*****************************************源码*************************************************
public void testPlaybackHeadPositionAfterInit() throws Exception { // constants for test final String TEST_NAME = "testPlaybackHeadPositionAfterInit"; final int TEST_SR = 22050; final int TEST_CONF = AudioFormat.CHANNEL_OUT_STEREO; final int TEST_FORMAT = AudioFormat.ENCODING_PCM_16BIT; final int TEST_MODE = AudioTrack.MODE_STREAM; final int TEST_STREAM_TYPE = AudioManager.STREAM_MUSIC; //-------- initialization -------------- AudioTrack track = new AudioTrack(TEST_STREAM_TYPE, TEST_SR, TEST_CONF, TEST_FORMAT, AudioTrack.getMinBufferSize(TEST_SR, TEST_CONF, TEST_FORMAT), TEST_MODE); //-------- test -------------- assumeTrue(TEST_NAME, track.getState() == AudioTrack.STATE_INITIALIZED); assertTrue(TEST_NAME, track.getPlaybackHeadPosition() == 0); //-------- tear down -------------- track.release(); **********************************************************************************************<br/> 源码路径：<br/> frameworks\base\media\tests\mediaframeworktest\src\com\android\mediaframeworktest\functional\MediaAudioTrackTest.java<br/> #######################说明################################<br/> <textarea readonly="" name="code" class="cpp"> //Test case 1: getPlaybackHeadPosition() at 0 after initialization public void testPlaybackHeadPositionAfterInit() throws Exception { // constants for test final String TEST_NAME = "testPlaybackHeadPositionAfterInit"; final int TEST_SR = 22050; final int TEST_CONF = AudioFormat.CHANNEL_OUT_STEREO; final int TEST_FORMAT = AudioFormat.ENCODING_PCM_16BIT; final int TEST_MODE = AudioTrack.MODE_STREAM; final int TEST_STREAM_TYPE = AudioManager.STREAM_MUSIC; //-------- initialization -------------- AudioTrack track = new AudioTrack(TEST_STREAM_TYPE, TEST_SR, TEST_CONF, TEST_FORMAT, AudioTrack.getMinBufferSize(TEST_SR, TEST_CONF, TEST_FORMAT), TEST_MODE); //-------- test -------------- assumeTrue(TEST_NAME, track.getState() == AudioTrack.STATE_INITIALIZED); // 今天要看的，就是下面的getPlaybackHeadPosition函数 // 由于在getPlaybackHeadPosition之前，并没有调用play函数，也就是没有开始播放，所以获取的position应该为0 assertTrue(TEST_NAME, track.getPlaybackHeadPosition() == 0); // +++++++++++++++++++++++++++++++getPlaybackHeadPosition+++++++++++++++++++++++++++++++++ * Returns the playback head position expressed in frames public int getPlaybackHeadPosition() { // 很直接 // 调用的是函数android_media_AudioTrack_get_position return native_get_position(); // ++++++++++++++++++++++++++++++android_media_AudioTrack_get_position++++++++++++++++++++++++++++++++++ // 路径：frameworks\base\core\jni\android_media_AudioTrack.cpp static jint android_media_AudioTrack_get_position(JNIEnv *env, jobject thiz) { AudioTrack *lpTrack = (AudioTrack *)env->GetIntField( thiz, javaAudioTrackFields.nativeTrackInJavaObj); uint32_t position = 0; if (lpTrack) { lpTrack->getPosition(&position); return (jint)position; // ++++++++++++++++++++++++++++++++AudioTrack::getPosition++++++++++++++++++++++++++++++++ status_t AudioTrack::getPosition(uint32_t *position) if (position == 0) return BAD_VALUE; *position = mCblk->server; return NO_ERROR; // server是mCblk的成员变量，mCblk是audio_track_cblk_t对象。 // server在函数audio_track_cblk_t::stepServer中有被赋值。 // 另外，mCblk->server有在函数AudioTrack::setPosition中被赋值。 // +++++++++++++++++++++++++++++++++AudioTrack::setPosition+++++++++++++++++++++++++++++++ status_t AudioTrack::setPosition(uint32_t position) Mutex::Autolock _l(mCblk->lock); if (!stopped()) return INVALID_OPERATION; if (position > mCblk->user) return BAD_VALUE; mCblk->server = position; mCblk->flags |= CBLK_FORCEREADY_ON; return NO_ERROR; // android_media_AudioTrack.cpp文件中的以下两个函数调用了AudioTrack::setPosition： // android_media_AudioTrack_set_pos_update_period函数 // android_media_AudioTrack_set_position函数 // 这些接口上给java层用的。 // 想象一下使用场景，拖动当前光标？ // ---------------------------------AudioTrack::setPosition------------------------------- // +++++++++++++++++++++++++++++++++audio_track_cblk_t::stepServer+++++++++++++++++++++++++++++++ bool audio_track_cblk_t::stepServer(uint32_t frameCount) // the code below simulates lock-with-timeout // we MUST do this to protect the AudioFlinger server // as this lock is shared with the client. status_t err; err = lock.tryLock(); if (err == -EBUSY) { // just wait a bit usleep(1000); err = lock.tryLock(); if (err != NO_ERROR) { // probably, the client just died. return false; uint64_t s = this->server; s += frameCount; if (flags & CBLK_DIRECTION_MSK) { // Mark that we have read the first buffer so that next time stepUser() is called // we switch to normal obtainBuffer() timeout period if (bufferTimeoutMs == MAX_STARTUP_TIMEOUT_MS) { bufferTimeoutMs = MAX_STARTUP_TIMEOUT_MS - 1; // It is possible that we receive a flush() // while the mixer is processing a block: in this case, // stepServer() is called After the flush() has reset u & s and // we have s > u if (s > this->user) { LOGW("stepServer occured after track reset"); s = this->user; if (s >= loopEnd) { LOGW_IF(s > loopEnd, "stepServer: s %llu > loopEnd %llu", s, loopEnd); s = loopStart; if (--loopCount == 0) { loopEnd = ULLONG_MAX; loopStart = ULLONG_MAX; if (s >= serverBase + this->frameCount) { serverBase += this->frameCount; this->server = s; cv.signal(); lock.unlock(); return true; // 函数AudioFlinger::ThreadBase::TrackBase::step中调用了函数audio_track_cblk_t::stepServer。 // ++++++++++++++++++++++++++++++AudioFlinger::ThreadBase::TrackBase::step++++++++++++++++++++++++++++++++++ bool AudioFlinger::ThreadBase::TrackBase::step() { bool result; audio_track_cblk_t* cblk = this->cblk(); result = cblk->stepServer(mFrameCount); if (!result) { LOGV("stepServer failed acquiring cblk mutex"); mFlags |= STEPSERVER_FAILED; return result; // 函数AudioFlinger::PlaybackThread::Track::getNextBuffer和函数AudioFlinger::RecordThread::RecordTrack::getNextBuffer // 调用了函数AudioFlinger::ThreadBase::TrackBase::step。 // 我们这儿只看函数AudioFlinger::PlaybackThread::Track::getNextBuffer。 // +++++++++++++++++++++++++++++AudioFlinger::PlaybackThread::Track::getNextBuffer+++++++++++++++++++++++++++++++++++ status_t AudioFlinger::PlaybackThread::Track::getNextBuffer(AudioBufferProvider::Buffer* buffer) audio_track_cblk_t* cblk = this->cblk(); uint32_t framesReady; uint32_t framesReq = buffer->frameCount; // Check if last stepServer failed, try to step now if (mFlags & TrackBase::STEPSERVER_FAILED) { if (!step()) goto getNextBuffer_exit; LOGV("stepServer recovered"); mFlags &= ~TrackBase::STEPSERVER_FAILED; framesReady = cblk->framesReady(); if (LIKELY(framesReady)) { uint64_t s = cblk->server; uint64_t bufferEnd = cblk->serverBase + cblk->frameCount; bufferEnd = (cblk->loopEnd < bufferEnd) ? cblk->loopEnd : bufferEnd; if (framesReq > framesReady) { framesReq = framesReady; if (s + framesReq > bufferEnd) { framesReq = bufferEnd - s; buffer->raw = getBuffer(s, framesReq); if (buffer->raw == 0) goto getNextBuffer_exit; buffer->frameCount = framesReq; return NO_ERROR; getNextBuffer_exit: buffer->raw = 0; buffer->frameCount = 0; LOGV("getNextBuffer() no more data for track %d on thread %p", mName, mThread.unsafe_get()); return NOT_ENOUGH_DATA; // 该函数在看framesReady的时候已经看过了，此时就不再多说了。 // -----------------------------AudioFlinger::PlaybackThread::Track::getNextBuffer----------------------------------- // ------------------------------AudioFlinger::ThreadBase::TrackBase::step---------------------------------- // ---------------------------------audio_track_cblk_t::stepServer------------------------------- // --------------------------------AudioTrack::getPosition-------------------------------- } else { jniThrowException(env, "java/lang/IllegalStateException", "Unable to retrieve AudioTrack pointer for getPosition()"); return AUDIOTRACK_ERROR; // ------------------------------android_media_AudioTrack_get_position---------------------------------- // -------------------------------getPlaybackHeadPosition--------------------------------- //-------- tear down -------------- track.release(); ###########################################################<br/> get position其实就是取得audio_track_cblk_t对象中server的位置。<br/> server的位置在set position或者step server的时候会被改变。<br/> 应用程序会set position。<br/> Get next buffer的时候会step server。<br/>