AudioTrack

AudioTrack 在Java应用中，管理和播放一个单一的语音资源

The AudioTrack class manages and plays a single audio resource for Java applications.
 * It allows streaming of PCM audio buffers to the audio sink for playback. This is
 * achieved by "pushing" the data to the AudioTrack object using one of the
 *  {@link #write(byte[], int, int)} and {@link #write(short[], int, int)} methods.
 

一个AudioTrack 实例可以在两种模式下运行：static和streaming模式

 

在Streaming模式下，应用调用write()方法，向AudioTrack中写入一段连续的数据流。

这个操作处于阻塞状态，直到数据从java层传递到native层，并且加入播放队列。才返回

streaming模式最适用于   以下音频数据块：

（1）音乐长度太长。导致太大放不进内存

（2）音乐质量太高，导致太大放不进内存。（高取样率，采样位数）

（3）在队列中的前一个audio正在播放时，接收到或生成。

 

 * <p>An AudioTrack instance can operate under two modes: static or streaming.<br>
 * In Streaming mode, the application writes a continuous stream of data to the AudioTrack, using
 * one of the {@code write()} methods. These are blocking and return when the data has been
 * transferred from the Java layer to the native layer and queued for playback. The streaming
 * mode is most useful when playing blocks of audio data that for instance are:
 *
 * <ul>
 *   <li>too big to fit in memory because of the duration of the sound to play,</li>
 *   <li>too big to fit in memory because of the characteristics of the audio data
 *         (high sampling rate, bits per sample ...)</li>
 *   <li>received or generated while previously queued audio is playing.</li>
 * </ul>
 *

static模式适用于  处理可以放在内存中的较短，且需要小的播放开销的声音片段

因此static模式适用于UI声音 和 游戏声音这种经常被播放的情况，开销很小。

 * The static mode should be chosen when dealing with short sounds that fit in memory and
 * that need to be played with the smallest latency possible. The static mode will
 * therefore be preferred for UI and game sounds that are played often, and with the
 * smallest overhead possible.
 

被创建后，一个AudioTrack对象初始化和它相关的音频缓存

缓存的大小在构造方法中被详细指定，决定了一个AudioTrack在用完数据之前能播放多长

对于一个使用static模式的AudioTrack，定义的size大小，是它能播放的声音片段大小的最大值

对使用streaming模式的， 写入audio sink（音频信宿）中的数据总和小于等于缓存大小。

AudioTrack 不是 final的，所以可以有子类，但是不推荐这么做

 * <p>Upon creation, an AudioTrack object initializes its associated audio buffer.
 * The size of this buffer, specified during the construction, determines how long an AudioTrack
 * can play before running out of data.<br>
 * For an AudioTrack using the static mode, this size is the maximum size of the sound that can
 * be played from it.<br>
 * For the streaming mode, data will be written to the audio sink in chunks of
 * sizes less than or equal to the total buffer size.
 *
 * AudioTrack is not final and thus permits subclasses, but such use is not recommended.

 

 

 

 

 

   /**
     * State of an AudioTrack that was not successfully initialized upon creation.
     */
    public static final int STATE_UNINITIALIZED = 0;　　未成功初始化
    /**
     * State of an AudioTrack that is ready to be used.
     */
    public static final int STATE_INITIALIZED   = 1;　　成功初始化
    /**
     * State of a successfully initialized AudioTrack that uses static data,
     * but that hasn't received that data yet.
     */
    public static final int STATE_NO_STATIC_DATA = 2;　　使用static模式，成功初始化。但还没有收到任何音频数据

 

    /**
     * Indicates the state of the AudioTrack instance.
     */
    private int mState = STATE_UNINITIALIZED;　　　　　mState 记录当前AudioTrack 实例的状态

 

    /**
     * Indicates the play state of the AudioTrack instance.
     */
    private int mPlayState = PLAYSTATE_STOPPED;　　 mPlayState 记录当前播放状态  初始化为 stopped

 

    /**
     * Lock to make sure mPlayState updates are reflecting the actual state of the object.
     */
    private final Object mPlayStateLock = new Object();　　　 mPlayState的锁 为保证反应播放真实状态

 

    /**
     * Looper associated with the thread that creates the AudioTrack instance.
     */
    private final Looper mInitializationLooper;　　　　创建AudioTrack实例的线程的Looper

 

 

    /**
     * The audio data source sampling rate in Hz.
     */
    private int mSampleRate; // initialized by all constructors　　　　音频数据资源的采样率（Hz）

 

    /**
     * The audio channel mask.
     */
    private int mChannels = AudioFormat.CHANNEL_OUT_MONO;　　　　初始化为单声道

 

    /**
     * The type of the audio stream to play. See
     *   {@link AudioManager#STREAM_VOICE_CALL}, {@link AudioManager#STREAM_SYSTEM},　　　　 通话　　系统
     *   {@link AudioManager#STREAM_RING}, {@link AudioManager#STREAM_MUSIC},　　　　　　　　  铃声　　音乐
     *   {@link AudioManager#STREAM_ALARM}, {@link AudioManager#STREAM_NOTIFICATION}, and　　闹铃　　通知
     *   {@link AudioManager#STREAM_DTMF}.　　　　　　　　　　　　　　　　　　　　　　　　　　　DTMF多音双频
     */
    private int mStreamType = AudioManager.STREAM_MUSIC;　　　　　　初始化为音乐声道
    /**
     * The way audio is consumed by the audio sink, streaming or static.
     */
    private int mDataLoadMode = MODE_STREAM;　　　　　　　　　　　  数据在音频信宿中的加载模式
    /**
     * The current audio channel configuration.
     */
    private int mChannelConfiguration = AudioFormat.CHANNEL_OUT_MONO;　　当前音频通道配置
    /**
     * The encoding of the audio samples.
     * @see AudioFormat#ENCODING_PCM_8BIT
     * @see AudioFormat#ENCODING_PCM_16BIT
     */
    private int mAudioFormat = AudioFormat.ENCODING_PCM_16BIT;　　　　 音频格式 初始化为16bits per sample（设备保证支持的方式）
    /**
     * Audio session ID
     */
    private int mSessionId = 0;

 

 

    public AudioTrack(int streamType, int sampleRateInHz, int channelConfig, int audioFormat,
            int bufferSizeInBytes, int mode)
    　　throws IllegalArgumentException {
       　　 this(streamType, sampleRateInHz, channelConfig, audioFormat,
             　　   bufferSizeInBytes, mode, 0 /*session*/);
    }

 

当AudioTrack必须和一个特殊的音频session绑定时。使用这个构造函数。

session ID的首要用途是


    /**
     * Class constructor with audio session. Use this constructor when the AudioTrack must be
     * attached to a particular audio session. The primary use of the audio session ID is to
     * associate audio effects to a particular instance of AudioTrack: if an audio session ID
     * is provided when creating an AudioEffect, this effect will be applied only to audio tracks
     * and media players in the same session and not to the output mix.
     * When an AudioTrack is created without specifying a session, it will create its own session
     * which can be retrieved by calling the {@link #getAudioSessionId()} method.
     * If a non-zero session ID is provided, this AudioTrack will share effects attached to this
     * session
     * with all other media players or audio tracks in the same session, otherwise a new session
     * will be created for this track if none is supplied.
     * @param streamType the type of the audio stream. See
     *   {@link AudioManager#STREAM_VOICE_CALL}, {@link AudioManager#STREAM_SYSTEM},
     *   {@link AudioManager#STREAM_RING}, {@link AudioManager#STREAM_MUSIC},
     *   {@link AudioManager#STREAM_ALARM}, and {@link AudioManager#STREAM_NOTIFICATION}.
     * @param sampleRateInHz the initial source sample rate expressed in Hz.
     * @param channelConfig describes the configuration of the audio channels.
     *   See {@link AudioFormat#CHANNEL_OUT_MONO} and
     *   {@link AudioFormat#CHANNEL_OUT_STEREO}
     * @param audioFormat the format in which the audio data is represented.
     *   See {@link AudioFormat#ENCODING_PCM_16BIT} and
     *   {@link AudioFormat#ENCODING_PCM_8BIT}

     * @param bufferSizeInBytes the total size (in bytes) of the buffer where audio data is read　　　　
     *   from for playback. If using the AudioTrack in streaming mode, you can write data into
     *   this buffer in smaller chunks than this size. If using the AudioTrack in static mode,
     *   this is the maximum size of the sound that will be played for this instance.
134　　　　　　If track's creation mode is {@link #MODE_STREAM}, you can write data into
     *   this buffer in chunks less than or equal to this size, and it is typical to use
     *   chunks of 1/2 of the total size to permit double-buffering.
     *   If the track's creation mode is {@link #MODE_STATIC},
     *   this is the maximum length sample, or audio clip, that can be played by this instance.
     *   See {@link #getMinBufferSize(int, int, int)} to determine the minimum required buffer size
     *   for the successful creation of an AudioTrack instance in streaming mode. Using values
     *   smaller than getMinBufferSize() will result in an initialization failure.

144 　　bufferSizeInBytes  是音频缓存区的总大小。从中读出音频数据来播放
146 　　streaming模式中。写入的数据要小于等于这个值。通常传1/2buffer大小的数据。保证双重缓冲区处理
148 　　static模式中。这是能播放的音频大小的最大值。
150 　　通过getMinBufferSize来获得一个AudioTrack实例在streaming模式下正确初始化需要的缓存区最小值。
152 　　如果buffer比这个最小值还小。将会导致初始化失败


     * @param mode streaming or static buffer. See {@link #MODE_STATIC} and {@link #MODE_STREAM}
     * @param sessionId Id of audio session the AudioTrack must be attached to
     * @throws java.lang.IllegalArgumentException
     */
    public AudioTrack(int streamType, int sampleRateInHz, int channelConfig, int audioFormat,
            int bufferSizeInBytes, int mode, int sessionId)

    throws IllegalArgumentException {
        // mState already == STATE_UNINITIALIZED

        // remember which looper is associated with the AudioTrack instantiation
        Looper looper;
        if ((looper = Looper.myLooper()) == null) {
            looper = Looper.getMainLooper();
        }
        mInitializationLooper = looper;　　　　　　记录创建实例时  所在looper

        audioParamCheck(streamType, sampleRateInHz, channelConfig, audioFormat, mode);　　　　检查参数合法性

        audioBuffSizeCheck(bufferSizeInBytes);　　　　　　检查缓冲区大小

        if (sessionId < 0) {
            throw new IllegalArgumentException("Invalid audio session ID: "+sessionId);
        }

        int[] session = new int[1];
        session[0] = sessionId;
        // native initialization
        int initResult = native_setup(new WeakReference<AudioTrack>(this),
                mStreamType, mSampleRate, mChannels, mAudioFormat,
                mNativeBufferSizeInBytes, mDataLoadMode, session);
        if (initResult != SUCCESS) {
            loge("Error code "+initResult+" when initializing AudioTrack.");
            return; // with mState == STATE_UNINITIALIZED
        }

        mSessionId = session[0];

        if (mDataLoadMode == MODE_STATIC) {
            mState = STATE_NO_STATIC_DATA;
        } else {
            mState = STATE_INITIALIZED;
        }
    }

    // mask of all the channels supported by this implementation　　　　　　　　按位或　　也就是将1叠加。就是支持的所有通道模式
    private static final int SUPPORTED_OUT_CHANNELS =
            AudioFormat.CHANNEL_OUT_FRONT_LEFT |
            AudioFormat.CHANNEL_OUT_FRONT_RIGHT |
            AudioFormat.CHANNEL_OUT_FRONT_CENTER |
            AudioFormat.CHANNEL_OUT_LOW_FREQUENCY |
            AudioFormat.CHANNEL_OUT_BACK_LEFT |
            AudioFormat.CHANNEL_OUT_BACK_RIGHT |
            AudioFormat.CHANNEL_OUT_BACK_CENTER;

    // Convenience method for the constructor's parameter checks.
    // This is where constructor IllegalArgumentException-s are thrown
    // postconditions:
    //    mStreamType is valid
    //    mChannelCount is valid
    //    mChannels is valid
    //    mAudioFormat is valid
    //    mSampleRate is valid
    //    mDataLoadMode is valid
    private void audioParamCheck(int streamType, int sampleRateInHz,　　　　　　　　用于构造函数参数检查
                                 int channelConfig, int audioFormat, int mode) {

        //--------------
        // stream type
        if( (streamType != AudioManager.STREAM_ALARM) && (streamType != AudioManager.STREAM_MUSIC)　　
           && (streamType != AudioManager.STREAM_RING) && (streamType != AudioManager.STREAM_SYSTEM)
           && (streamType != AudioManager.STREAM_VOICE_CALL)
           && (streamType != AudioManager.STREAM_NOTIFICATION)
           && (streamType != AudioManager.STREAM_BLUETOOTH_SCO)
           && (streamType != AudioManager.STREAM_DTMF)) {
            throw new IllegalArgumentException("Invalid stream type.");
        }
        mStreamType = streamType;

        //--------------
        // sample rate, note these values are subject to change
        if ( (sampleRateInHz < 4000) || (sampleRateInHz > 48000) ) {　　　　　　4000Hz <= 采样率 <= 48000Hz  合法
            throw new IllegalArgumentException(sampleRateInHz
                    + "Hz is not a supported sample rate.");
        }
        mSampleRate = sampleRateInHz;

        //--------------
        // channel config
        mChannelConfiguration = channelConfig;

        switch (channelConfig) {
        case AudioFormat.CHANNEL_OUT_DEFAULT: //AudioFormat.CHANNEL_CONFIGURATION_DEFAULT
        case AudioFormat.CHANNEL_OUT_MONO:
        case AudioFormat.CHANNEL_CONFIGURATION_MONO:
            mChannelCount = 1;　　　　　　　　　　　　　　　　　　　　　　声道数：1
            mChannels = AudioFormat.CHANNEL_OUT_MONO;　　　　　　　　单声道
            break;
        case AudioFormat.CHANNEL_OUT_STEREO:
        case AudioFormat.CHANNEL_CONFIGURATION_STEREO:
            mChannelCount = 2;　　　　　　　　　　　　　　　　　　　　　    声道数：2
            mChannels = AudioFormat.CHANNEL_OUT_STEREO;　　　　　　　双声道
            break;
        default:
            if (!isMultichannelConfigSupported(channelConfig)) {　　　　　　　　不支持多声道   抛异常
                // input channel configuration features unsupported channels
                throw new IllegalArgumentException("Unsupported channel configuration.");
            }
            mChannels = channelConfig;
            mChannelCount = Integer.bitCount(channelConfig);　　　　　　　　  计算1的位数　　即为声道数母
        }

        //--------------
        // audio format
        switch (audioFormat) {
        case AudioFormat.ENCODING_DEFAULT:　　　　　　　　　　　　默认16bit
            mAudioFormat = AudioFormat.ENCODING_PCM_16BIT;
            break;
        case AudioFormat.ENCODING_PCM_16BIT:
        case AudioFormat.ENCODING_PCM_8BIT:
            mAudioFormat = audioFormat;
            break;
        default:
            throw new IllegalArgumentException("Unsupported sample encoding."
                + " Should be ENCODING_PCM_8BIT or ENCODING_PCM_16BIT.");
        }

        //--------------
        // audio load mode
        if ( (mode != MODE_STREAM) && (mode != MODE_STATIC) ) {　　不是stream或者static  抛异常
            throw new IllegalArgumentException("Invalid mode.");
        }
        mDataLoadMode = mode;
    }

 

    // Convenience method for the constructor's audio buffer size check.
    // preconditions:
    //    mChannelCount is valid
    //    mAudioFormat is valid
    // postcondition:
    //    mNativeBufferSizeInBytes is valid (multiple of frame size, positive)
    private void audioBuffSizeCheck(int audioBufferSize) {
        // NB: this section is only valid with PCM data.
        //     To update when supporting compressed formats
        int frameSizeInBytes = mChannelCount
                * (mAudioFormat == AudioFormat.ENCODING_PCM_8BIT ? 1 : 2);　　　　　　帧大小 = 信道数  * （8bits per sample ：1，16bits per sample ：2）
        if ((audioBufferSize % frameSizeInBytes != 0) || (audioBufferSize < 1)) {　　　　　　 不整 或<1。抛异常
            throw new IllegalArgumentException("Invalid audio buffer size.");
        }

        mNativeBufferSizeInBytes = audioBufferSize;
        mNativeBufferSizeInFrames = audioBufferSize / frameSizeInBytes;　　　　　　　 　 帧数
    }

 

    /**
     * Convenience method to check that the channel configuration (a.k.a channel mask) is supported
     * @param channelConfig the mask to validate
     * @return false if the AudioTrack can't be used with such a mask
     */
    private static boolean isMultichannelConfigSupported(int channelConfig) {
        // check for unsupported channels
        if ((channelConfig & SUPPORTED_OUT_CHANNELS) != channelConfig) {　　　　不相等，也就是某个1被置0了。说明包含不支持的声道模式
            loge("Channel configuration features unsupported channels");
            return false;
        }
        // check for unsupported multichannel combinations:
        // - FL/FR must be present
        // - L/R channels must be paired (e.g. no single L channel)
        final int frontPair =
                AudioFormat.CHANNEL_OUT_FRONT_LEFT | AudioFormat.CHANNEL_OUT_FRONT_RIGHT;　　前置
        if ((channelConfig & frontPair) != frontPair) {
                loge("Front channels must be present in multichannel configurations");
                return false;
        }
        final int backPair =
                AudioFormat.CHANNEL_OUT_BACK_LEFT | AudioFormat.CHANNEL_OUT_BACK_RIGHT;　　后置
        if ((channelConfig & backPair) != 0) {
            if ((channelConfig & backPair) != backPair) {　　　　　　　　支持后置，但是只支持其中一个。
                loge("Rear channels can't be used independently");
                return false;
            }
        }　　　　　　　　　　　　　　　　　　　　　　　　　　　　==0情况下返回true。也就是不支持后置时，返回的是true
        return true;
    }

 


    /**
     * Releases the native AudioTrack resources.　　　　　　　　释放native层的资源
     */
    public void release() {
        // even though native_release() stops the native AudioTrack, we need to stop　　　即使native_release(); 中停止了native层的AudioTrack。我们还是需要调用stop去停止AudioTrack的子类
        // AudioTrack subclasses too.
        try {
            stop();
        } catch(IllegalStateException ise) {
            // don't raise an exception, we're releasing the resources.
        }
        native_release();
        mState = STATE_UNINITIALIZED;　　　　　　状态设置为  未初始化
    }

    @Override
    protected void finalize() {
        native_finalize();
    }

    //--------------------------------------------------------------------------
    // Getters
    //--------------------
    /**
     * Returns the minimum valid volume value. Volume values set under this one will　　　　如果音量设置比MIN小，则置为0.1
     * be clamped at this value.
     * @return the minimum volume expressed as a linear attenuation.
     */
    static public float getMinVolume() {
        return VOLUME_MIN;
    }

    /**
     * Returns the maximum valid volume value. Volume values set above this one will　　　 如果音量设置比MAX大，则置为1.0
     * be clamped at this value.
     * @return the maximum volume expressed as a linear attenuation.
     */
    static public float getMaxVolume() {
        return VOLUME_MAX;
    }

 

 

    /**
     * Returns the playback state of the AudioTrack instance.
     * @see #PLAYSTATE_STOPPED
     * @see #PLAYSTATE_PAUSED
     * @see #PLAYSTATE_PLAYING
     */
    public int getPlayState() {
        synchronized (mPlayStateLock) {　　　　　　　　　　　　互斥锁用在这儿
            return mPlayState;
        }
    }

 

406 返回stream模式下。成功创建需要的buffer大小
407 
408 注意：这个大小不保证  音频加载后顺利播放。
409 
410 buffer 被数据重复填充的期望频率。需要选择更大的buffer值。
411 
412 例如，如果你想要动态设置一个比初始值大的AudioTrack资源采样率。要保证计算时，使用你想设置的
413 
414 最大采样率去计算buffer大小

    /**
     * Returns the minimum buffer size required for the successful creation of an AudioTrack
     * object to be created in the {@link #MODE_STREAM} mode. Note that this size doesn't
     * guarantee a smooth playback under load, and higher values should be chosen according to
     * the expected frequency at which the buffer will be refilled with additional data to play.
     * For example, if you intend to dynamically set the source sample rate of an AudioTrack
     * to a higher value than the initial source sample rate, be sure to configure the buffer size
     * based on the highest planned sample rate.
     * @param sampleRateInHz the source sample rate expressed in Hz.
     * @param channelConfig describes the configuration of the audio channels.
     *   See {@link AudioFormat#CHANNEL_OUT_MONO} and
     *   {@link AudioFormat#CHANNEL_OUT_STEREO}
     * @param audioFormat the format in which the audio data is represented.
     *   See {@link AudioFormat#ENCODING_PCM_16BIT} and
     *   {@link AudioFormat#ENCODING_PCM_8BIT}
     * @return {@link #ERROR_BAD_VALUE} if an invalid parameter was passed,
     *   or {@link #ERROR} if unable to query for output properties,
     *   or the minimum buffer size expressed in bytes.
     */
    static public int getMinBufferSize(int sampleRateInHz, int channelConfig, int audioFormat) {
        int channelCount = 0;
        switch(channelConfig) {　　　　　　　　　　　　　　　　　　　　　　计算信道数目
        case AudioFormat.CHANNEL_OUT_MONO:
        case AudioFormat.CHANNEL_CONFIGURATION_MONO:
            channelCount = 1;
            break;
        case AudioFormat.CHANNEL_OUT_STEREO:
        case AudioFormat.CHANNEL_CONFIGURATION_STEREO:
            channelCount = 2;
            break;
        default:
            if ((channelConfig & SUPPORTED_OUT_CHANNELS) != channelConfig) {
                // input channel configuration features unsupported channels
                loge("getMinBufferSize(): Invalid channel configuration.");
                return ERROR_BAD_VALUE;
            } else {
                channelCount = Integer.bitCount(channelConfig);
            }
        }

        if ((audioFormat != AudioFormat.ENCODING_PCM_16BIT)　　　　　　　非法音频格式
            && (audioFormat != AudioFormat.ENCODING_PCM_8BIT)) {
            loge("getMinBufferSize(): Invalid audio format.");
            return ERROR_BAD_VALUE;
        }

        // sample rate, note these values are subject to change
        if ( (sampleRateInHz < SAMPLE_RATE_HZ_MIN) || (sampleRateInHz > SAMPLE_RATE_HZ_MAX) ) {　　非法采样率
            loge("getMinBufferSize(): " + sampleRateInHz + " Hz is not a supported sample rate.");
            return ERROR_BAD_VALUE;
        }

        int size = native_get_min_buff_size(sampleRateInHz, channelCount, audioFormat);　　　　调用native
        if (size <= 0) {
            loge("getMinBufferSize(): error querying hardware");
            return ERROR;
        }
        else {
            return size;
        }
    }

 

 



    /**　　　　

486 设置播放的开始位置（帧）。但是此时播放必须处于停止或暂停状态。
487 
488 并且必须是static 模式下。
489 
490  0 <= position <= buffer 可以容纳的帧总数


     * Sets the playback head position.
     * The track must be stopped or paused for the position to be changed,
     * and must use the {@link #MODE_STATIC} mode.
     * @param positionInFrames playback head position expressed in frames
     * Zero corresponds to start of buffer.
     * The position must not be greater than the buffer size in frames, or negative.
     * @return error code or success, see {@link #SUCCESS}, {@link #ERROR_BAD_VALUE},
     *    {@link #ERROR_INVALID_OPERATION}
     */
    public int setPlaybackHeadPosition(int positionInFrames) {
        if (mDataLoadMode == MODE_STREAM || mState != STATE_INITIALIZED ||
                getPlayState() == PLAYSTATE_PLAYING) {
            return ERROR_INVALID_OPERATION;
        }
        if (!(0 <= positionInFrames && positionInFrames <= mNativeBufferSizeInFrames)) {
            return ERROR_BAD_VALUE;
        }
        return native_set_position(positionInFrames);
    }

 


    /**

518 设置循环起始位置，终止位置和循环次数。可以无限循环
519 
520 和上一方法一样。前提必须停止 或者暂停。且为static模式
521 
522 起始帧位置。0指代buffer的开头。 start  不能 >= buffer帧总数结束帧位置。end 不能 > buffer帧总数
523 
524 为了循环。 start < end
525 
526 start end 和 loopcount 可以都为0


     * Sets the loop points and the loop count. The loop can be infinite.
     * Similarly to setPlaybackHeadPosition,
     * the track must be stopped or paused for the loop points to be changed,
     * and must use the {@link #MODE_STATIC} mode.
     * @param startInFrames loop start marker expressed in frames
     * Zero corresponds to start of buffer.
     * The start marker must not be greater than or equal to the buffer size in frames, or negative.
     * @param endInFrames loop end marker expressed in frames
     * The total buffer size in frames corresponds to end of buffer.
     * The end marker must not be greater than the buffer size in frames.
     * For looping, the end marker must not be less than or equal to the start marker,
     * but to disable looping
     * it is permitted for start marker, end marker, and loop count to all be 0.
     * @param loopCount the number of times the loop is looped.
     *    A value of -1 means infinite looping, and 0 disables looping.
     * @return error code or success, see {@link #SUCCESS}, {@link #ERROR_BAD_VALUE},
     *    {@link #ERROR_INVALID_OPERATION}
     */
    public int setLoopPoints(int startInFrames, int endInFrames, int loopCount) {
        if (mDataLoadMode == MODE_STREAM || mState != STATE_INITIALIZED ||
                getPlayState() == PLAYSTATE_PLAYING) {
            return ERROR_INVALID_OPERATION;
        }
        if (loopCount == 0) {
            ;   // explicitly allowed as an exception to the loop region range check
        } else if (!(0 <= startInFrames && startInFrames < mNativeBufferSizeInFrames &&
                startInFrames < endInFrames && endInFrames <= mNativeBufferSizeInFrames)) {
            return ERROR_BAD_VALUE;
        }
        return native_set_loop(startInFrames, endInFrames, loopCount);
    }

 


    /**　　　　不让用只有子类让用。但是不建议有子类。所以此方法荒废
     * Sets the initialization state of the instance. This method was originally intended to be used
     * in an AudioTrack subclass constructor to set a subclass-specific post-initialization state.
     * However, subclasses of AudioTrack are no longer recommended, so this method is obsolete.
     * @param state the state of the AudioTrack instance
     * @deprecated Only accessible by subclasses, which are not recommended for AudioTrack.
     */
    @Deprecated
    protected void setState(int state) {
        mState = state;
    }

 

    //---------------------------------------------------------
    // Transport control methods　　　　　　以下三个方法格式基本相同   play  stop pause
    //--------------------
    /**
     * Starts playing an AudioTrack.
     * If track's creation mode is {@link #MODE_STATIC}, you must have called write() prior.　　　　static 模式下，要先调用write()
     *
     * @throws IllegalStateException
     */
    public void play()
    throws IllegalStateException {
        if (mState != STATE_INITIALIZED) {
            throw new IllegalStateException("play() called on uninitialized AudioTrack.");
        }

        synchronized(mPlayStateLock) {
            native_start();
            mPlayState = PLAYSTATE_PLAYING;
        }
    }

    /**
601 stream模式下创建的实例，音频会在buffer写入的数据都被播放后才会停止。
602 
603 如果想让它立即停止需要调用  pause() 然后调用flush() 来清除buffer中没被播放的数据

     * Stops playing the audio data.
     * When used on an instance created in {@link #MODE_STREAM} mode, audio will stop playing
     * after the last buffer that was written has been played. For an immediate stop, use
     * {@link #pause()}, followed by {@link #flush()} to discard audio data that hasn't been played
     * back yet.
     * @throws IllegalStateException
     */
    public void stop()
    throws IllegalStateException {
        if (mState != STATE_INITIALIZED) {
            throw new IllegalStateException("stop() called on uninitialized AudioTrack.");
        }

        // stop playing
        synchronized(mPlayStateLock) {
            native_stop();
            mPlayState = PLAYSTATE_STOPPED;
        }
    }

    /**

626 暂停。还没被播放的数据不会被消除。如果再调用play() 会继续播放。
627 
628 使用flush() 清除缓存数据


     * Pauses the playback of the audio data. Data that has not been played
     * back will not be discarded. Subsequent calls to {@link #play} will play
     * this data back. See {@link #flush()} to discard this data.
     *
     * @throws IllegalStateException
     */
    public void pause()
    throws IllegalStateException {
        if (mState != STATE_INITIALIZED) {
            throw new IllegalStateException("pause() called on uninitialized AudioTrack.");
        }
        //logd("pause()");

        // pause playback
        synchronized(mPlayStateLock) {
            native_pause();
            mPlayState = PLAYSTATE_PAUSED;
        }
    }


    //---------------------------------------------------------
    // Audio data supply
    //--------------------

    /**

658 清除队列中等待播放的音频数据。所有都会被清除。
659 
660 如果没有停止或暂停。或者当前模式不是stream。操作会失败

    * Flushes the audio data currently queued for playback. Any data that has
     * not been played back will be discarded.  No-op if not stopped or paused,
     * or if the track's creation mode is not {@link #MODE_STREAM}.
     */
    public void flush() {
        if (mState == STATE_INITIALIZED) {
            // flush the data in native layer
            native_flush();
        }

    }

 

 

    //--------------------------------------------------------------------------
    // Audio effects management　　　　　　音频效果管理
    //--------------------

    /**

684 为AudioTrack增加一种辅助效果。一种典型辅助效果是混响，可以应用于所有音频资源。导致
685 
686 对这种辅助效果  造成一个具有确切数值的影响。这个数值被setAuxEffectSendLevel()指定
687 
688 创建一个辅助效果后，通过AudioEffect.gerId()来获得它的ID，在使用这个ID调用此方法。
689 
690 当你想去掉这种辅助效果。再调一次这个方法，此时ID传null


     * Attaches an auxiliary effect to the audio track. A typical auxiliary
     * effect is a reverberation effect which can be applied on any sound source
     * that directs a certain amount of its energy to this effect. This amount
     * is defined by setAuxEffectSendLevel().
     * {@see #setAuxEffectSendLevel(float)}.
     * <p>After creating an auxiliary effect (e.g.
     * {@link android.media.audiofx.EnvironmentalReverb}), retrieve its ID with
     * {@link android.media.audiofx.AudioEffect#getId()} and use it when calling
     * this method to attach the audio track to the effect.
     * <p>To detach the effect from the audio track, call this method with a
     * null effect id.
     *
     * @param effectId system wide unique id of the effect to attach　　　　这个效果id，在系统内是唯一的
     * @return error code or success, see {@link #SUCCESS},
     *    {@link #ERROR_INVALID_OPERATION}, {@link #ERROR_BAD_VALUE}
     */
    public int attachAuxEffect(int effectId) {
        if (mState == STATE_UNINITIALIZED) {
            return ERROR_INVALID_OPERATION;
        }
        return native_attachAuxEffect(effectId);
    }

    /**

718 设置辅助效果的level。从0.0f 到1.0f。超出会被卡掉。默认为0.0f
719 
720 所以即使已经指定了效果。也需要调用这个方法，效果才会被应用
721 
722 注意：这个值是个未加工过的度量。UI操作需要被对数拉伸
723 
724 audio 框架中的增益 从-72dB到0dB。所以从线性UI的输入x到这个level的一个合适的转换为：
725 
726 x == 0 则 level==0
727 
728 0<x<=R 则 level = 10^(72*(x-R)/20/R)


     * Sets the send level of the audio track to the attached auxiliary effect
     * {@link #attachAuxEffect(int)}.  The level value range is 0.0f to 1.0f.
     * Values are clamped to the (0.0f, 1.0f) interval if outside this range.
     * <p>By default the send level is 0.0f, so even if an effect is attached to the player
     * this method must be called for the effect to be applied.
     * Note that the passed level value is a raw scalar. UI controls should be scaled logarithmically:

the gain applied by audio framework ranges from -72dB to 0dB, so an appropriate conversion from

linear UI input x to level is: x == 0 -> level = 0 0 < x <= R -> level = 10^(72*(x-R)/20/R)
     *
     * @param level send level scalar
     * @return error code or success, see {@link #SUCCESS},
     *    {@link #ERROR_INVALID_OPERATION}
     */
    public int setAuxEffectSendLevel(float level) {
        if (mState == STATE_UNINITIALIZED) {
            return ERROR_INVALID_OPERATION;
        }
        // clamp the level
        if (level < getMinVolume()) {
            level = getMinVolume();
        }
        if (level > getMaxVolume()) {
            level = getMaxVolume();
        }
        native_setAuxEffectSendLevel(level);
        return SUCCESS;
    }