Android Audio程式碼分析19 - setPosition函數

之前我們看過get position，今天來看看set position。

*****************************************源碼*************************************************
    //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();
    }
**********************************************************************************************
源碼路徑：
frameworks\base\media\tests\mediaframeworktest\src\com\android\mediaframeworktest\functional\MediaAudioTrackTest.java

#######################說明################################
    //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改變的其實是audio_track_cblk_t結構體中的server成員。
也就是AudioMixer中從audio_track_cblk_t中copy資料時的起始地址。
這兒設定position有個限制，就是不能跑到user前面去，
即不能讓取資料的其真實位址超出已寫資料的範圍。
在播放器中，大幅度拖動進度條，改變的應該是讀取檔案時的地址，
而不是audio_track_cblk_t對象中的地址。

另外，只能在stop的狀態下才能改變position。
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摘自：江風的專欄