStatus = PERMISSION_DENIED;
        goto Exit;
    }
    // check recording permission for visualizer
    if ((memcmp(&pDesc->type, SL_IID_VISUALIZATION, sizeof(effect_uuid_t)) == 0 ||
         memcmp(&pDesc->uuid, &VISUALIZATION_UUID_, sizeof(effect_uuid_t)) == 0) &&
        !recordingAllowed()) {
        lStatus = PERMISSION_DENIED;
        goto Exit;
    }
    if (output == 0) {
        if (sessionId == AudioSystem::SESSION_OUTPUT_STAGE) {
  // 這種情況下，output應該由AudioPolicyManager來指定
            // output must be specified by AudioPolicyManager when using session
            // AudioSystem::SESSION_OUTPUT_STAGE
            lStatus = BAD_VALUE;
            goto Exit;
        } else if (sessionId == AudioSystem::SESSION_OUTPUT_MIX) {
            // if the output returned by getOutputForEffect() is removed before we lock the
            // mutex below, the call to checkPlaybackThread_l(output) below will detect it
            // and we will exit safely
// ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
audio_io_handle_t AudioSystem::getOutputForEffect(effect_descriptor_t *desc)
{
    const sp<IAudioPolicyService>& aps = AudioSystem::get_audio_policy_service();
    if (aps == 0) return PERMISSION_DENIED;
// ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
    virtual audio_io_handle_t getOutputForEffect(effect_descriptor_t *desc)
    {
        Parcel data, reply;
        data.writeInterfaceToken(IAudioPolicyService::getInterfaceDescriptor());
        data.write(desc, sizeof(effect_descriptor_t));
// ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
status_t BnAudioPolicyService::onTransact(
    uint32_t code, const Parcel& data, Parcel* reply, uint32_t flags)
{
...
        case GET_OUTPUT_FOR_EFFECT: {
            CHECK_INTERFACE(IAudioPolicyService, data, reply);
            effect_descriptor_t desc;
            data.read(&desc, sizeof(effect_descriptor_t));
// ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
audio_io_handle_t AudioPolicyService::getOutputForEffect(effect_descriptor_t *desc)
{
    if (mpPolicyManager == NULL) {
        return NO_INIT;
    }
    Mutex::Autolock _l(mLock);
// ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
audio_io_handle_t AudioPolicyManagerBase::getOutputForEffect(effect_descriptor_t *desc)
{
    LOGV("getOutputForEffect()");
 // 函式getOutput以前有看過，不過當時由於篇幅問題，未列出程式碼，今天列出來看看
// ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
audio_io_handle_t AudioPolicyManagerBase::getOutput(AudioSystem::stream_type stream,
                                    uint32_t samplingRate,
                                    uint32_t format,
                                    uint32_t channels,
                                    AudioSystem::output_flags flags)
{
    audio_io_handle_t output = 0;
    uint32_t latency = 0;
 // 獲取策略
// ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
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;
    case AudioSystem::RING:
    case AudioSystem::NOTIFICATION:
    case AudioSystem::ALARM:
    case AudioSystem::ENFORCED_AUDIBLE:
        return STRATEGY_SONIFICATION;
    case AudioSystem::DTMF:
        return STRATEGY_DTMF;
    default:
        LOGE("unknown stream type");
    case AudioSystem::SYSTEM:
        // NOTE: SYSTEM stream uses MEDIA strategy because muting music and switching outputs
        // while key clicks are played produces a poor result
    case AudioSystem::TTS:
    case AudioSystem::MUSIC:
        return STRATEGY_MEDIA;
    }
}
// ----------------------------------------------------------------
    routing_strategy strategy = getStrategy((AudioSystem::stream_type)stream);
 // 獲取device
// ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
uint32_t AudioPolicyManagerBase::getDeviceForStrategy(routing_strategy strategy, bool fromCache)
{
    uint32_t device = 0;
    if (fromCache) {
        LOGV("getDeviceForStrategy() from cache strategy %d, device %x", strategy, mDeviceForStrategy[strategy]);
  // mDeviceForStrategy在以下函式中賦值
// ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
void AudioPolicyManagerBase::updateDeviceForStrategy()
{
    for (int i = 0; i < NUM_STRATEGIES; i++) {
        mDeviceForStrategy[i] = getDeviceForStrategy((routing_strategy)i, false);
    }
}
// ----------------------------------------------------------------
        return mDeviceForStrategy[strategy];
    }
    switch (strategy) {
    case STRATEGY_DTMF:
        if (!isInCall()) {
            // when off call, DTMF strategy follows the same rules as MEDIA strategy
            device = getDeviceForStrategy(STRATEGY_MEDIA, false);
            break;
        }
        // when in call, DTMF and PHONE strategies follow the same rules
        // FALL THROUGH
    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]) {
        case AudioSystem::FORCE_BT_SCO:
            if (!isInCall() || strategy != STRATEGY_DTMF) {
   // 所以device的型別
// ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
    // output devices
    public static final int DEVICE_OUT_EARPIECE = 0x1;
    public static final int DEVICE_OUT_SPEAKER = 0x2;
    public static final int DEVICE_OUT_WIRED_HEADSET = 0x4;
    public static final int DEVICE_OUT_WIRED_HEADPHONE = 0x8;
    public static final int DEVICE_OUT_BLUETOOTH_SCO = 0x10;
    public static final int DEVICE_OUT_BLUETOOTH_SCO_HEADSET = 0x20;
    public static final int DEVICE_OUT_BLUETOOTH_SCO_CARKIT = 0x40;
    public static final int DEVICE_OUT_BLUETOOTH_A2DP = 0x80;
    public static final int DEVICE_OUT_BLUETOOTH_A2DP_HEADPHONES = 0x100;
    public static final int DEVICE_OUT_BLUETOOTH_A2DP_SPEAKER = 0x200;
    public static final int DEVICE_OUT_AUX_DIGITAL = 0x400;
    public static final int DEVICE_OUT_WIRED_HDMI = 0x4000;
    public static final int DEVICE_OUT_DEFAULT = 0x8000;
    // input devices
    public static final int DEVICE_IN_COMMUNICATION = 0x10000;
    public static final int DEVICE_IN_AMBIENT = 0x20000;
    public static final int DEVICE_IN_BUILTIN_MIC1 = 0x40000;
    public static final int DEVICE_IN_BUILTIN_MIC2 = 0x80000;
    public static final int DEVICE_IN_MIC_ARRAY = 0x100000;
    public static final int DEVICE_IN_BLUETOOTH_SCO_HEADSET = 0x200000;
    public static final int DEVICE_IN_WIRED_HEADSET = 0x400000;
    public static final int DEVICE_IN_AUX_DIGITAL = 0x800000;
    public static final int DEVICE_IN_DEFAULT = 0x80000000;
// ----------------------------------------------------------------
                device = mAvailableOutputDevices & AudioSystem::DEVICE_OUT_BLUETOOTH_SCO_CARKIT;
                if (device) break;
            }
            device = mAvailableOutputDevices & AudioSystem::DEVICE_OUT_BLUETOOTH_SCO_HEADSET;
            if (device) break;
            device = mAvailableOutputDevices & AudioSystem::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
            device = mAvailableOutputDevices & AudioSystem::DEVICE_OUT_WIRED_HEADPHONE;
            if (device) break;
            device = mAvailableOutputDevices & AudioSystem::DEVICE_OUT_WIRED_HEADSET;
            if (device) break;
#ifdef WITH_A2DP
            // when not in a phone call, phone strategy should route STREAM_VOICE_CALL to A2DP
            if (!isInCall()) {
                device = mAvailableOutputDevices & AudioSystem::DEVICE_OUT_BLUETOOTH_A2DP;
                if (device) break;
                device = mAvailableOutputDevices & AudioSystem::DEVICE_OUT_BLUETOOTH_A2DP_HEADPHONES;
                if (device) break;
            }
#endif
            device = mAvailableOutputDevices & AudioSystem::DEVICE_OUT_EARPIECE;
            if (device == 0) {
                LOGE("getDeviceForStrategy() earpiece device not found");
            }
            break;
        case AudioSystem::FORCE_SPEAKER:
            if (!isInCall() || strategy != STRATEGY_DTMF) {
                device = mAvailableOutputDevices & AudioSystem::DEVICE_OUT_BLUETOOTH_SCO_CARKIT;
                if (device) break;
            }
#ifdef WITH_A2DP
            // when not in a phone call, phone strategy should route STREAM_VOICE_CALL to
            // A2DP speaker when forcing to speaker output
            if (!isInCall()) {
                device = mAvailableOutputDevices & AudioSystem::DEVICE_OUT_BLUETOOTH_A2DP_SPEAKER;
                if (device) break;
            }
#endif
            device = mAvailableOutputDevices & AudioSystem::DEVICE_OUT_SPEAKER;
            if (device == 0) {
                LOGE("getDeviceForStrategy() speaker device not found");
            }
            break;
        }
    break;
    case STRATEGY_SONIFICATION:
        // If incall, just select the STRATEGY_PHONE device: The rest of the behavior is handled by
        // handleIncallSonification().
        if (isInCall()) {
            device = getDeviceForStrategy(STRATEGY_PHONE, false);
            break;
        }
        device = mAvailableOutputDevices & AudioSystem::DEVICE_OUT_SPEAKER;
        if (device == 0) {
            LOGE("getDeviceForStrategy() speaker device not found");
        }
        // The second device used for sonification is the same as the device used by media strategy
        // FALL THROUGH
    case STRATEGY_MEDIA: {
        uint32_t device2 = mAvailableOutputDevices & AudioSystem::DEVICE_OUT_AUX_DIGITAL;
 
        if (device2 == 0) {
            device2 = mAvailableOutputDevices & AudioSystem::DEVICE_OUT_WIRED_HDMI;
        }
        if (device2 == 0) {
            device2 = mAvailableOutputDevices & AudioSystem::DEVICE_OUT_WIRED_HEADPHONE;
        }
        if (device2 == 0) {
            device2 = mAvailableOutputDevices & AudioSystem::DEVICE_OUT_WIRED_HEADSET;
        }
#ifdef WITH_A2DP
        if (mA2dpOutput != 0) {
            if (strategy == STRATEGY_SONIFICATION && !a2dpUsedForSonification()) {
                break;
            }
            if (device2 == 0) {
                device2 = mAvailableOutputDevices & AudioSystem::DEVICE_OUT_BLUETOOTH_A2DP;
            }
            if (device2 == 0) {
                device2 = mAvailableOutputDevices & AudioSystem::DEVICE_OUT_BLUETOOTH_A2DP_HEADPHONES;
            }
            if (device2 == 0) {
                device2 = mAvailableOutputDevices & AudioSystem::DEVICE_OUT_BLUETOOTH_A2DP_SPEAKER;
            }
        }
#endif
        if (device2 == 0) {
            device2 = mAvailableOutputDevices & AudioSystem::DEVICE_OUT_SPEAKER;
        }
        // device is DEVICE_OUT_SPEAKER if we come from case STRATEGY_SONIFICATION, 0 otherwise
        device |= device2;
        if (device == 0) {
            LOGE("getDeviceForStrategy() speaker device not found");
        }
        } break;
    default:
        LOGW("getDeviceForStrategy() unknown strategy: %d", strategy);
        break;
    }
    LOGV("getDeviceForStrategy() strategy %d, device %x", strategy, device);
    return device;
}
// ----------------------------------------------------------------
    uint32_t device = getDeviceForStrategy(strategy);
    LOGV("getOutput() stream %d, samplingRate %d, format %d, channels %x, flags %x", stream, samplingRate, format, channels, flags);
#ifdef AUDIO_POLICY_TEST
    if (mCurOutput != 0) {
        LOGV("getOutput() test output mCurOutput %d, samplingRate %d, format %d, channels %x, mDirectOutput %d",
                mCurOutput, mTestSamplingRate, mTestFormat, mTestChannels, mDirectOutput);
        if (mTestOutputs[mCurOutput] == 0) {
            LOGV("getOutput() opening test output");
            AudioOutputDescriptor *outputDesc = new AudioOutputDescriptor();
            outputDesc->mDevice = mTestDevice;
            outputDesc->mSamplingRate = mTestSamplingRate;
            outputDesc->mFormat = mTestFormat;
            outputDesc->mChannels = mTestChannels;
            outputDesc->mLatency = mTestLatencyMs;
            outputDesc->mFlags = (AudioSystem::output_flags)(mDirectOutput ? AudioSystem::OUTPUT_FLAG_DIRECT : 0);
            outputDesc->mRefCount[stream] = 0;
   // mpClientInterface在建構函式中被賦值。
// ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
AudioPolicyManagerBase::AudioPolicyManagerBase(AudioPolicyClientInterface *clientInterface)
    :
#ifdef AUDIO_POLICY_TEST
    Thread(false),
#endif //AUDIO_POLICY_TEST
    mPhoneState(AudioSystem::MODE_NORMAL), mRingerMode(0),
    mMusicStopTime(0), mLimitRingtoneVolume(false), mLastVoiceVolume(-1.0f),
    mTotalEffectsCpuLoad(0), mTotalEffectsMemory(0),
    mA2dpSuspended(false)
{
    mpClientInterface = clientInterface;
    for (int i = 0; i < AudioSystem::NUM_FORCE_USE; i++) {
        mForceUse[i] = AudioSystem::FORCE_NONE;
    }
    // devices available by default are speaker, ear piece and microphone
    mAvailableOutputDevices = AudioSystem::DEVICE_OUT_EARPIECE |
                        AudioSystem::DEVICE_OUT_SPEAKER;
    mAvailableInputDevices = AudioSystem::DEVICE_IN_BUILTIN_MIC;
#ifdef WITH_A2DP
    mA2dpOutput = 0;
    mDuplicatedOutput = 0;
    mA2dpDeviceAddress = String8("");
#endif
    mScoDeviceAddress = String8("");
    // open hardware output
    AudioOutputDescriptor *outputDesc = new AudioOutputDescriptor();
    outputDesc->mDevice = (uint32_t)AudioSystem::DEVICE_OUT_SPEAKER;
    mHardwareOutput = mpClientInterface->openOutput(&outputDesc->mDevice,
                                    &outputDesc->mSamplingRate,
                                    &outputDesc->mFormat,
                                    &outputDesc->mChannels,
                                    &outputDesc->mLatency,
                                    outputDesc->mFlags);
    if (mHardwareOutput == 0) {
        LOGE("Failed to initialize hardware output stream, samplingRate: %d, format %d, channels %d",
                outputDesc->mSamplingRate, outputDesc->mFormat, outputDesc->mChannels);
    } else {
        addOutput(mHardwareOutput, outputDesc);
        setOutputDevice(mHardwareOutput, (uint32_t)AudioSystem::DEVICE_OUT_SPEAKER, true);
        //TODO: configure audio effect output stage here
    }
    updateDeviceForStrategy();
#ifdef AUDIO_POLICY_TEST
    AudioParameter outputCmd = AudioParameter();
    outputCmd.addInt(String8("set_id"), 0);
    mpClientInterface->setParameters(mHardwareOutput, outputCmd.toString());
    mTestDevice = AudioSystem::DEVICE_OUT_SPEAKER;
    mTestSamplingRate = 44100;
    mTestFormat = AudioSystem::PCM_16_BIT;
    mTestChannels =  AudioSystem::CHANNEL_OUT_STEREO;
    mTestLatencyMs = 0;
    mCurOutput = 0;
    mDirectOutput = false;
    for (int i = 0; i < NUM_TEST_OUTPUTS; i++) {
        mTestOutputs[i] = 0;
    }
    const size_t SIZE = 256;
    char buffer[SIZE];
    snprintf(buffer, SIZE, "AudioPolicyManagerTest");
    run(buffer, ANDROID_PRIORITY_AUDIO);
#endif //AUDIO_POLICY_TEST
}
// ----------------------------------------------------------------
// AudioPolicyManagerBase物件在AudioPolicyService的建構函式中被建立
// ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
AudioPolicyService::AudioPolicyService()
    : BnAudioPolicyService() , mpPolicyManager(NULL)
{
    char value[PROPERTY_VALUE_MAX];
    // start tone playback thread
    mTonePlaybackThread = new AudioCommandThread(String8(""));
    // start audio commands thread
    mAudioCommandThread = new AudioCommandThread(String8("ApmCommandThread"));
#if (defined GENERIC_AUDIO) || (defined AUDIO_POLICY_TEST)
    mpPolicyManager = new AudioPolicyManagerBase(this);
    LOGV("build for GENERIC_AUDIO - using generic audio policy");
#else
    // if running in emulation - use the emulator driver
    if (property_get("ro.kernel.qemu", value, 0)) {
        LOGV("Running in emulation - using generic audio policy");
        mpPolicyManager = new AudioPolicyManagerBase(this);
    }
    else {
        LOGV("Using hardware specific audio policy");
  // 我們用到的肯定是這兒的
  // 函式createAudioPolicyManager的實現都在hardware中
  // 我們使用到的是alsa中的
// ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
extern "C" AudioPolicyInterface* createAudioPolicyManager(AudioPolicyClientInterface *clientInterface)
{
// AudioPolicyManagerALSA的建構函式中沒作什麼特殊處理
// ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
// Nothing currently different between the Base implementation.
// 註釋說它和base沒什麼差別
// getOutput函式中呼叫的openOutput介面，應該是在AudioPolicyService中實現的了
AudioPolicyManagerALSA::AudioPolicyManagerALSA(AudioPolicyClientInterface *clientInterface)
    : AudioPolicyManagerBase(clientInterface)
{
}
// ----------------------------------------------------------------
    return new AudioPolicyManagerALSA(clientInterface);
}
// ----------------------------------------------------------------
        mpPolicyManager = createAudioPolicyManager(this);
    }
#endif
    // load properties
    property_get("ro.camera.sound.forced", value, "0");
    mpPolicyManager->setSystemProperty("ro.camera.sound.forced", value);
}
// ----------------------------------------------------------------
// ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
audio_io_handle_t AudioPolicyService::openOutput(uint32_t *pDevices,
                                uint32_t *pSamplingRate,
                                uint32_t *pFormat,
                                uint32_t *pChannels,
                                uint32_t *pLatencyMs,
                                AudioSystem::output_flags flags)
{
    sp<IAudioFlinger> af = AudioSystem::get_audio_flinger();
    if (af == 0) {
        LOGW("openOutput() could not get AudioFlinger");
        return 0;
    }
 // 又調到了audioflinger中
// ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
int AudioFlinger::openOutput(uint32_t *pDevices,
                                uint32_t *pSamplingRate,
                                uint32_t *pFormat,
                                uint32_t *pChannels,
                                uint32_t *pLatencyMs,
                                uint32_t flags)
{
    status_t status;
    PlaybackThread *thread = NULL;
    mHardwareStatus = AUDIO_HW_OUTPUT_OPEN;
    uint32_t samplingRate = pSamplingRate ? *pSamplingRate : 0;
    uint32_t format = pFormat ? *pFormat : 0;
    uint32_t channels = pChannels ? *pChannels : 0;
    uint32_t latency = pLatencyMs ? *pLatencyMs : 0;
    LOGV("openOutput(), Device %x, SamplingRate %d, Format %d, Channels %x, flags %x",
            pDevices ? *pDevices : 0,
            samplingRate,
            format,
            channels,
            flags);
    if (pDevices == NULL || *pDevices == 0) {
        return 0;
    }
    Mutex::Autolock _l(mLock);
// 又調到了HAL層的openOutputStream函式
    AudioStreamOut *output = mAudioHardware->openOutputStream(*pDevices,
                                                             (int *)&format,
                                                             &channels,
                                                             &samplingRate,
                                                             &status);
    LOGV("openOutput() openOutputStream returned output %p, SamplingRate %d, Format %d, Channels %x, status %d",
            output,
            samplingRate,
            format,
            channels,
            status);
    mHardwareStatus = AUDIO_HW_IDLE;
    if (output != 0) {
        int id = nextUniqueId();
        if ((flags & AudioSystem::OUTPUT_FLAG_DIRECT) ||
            (format != AudioSystem::PCM_16_BIT) ||
            (channels != AudioSystem::CHANNEL_OUT_STEREO)) {
            thread = new DirectOutputThread(this, output, id, *pDevices);
            LOGV("openOutput() created direct output: ID %d thread %p", id, thread);
        } else {
            thread = new MixerThread(this, output, id, *pDevices);
            LOGV("openOutput() created mixer output: ID %d thread %p", id, thread);
#ifdef LVMX
            unsigned bitsPerSample =
                (format == AudioSystem::PCM_16_BIT) ? 16 :
                    ((format == AudioSystem::PCM_8_BIT) ? 8 : 0);
            unsigned channelCount = (channels == AudioSystem::CHANNEL_OUT_STEREO) ? 2 : 1;
            int audioOutputType = LifeVibes::threadIdToAudioOutputType(thread->id());
            LifeVibes::init_aot(audioOutputType, samplingRate, bitsPerSample, channelCount);
            LifeVibes::setDevice(audioOutputType, *pDevices);
#endif
        }
        mPlaybackThreads.add(id, thread);
        if (pSamplingRate) *pSamplingRate = samplingRate;
        if (pFormat) *pFormat = format;
        if (pChannels) *pChannels = channels;
        if (pLatencyMs) *pLatencyMs = thread->latency();
        // notify client processes of the new output creation
        thread->audioConfigChanged_l(AudioSystem::OUTPUT_OPENED);
        return id;
    }
    return 0;
}
// ----------------------------------------------------------------
    return af->openOutput(pDevices,
                          pSamplingRate,
                          (uint32_t *)pFormat,
                          pChannels,
                          pLatencyMs,
                          flags);
}
// ----------------------------------------------------------------
            mTestOutputs[mCurOutput] = mpClientInterface->openOutput(&outputDesc->mDevice,
                                            &outputDesc->mSamplingRate,
                                            &outputDesc->mFormat,
                                            &outputDesc->mChannels,
                                            &outputDesc->mLatency,
                                            outputDesc->mFlags);
            if (mTestOutputs[mCurOutput]) {
                AudioParameter outputCmd = AudioParameter();
                outputCmd.addInt(String8("set_id"),mCurOutput);
                mpClientInterface->setParameters(mTestOutputs[mCurOutput],outputCmd.toString());
                addOutput(mTestOutputs[mCurOutput], outputDesc);
            }
        }
        return mTestOutputs[mCurOutput];
    }
#endif //AUDIO_POLICY_TEST
    // open a direct output if required by specified parameters
    if (needsDirectOuput(stream, samplingRate, format, channels, flags, device)) {
        LOGV("getOutput() opening direct output device %x", device);
        AudioOutputDescriptor *outputDesc = new AudioOutputDescriptor();
        outputDesc->mDevice = device;
        outputDesc->mSamplingRate = samplingRate;
        outputDesc->mFormat = format;
        outputDesc->mChannels = channels;
        outputDesc->mLatency = 0;
        outputDesc->mFlags = (AudioSystem::output_flags)(flags | AudioSystem::OUTPUT_FLAG_DIRECT);
        outputDesc->mRefCount[stream] = 0;
        output = mpClientInterface->openOutput(&outputDesc->mDevice,
                                        &outputDesc->mSamplingRate,
                                        &outputDesc->mFormat,
                                        &outputDesc->mChannels,
                                        &outputDesc->mLatency,
                                        outputDesc->mFlags);
        // only accept an output with the requeted parameters
        if (output == 0 ||
            (samplingRate != 0 && samplingRate != outputDesc->mSamplingRate) ||
            (format != 0 && format != outputDesc->mFormat) ||
            (channels != 0 && channels != outputDesc->mChannels)) {
            LOGV("getOutput() failed opening direct output: samplingRate %d, format %d, channels %d",
                    samplingRate, format, channels);
            if (output != 0) {
                mpClientInterface->closeOutput(output);
            }
            delete outputDesc;
            return 0;
        }
        addOutput(output, outputDesc);
        return output;
    }
    if (channels != 0 && channels != AudioSystem::CHANNEL_OUT_MONO &&
        channels != AudioSystem::CHANNEL_OUT_STEREO) {
        return 0;
    }
    // open a non direct output
    // get which output is suitable for the specified stream. The actual routing change will happen
    // when startOutput() will be called
    uint32_t a2dpDevice = device & AudioSystem::DEVICE_OUT_ALL_A2DP;
    if (AudioSystem::popCount((AudioSystem::audio_devices)device) == 2) {
#ifdef WITH_A2DP
        if (a2dpUsedForSonification() && a2dpDevice != 0) {
            // if playing on 2 devices among which one is A2DP, use duplicated output
            LOGV("getOutput() using duplicated output");
            LOGW_IF((mA2dpOutput == 0), "getOutput() A2DP device in multiple %x selected but A2DP output not opened", device);
            output = mDuplicatedOutput;
        } else
#endif
        {
            // if playing on 2 devices among which none is A2DP, use hardware output
            output = mHardwareOutput;
        }
        LOGV("getOutput() using output %d for 2 devices %x", output, device);
    } else {
#ifdef WITH_A2DP
        if (a2dpDevice != 0) {
            // if playing on A2DP device, use a2dp output
            LOGW_IF((mA2dpOutput == 0), "getOutput() A2DP device %x selected but A2DP output not opened", device);
            output = mA2dpOutput;
        } else
#endif
        {
            // if playing on not A2DP device, use hardware output
            output = mHardwareOutput;
        }
    }

    LOGW_IF((output ==0), "getOutput() could not find output for stream %d, samplingRate %d, format %d, channels %x, flags %x",
                stream, samplingRate, format, channels, flags);
    return output;
}
// ----------------------------------------------------------------
    // apply simple rule where global effects are attached to the same output as MUSIC streams
    return getOutput(AudioSystem::MUSIC);
}
原來到最後只是獲取了MUSIC的output。
// ----------------------------------------------------------------
    return mpPolicyManager->getOutputForEffect(desc);
}
// ----------------------------------------------------------------
            audio_io_handle_t output = getOutputForEffect(&desc);
            reply->writeInt32(static_cast <int>(output));
            return NO_ERROR;
        } break;
...
}
// ----------------------------------------------------------------
        remote()->transact(GET_OUTPUT_FOR_EFFECT, data, &reply);
        return static_cast <audio_io_handle_t> (reply.readInt32());
    }
// ----------------------------------------------------------------
    return aps->getOutputForEffect(desc);
}
// ----------------------------------------------------------------
            output = AudioSystem::getOutputForEffect(&desc);
        }
    }
    {
        Mutex::Autolock _l(mLock);

        if (!EffectIsNullUuid(&pDesc->uuid)) {
            // if uuid is specified, request effect descriptor
            lStatus = EffectGetDescriptor(&pDesc->uuid, &desc);
            if (lStatus < 0) {
                LOGW("createEffect() error %d from EffectGetDescriptor", lStatus);
                goto Exit;
            }
        } else {
            // if uuid is not specified, look for an available implementation
            // of the required type in effect factory
            if (EffectIsNullUuid(&pDesc->type)) {
                LOGW("createEffect() no effect type");
                lStatus = BAD_VALUE;
                goto Exit;
            }
            uint32_t numEffects = 0;
            effect_descriptor_t d;
            bool found = false;
   // 得到Effect的個數
            lStatus = EffectQueryNumberEffects(&numEffects);
            if (lStatus < 0) {
                LOGW("createEffect() error %d from EffectQueryNumberEffects", lStatus);
                goto Exit;
            }
   // 尋找匹配的effect
            for (uint32_t i = 0; i < numEffects; i++) {
                lStatus = EffectQueryEffect(i, &desc);
                if (lStatus < 0) {
                    LOGW("createEffect() error %d from EffectQueryEffect", lStatus);
                    continue;
                }
                if (memcmp(&desc.type, &pDesc->type, sizeof(effect_uuid_t)) == 0) {
                    // If matching type found save effect descriptor. If the session is
                    // 0 and the effect is not auxiliary, continue enumeration in case
                    // an auxiliary version of this effect type is available
                    found = true;
                    memcpy(&d, &desc, sizeof(effect_descriptor_t));
                    if (sessionId != AudioSystem::SESSION_OUTPUT_MIX ||
                            (desc.flags & EFFECT_FLAG_TYPE_MASK) == EFFECT_FLAG_TYPE_AUXILIARY) {
                        break;
                    }
                }
            }
            if (!found) {
                lStatus = BAD_VALUE;
                LOGW("createEffect() effect not found");
                goto Exit;
            }
            // For same effect type, chose auxiliary version over insert version if
            // connect to output mix (Compliance to OpenSL ES)
            if (sessionId == AudioSystem::SESSION_OUTPUT_MIX &&
                    (d.flags & EFFECT_FLAG_TYPE_MASK) != EFFECT_FLAG_TYPE_AUXILIARY) {
                memcpy(&desc, &d, sizeof(effect_descriptor_t));
            }
        }
        // Do not allow auxiliary effects on a session different from 0 (output mix)
        if (sessionId != AudioSystem::SESSION_OUTPUT_MIX &&
             (desc.flags & EFFECT_FLAG_TYPE_MASK) == EFFECT_FLAG_TYPE_AUXILIARY) {
            lStatus = INVALID_OPERATION;
            goto Exit;
        }
        // return effect descriptor
        memcpy(pDesc, &desc, sizeof(effect_descriptor_t));
        // If output is not specified try to find a matching audio session ID in one of the
        // output threads.
        // If output is 0 here, sessionId is neither SESSION_OUTPUT_STAGE nor SESSION_OUTPUT_MIX
        // because of code checking output when entering the function.
        if (output == 0) {
             // look for the thread where the specified audio session is present
            for (size_t i = 0; i < mPlaybackThreads.size(); i++) {
                if (mPlaybackThreads.valueAt(i)->hasAudioSession(sessionId) != 0) {
                    output = mPlaybackThreads.keyAt(i);
                    break;
                }
            }
            // If no output thread contains the requested session ID, default to
            // first output. The effect chain will be moved to the correct output
            // thread when a track with the same session ID is created
            if (output == 0 && mPlaybackThreads.size()) {
                output = mPlaybackThreads.keyAt(0);
            }
        }
        LOGV("createEffect() got output %d for effect %s", output, desc.name);
        PlaybackThread *thread = checkPlaybackThread_l(output);
        if (thread == NULL) {
            LOGE("createEffect() unknown output thread");
            lStatus = BAD_VALUE;
            goto Exit;
        }
        // TODO: allow attachment of effect to inputs
        wclient = mClients.valueFor(pid);
        if (wclient != NULL) {
            client = wclient.promote();
        } else {
            client = new Client(this, pid);
            mClients.add(pid, client);
        }
  // 在函式AudioEffect::Set函式中建立的EffectClient物件(effectClient)，在此處並未作太多處理，而是直接傳給了函式thread->createEffect_l
// ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
// PlaybackThread::createEffect_l() must be called with AudioFlinger::mLock held
sp<AudioFlinger::EffectHandle> AudioFlinger::PlaybackThread::createEffect_l(
        const sp<AudioFlinger::Client>& client,
        const sp<IEffectClient>& effectClient,
        int32_t priority,
        int sessionId,
        effect_descriptor_t *desc,
        int *enabled,
        status_t *status
        )
{
    sp<EffectModule> effect;
    sp<EffectHandle> handle;
    status_t lStatus;
    sp<Track> track;
    sp<EffectChain> chain;
    bool chainCreated = false;
    bool effectCreated = false;
    bool effectRegistered = false;
    if (mOutput == 0) {
        LOGW("createEffect_l() Audio driver not initialized.");
        lStatus = NO_INIT;
        goto Exit;
    }
    // Do not allow auxiliary effect on session other than 0
    if ((desc->flags & EFFECT_FLAG_TYPE_MASK) == EFFECT_FLAG_TYPE_AUXILIARY &&
        sessionId != AudioSystem::SESSION_OUTPUT_MIX) {
        LOGW("createEffect_l() Cannot add auxiliary effect %s to session %d",
                desc->name, sessionId);
        lStatus = BAD_VALUE;
        goto Exit;
    }
    // Do not allow effects with session ID 0 on direct output or duplicating threads
    // TODO: add rule for hw accelerated effects on direct outputs with non PCM format
    if (sessionId == AudioSystem::SESSION_OUTPUT_MIX && mType != MIXER) {
        LOGW("createEffect_l() Cannot add auxiliary effect %s to session %d",
                desc->name, sessionId);
        lStatus = BAD_VALUE;
        goto Exit;
    }
    LOGV("createEffect_l() thread %p effect %s on session %d", this, desc->name, sessionId);
    { // scope for mLock
        Mutex::Autolock _l(mLock);
  // 先看是否存在指定session的effect鏈，如果不存在，建立一個
        // check for existing effect chain with the requested audio session
        chain = getEffectChain_l(sessionId);
        if (chain == 0) {
            // create a new chain for this session
            LOGV("createEffect_l() new effect chain for session %d", sessionId);
            chain = new EffectChain(this, sessionId);
            addEffectChain_l(chain);
            chain->setStrategy(getStrategyForSession_l(sessionId));
            chainCreated = true;
        } else {
            effect = chain->getEffectFromDesc_l(desc);
        }
        LOGV("createEffect_l() got effect %p on chain %p", effect == 0 ? 0 : effect.get(), chain.get());
  // 是否存在相同描述的effect，不存在的話，建立一個
        if (effect == 0) {
            int id = mAudioFlinger->nextUniqueId();
            // Check CPU and memory usage
            lStatus = AudioSystem::registerEffect(desc, mId, chain->strategy(), sessionId, id);
            if (lStatus != NO_ERROR) {
                goto Exit;
            }
            effectRegistered = true;
            // create a new effect module if none present in the chain
            effect = new EffectModule(this, chain, desc, id, sessionId);
            lStatus = effect->status();
            if (lStatus != NO_ERROR) {
                goto Exit;
            }
            lStatus = chain->addEffect_l(effect);
            if (lStatus != NO_ERROR) {
                goto Exit;
            }
            effectCreated = true;
            effect->setDevice(mDevice);
            effect->setMode(mAudioFlinger->getMode());
        }
  // 看看下面EffectHandle的註釋，便於理解
// ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
    // The EffectHandle class implements the IEffect interface. It provides resources
    // to receive parameter updates, keeps track of effect control
    // ownership and state and has a pointer to the EffectModule object it is controlling.
    // There is one EffectHandle object for each application controlling (or using)
    // an effect module.
    // The EffectHandle is obtained by calling AudioFlinger::createEffect().
// ----------------------------------------------------------------
        // create effect handle and connect it to effect module
        handle = new EffectHandle(effect, client, effectClient, priority);
        lStatus = effect->addHandle(handle);
        if (enabled) {
            *enabled = (int)effect->isEnabled();
        }
    }
Exit:
    if (lStatus != NO_ERROR && lStatus != ALREADY_EXISTS) {
        Mutex::Autolock _l(mLock);
        if (effectCreated) {
            chain->removeEffect_l(effect);
        }
        if (effectRegistered) {
            AudioSystem::unregisterEffect(effect->id());
        }
        if (chainCreated) {
            removeEffectChain_l(chain);
        }
        handle.clear();
    }
    if(status) {
        *status = lStatus;
    }
    return handle;
}
// ----------------------------------------------------------------
        // create effect on selected output trhead
        handle = thread->createEffect_l(client, effectClient, priority, sessionId,
                &desc, enabled, &lStatus);
        if (handle != 0 && id != NULL) {
            *id = handle->id();
        }
    }
Exit:
    if(status) {
        *status = lStatus;
    }
    return handle;
}
// ----------------------------------------------------------------
    iEffect = audioFlinger->createEffect(getpid(), (effect_descriptor_t *)&mDescriptor,
            mIEffectClient, priority, output, mSessionId, &mStatus, &mId, &enabled);
    if (iEffect == 0 || (mStatus != NO_ERROR && mStatus != ALREADY_EXISTS)) {
        LOGE("set(): AudioFlinger could not create effect, status: %d", mStatus);
        return mStatus;
    }
    mEnabled = (volatile int32_t)enabled;
    mIEffect = iEffect;
    cblk = iEffect->getCblk();
    if (cblk == 0) {
        mStatus = NO_INIT;
        LOGE("Could not get control block");
        return mStatus;
    }
    mIEffect = iEffect;
    mCblkMemory = cblk;
    mCblk = static_cast<effect_param_cblk_t*>(cblk->pointer());
    int bufOffset = ((sizeof(effect_param_cblk_t) - 1) / sizeof(int) + 1) * sizeof(int);
    mCblk->buffer = (uint8_t *)mCblk + bufOffset;
    iEffect->asBinder()->linkToDeath(mIEffectClient);
    LOGV("set() %p OK effect: %s id: %d status %d enabled %d, ", this, mDescriptor.name, mId, mStatus, mEnabled);
    return mStatus