Android Audio代码分析19 - setPosition函数

之前我们看过get position，今天来看看set position。

*****************************************源码*************************************************

//Test case 3: setPlaybackHeadPosition() on paused track
@LargeTest
public void testSetPlaybackHeadPositionPaused() throws Exception {
// constants for test
final String TEST_NAME = "testSetPlaybackHeadPositionPaused";
final int TEST_SR = 22050;
final int TEST_CONF = AudioFormat.CHANNEL_OUT_MONO;
final int TEST_FORMAT = AudioFormat.ENCODING_PCM_16BIT;
final int TEST_MODE = AudioTrack.MODE_STREAM;
final int TEST_STREAM_TYPE = AudioManager.STREAM_MUSIC;

//-------- initialization --------------
int minBuffSize = AudioTrack.getMinBufferSize(TEST_SR, TEST_CONF, TEST_FORMAT);
AudioTrack track = new AudioTrack(TEST_STREAM_TYPE, TEST_SR, TEST_CONF, TEST_FORMAT,
2*minBuffSize, TEST_MODE);
byte data[] = new byte[minBuffSize];
//--------    test        --------------
assumeTrue(TEST_NAME, track.getState() == AudioTrack.STATE_INITIALIZED);
track.write(data, 0, data.length);
track.write(data, 0, data.length);
track.play();
track.pause();
assumeTrue(TEST_NAME, track.getPlayState() == AudioTrack.PLAYSTATE_PAUSED);
assertTrue(TEST_NAME, track.setPlaybackHeadPosition(10) == AudioTrack.SUCCESS);
//-------- tear down      --------------
track.release();
}

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

源码路径：

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

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

//Test case 3: setPlaybackHeadPosition() on paused track
@LargeTest
public void testSetPlaybackHeadPositionPaused() throws Exception {
// constants for test
final String TEST_NAME = "testSetPlaybackHeadPositionPaused";
final int TEST_SR = 22050;
final int TEST_CONF = AudioFormat.CHANNEL_OUT_MONO;
final int TEST_FORMAT = AudioFormat.ENCODING_PCM_16BIT;
final int TEST_MODE = AudioTrack.MODE_STREAM;
final int TEST_STREAM_TYPE = AudioManager.STREAM_MUSIC;

//-------- initialization --------------
int minBuffSize = AudioTrack.getMinBufferSize(TEST_SR, TEST_CONF, TEST_FORMAT);
AudioTrack track = new AudioTrack(TEST_STREAM_TYPE, TEST_SR, TEST_CONF, TEST_FORMAT,
2*minBuffSize, TEST_MODE);
byte data[] = new byte[minBuffSize];
//--------    test        --------------
assumeTrue(TEST_NAME, track.getState() == AudioTrack.STATE_INITIALIZED);
track.write(data, 0, data.length);
track.write(data, 0, data.length);
track.play();
track.pause();
assumeTrue(TEST_NAME, track.getPlayState() == AudioTrack.PLAYSTATE_PAUSED);
assertTrue(TEST_NAME, track.setPlaybackHeadPosition(10) == AudioTrack.SUCCESS);
// +++++++++++++++++++++++++++++setPlaybackHeadPosition+++++++++++++++++++++++++++++++++++
/**
* Sets the playback head position. The track must be stopped for the position to be changed.
* @param positionInFrames playback head position expressed in frames
* @return error code or success, see {@link #SUCCESS}, {@link #ERROR_BAD_VALUE},
*    {@link #ERROR_INVALID_OPERATION}
*/
public int setPlaybackHeadPosition(int positionInFrames) {
synchronized(mPlayStateLock) {
if ((mPlayState == PLAYSTATE_STOPPED) || (mPlayState == PLAYSTATE_PAUSED)) {
return native_set_position(positionInFrames);
// ++++++++++++++++++++++++++++android_media_AudioTrack_set_position++++++++++++++++++++++++++++++++++++
static jint android_media_AudioTrack_set_position(JNIEnv *env,  jobject thiz,
jint position) {

AudioTrack *lpTrack = (AudioTrack *)env->GetIntField(
thiz, javaAudioTrackFields.nativeTrackInJavaObj);

if (lpTrack) {
return android_media_translateErrorCode( lpTrack->setPosition(position) );
// ++++++++++++++++++++++++++++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;
}
// ----------------------------AudioTrack::setPosition------------------------------------
} else {
jniThrowException(env, "java/lang/IllegalStateException",
"Unable to retrieve AudioTrack pointer for setPosition()");
return AUDIOTRACK_ERROR;
}
}
// ----------------------------android_media_AudioTrack_set_position------------------------------------
} else {
return ERROR_INVALID_OPERATION;
}
}
}
// -----------------------------setPlaybackHeadPosition-----------------------------------
//-------- tear down      --------------
track.release();
}

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

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set position改变的其实是audio_track_cblk_t结构体中的server成员。

也就是AudioMixer中从audio_track_cblk_t中copy数据时的起始地址。

这儿设置position有个限制，就是不能跑到user前面去，

即不能让取数据的其实地址超出已写数据的范围。

在播放器中，大幅度拖动进度条，改变的应该是读取文件时的地址，

而不是audio_track_cblk_t对象中的地址。

另外，只能在stop的状态下才能改变position。

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