Android Audio代码分析9 - AudioTrack::write函数

在最初的代码testWriteByte中，创建完AudioTrack对象后，调用了AudioTrack对象的write函数实现播放。
今天就来看看write函数的实现。

*****************************************源码*************************************************
    public int write(byte[] audioData,int offsetInBytes, int sizeInBytes) {
        if ((mDataLoadMode == MODE_STATIC)
                && (mState == STATE_NO_STATIC_DATA)
                && (sizeInBytes > 0)) {
            mState = STATE_INITIALIZED;
        }

        if (mState != STATE_INITIALIZED) {
            return ERROR_INVALID_OPERATION;
        }

        if ( (audioData == null) || (offsetInBytes < 0 ) || (sizeInBytes < 0)
                || (offsetInBytes + sizeInBytes > audioData.length)) {
            return ERROR_BAD_VALUE;
        }

        return native_write_byte(audioData, offsetInBytes, sizeInBytes, mAudioFormat);
    }
**********************************************************************************************
源码路径：
frameworks\base\media\java\android\media\AudioTrack.java

#################说明################################################
    /**
     * Writes the audio data to the audio hardware for playback.
     * @param audioData the array that holds the data to play.
     * @param offsetInBytes the offset expressed in bytes in audioData where the data to play
     *    starts.
     * @param sizeInBytes the number of bytes to read in audioData after the offset.
     * @return the number of bytes that were written or {@link #ERROR_INVALID_OPERATION}
     *    if the object wasn't properly initialized, or {@link #ERROR_BAD_VALUE} if
     *    the parameters don't resolve to valid data and indexes.
     */
// 先看看注释，有一点需要注意，offsetInBytes是指要播放的数据是从参数audioData的哪个地方开始
    public int write(byte[] audioData,int offsetInBytes, int sizeInBytes) {
        if ((mDataLoadMode == MODE_STATIC)
                && (mState == STATE_NO_STATIC_DATA)
                && (sizeInBytes > 0)) {
            mState = STATE_INITIALIZED;
        }

        if (mState != STATE_INITIALIZED) {
            return ERROR_INVALID_OPERATION;
        }

        if ( (audioData == null) || (offsetInBytes < 0 ) || (sizeInBytes < 0)
                || (offsetInBytes + sizeInBytes > audioData.length)) {
            return ERROR_BAD_VALUE;
        }

// 前面主要检查了状态及参数，真正干活的在native中。
        return native_write_byte(audioData, offsetInBytes, sizeInBytes, mAudioFormat);
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
路径：frameworks\base\core\jni\android_media_AudioTrack.cpp
对应的native侧的函数为android_media_AudioTrack_native_write，其实现如下：
static jint android_media_AudioTrack_native_write(JNIEnv *env,  jobject thiz,
                                                  jbyteArray javaAudioData,
                                                  jint offsetInBytes, jint sizeInBytes,
                                                  jint javaAudioFormat) {
    jbyte* cAudioData = NULL;
    AudioTrack *lpTrack = NULL;
    //LOGV("android_media_AudioTrack_native_write(offset=%d, sizeInBytes=%d) called",
    //    offsetInBytes, sizeInBytes);
   
    // get the audio track to load with samples
// 我们创建AudioTrack对象的时间将其保存到了java侧，
// 现在要使用它了，所以把它取出来
    lpTrack = (AudioTrack *)env->GetIntField(thiz, javaAudioTrackFields.nativeTrackInJavaObj);
    if (lpTrack == NULL) {
        jniThrowException(env, "java/lang/IllegalStateException",
            "Unable to retrieve AudioTrack pointer for write()");
        return 0;
    }

    // get the pointer for the audio data from the java array
    if (javaAudioData) {
        cAudioData = (jbyte *)env->GetPrimitiveArrayCritical(javaAudioData, NULL);
        if (cAudioData == NULL) {
            LOGE("Error retrieving source of audio data to play, can't play");
            return 0; // out of memory or no data to load
        }
    } else {
        LOGE("NULL java array of audio data to play, can't play");
        return 0;
    }

    jint written = writeToTrack(lpTrack, javaAudioFormat, cAudioData, offsetInBytes, sizeInBytes);

++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
jint writeToTrack(AudioTrack* pTrack, jint audioFormat, jb

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