http streaming緩衝buffer更新進度的處理流程

對於http streaming，更新buffer進度的處理在AwesomePlayer::onBufferingUpdate()函數中，首先判斷mCachedSource != NULL，即是http://流媒體的情況下，通過getBitrate(&bitrate)函數取得bitrate，然後計算出buffer的進度，通過notifyListener_l(MEDIA_BUFFERING_UPDATE, percentage)發訊息，在MediaPlayer.cpp類對應MediaPlayer::notify函數接收到MEDIA_BUFFERING_UPDATE訊息，然後在應用程式層通過註冊MediaPlayer的setOnBufferingUpdateListener監聽，接收到MEDIA_BUFFERING_UPDATE訊息，即可以獲得buffer更新的進度值percent。

1.AwesomePlayer::onBufferingUpdate()函數

計算buffer更新的percent

            if (getBitrate(&bitrate)) {                LOGV("onBufferingUpdate: bitrate: %lld", bitrate);                size_t lowWaterThreshold = (size_t) ((bitrate * kLowWaterMarkUs                        * (1.0f + kThresholdPaddingFactor)) / 8000000ll);                size_t highWaterThreshold = (size_t) ((bitrate                        * kHighWaterMarkUs * (1.0f + kThresholdPaddingFactor))                        / 8000000ll);                mCachedSource->setThresholds(lowWaterThreshold,                        highWaterThreshold);                size_t cachedSize = mCachedSource->cachedSize();                int64_t cachedDurationUs = cachedSize * 8000000ll / bitrate;                int percentage = 100.0 * (double)cachedDurationUs / mDurationUs;                if (percentage > 100) {                    percentage = 100;                }                notifyListener_l(MEDIA_BUFFERING_UPDATE, percentage);            } else {

其中8000000ll為1byte以us為單位的bit數，單位long long類型，即1byte = 1byte * 8bits/byte * 1000000us = 8000000ll

 2.AwesomePlayer::getBitrate函數

bool AwesomePlayer::getBitrate(int64_t *bitrate) {    off64_t size;    if (mDurationUs >= 0 && mCachedSource != NULL            && mCachedSource->getSize(&size) == OK) {        *bitrate = size * 8000000ll / mDurationUs;  // in bits/sec        return true;    }    if (mBitrate >= 0) {        *bitrate = mBitrate;        return true;    }    *bitrate = 0;    return false;}

3. NuCachedSource2::getSize

調用其成員變數mSource的getSize函數取得當前更新的size大小，mSouce即為NuCachedSource2建構函式傳遞的參數，此參數為AwesomePlayer::finishSetDataSource_l()函數中建立mCachedSource對象時傳遞的類型為NuHTTPDataSource的參數mConnectingDataSource。

所以，mSource->getSize調用的是NuHTTPDataSource的getSize函數。

status_t NuCachedSource2::getSize(off64_t *size) {    return mSource->getSize(size);}

4.NuHTTPDataSource::getSize函數

此函數取得size值為mContentLength的值，取得mContentLength的值的處理在NuHTTPDataSource::connect函數，從http header中讀取的，即從"Content-Length"或者"Content-Range"頭域指定的值中計算出的。

NuCachedSource2和NuHTTPDataSource類以及getSize函數的關係圖如下：

 

 5.mediaplayer.cpp的MediaPlayer::notify函數

void MediaPlayer::notify(int msg, int ext1, int ext2){...    case MEDIA_BUFFERING_UPDATE:        LOGV("buffering %d", ext1);        break;...    sp<MediaPlayerListener> listener = mListener;    if (locked) mLock.unlock();    // this prevents re-entrant calls into client code    if ((listener != 0) && send) {        Mutex::Autolock _l(mNotifyLock);        LOGV("callback application");        listener->notify(msg, ext1, ext2); // 發送訊息給MediaPlayer.java，然後應用程式層就接收到        LOGV("back from callback");    }

6.MediaPlayer.java的setOnBufferingUpdateListener函數

在onBufferingUpdate函數中的參數percent即為buffer更新進度值0-100.

    /**     * Interface definition of a callback to be invoked indicating buffering     * status of a media resource being streamed over the network.     */    public interface OnBufferingUpdateListener    {        /**         * Called to update status in buffering a media stream.         *         * @param mp      the MediaPlayer the update pertains to         * @param percent the percentage (0-100) of the buffer         *                that has been filled thus far         */        void onBufferingUpdate(MediaPlayer mp, int percent);    }    /**     * Register a callback to be invoked when the status of a network     * stream's buffer has changed.     *     * @param listener the callback that will be run.     */    public void setOnBufferingUpdateListener(OnBufferingUpdateListener listener)    {        mOnBufferingUpdateListener = listener;    }