Android Audio代码分析13 - AudioTrack::getPosition函数

打算以测试代码中所使用的接口为点，以接口间调用关系为线，逐步撕开Android中Audio的面纱。

*****************************************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();
}

**********************************************************************************************

源码路径：

frameworks\base\media\tests\mediaframeworktest\src\com\android\mediaframeworktest\functional\MediaAudioTrackTest.java

#######################说明################################

//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();
}

###########################################################

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get position其实就是取得audio_track_cblk_t对象中server的位置。

server的位置在set position或者step server的时候会被改变。

应用程序会set position。

Get next buffer的时候会step server。

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