从Android 2.0，Google引进了Stagefright，并在android2.3时用Stagefright在Android中是以shared library的形式存在(libstagefright.so)，其中AwesomePlayer可用來播放video/audio。AwesomePlayer提供許多API，可以让上层的应用用程式(Java/JNI)來呼叫，我在这里简单说明一下video playback的流程（采用的是android2.2的源码）。

在Java中，若要播放一个影片，我們通常会这样写：

MediaPlayer mp = new MediaPlayer();
mp.setDataSource(PATH_TO_FILE);
mp.prepare()；
mp.start();

在Stagefright中，会看到如下的处理：

1.将影片文件的绝对路径指定给uri:

status_t AwesomePlayer::setDataSource(
        const char *uri, const KeyedVector<String8, String8> *headers) {
    Mutex::Autolock autoLock(mLock);
    return setDataSource_l(uri, headers);
}

status_t AwesomePlayer::setDataSource_l(
        const char *uri, const KeyedVector<String8, String8> *headers) {
    reset_l();

    mUri = uri;

    if (headers) {
        mUriHeaders = *headers;
    }

    // The actual work will be done during preparation in the call to
    // ::finishSetDataSource_l to avoid blocking the calling thread in
    // setDataSource for any significant time.

    return OK;
}

2.启动mQueue:

status_t AwesomePlayer::prepare() {
    Mutex::Autolock autoLock(mLock);
    return prepare_l();
}

status_t AwesomePlayer::prepare_l() {
    if (mFlags & PREPARED) {
        return OK;
    }

    if (mFlags & PREPARING) {
        return UNKNOWN_ERROR;
    }

    mIsAsyncPrepare = false;
    status_t err = prepareAsync_l();

    if (err != OK) {
        return err;
    }

    while (mFlags & PREPARING) {
        mPreparedCondition.wait(mLock);
    }

    return mPrepareResult;
}


status_t AwesomePlayer::prepareAsync() {
    Mutex::Autolock autoLock(mLock);

    if (mFlags & PREPARING) {
        return UNKNOWN_ERROR;  // async prepare already pending
    }

    mIsAsyncPrepare = true;
    return prepareAsync_l();
}

status_t AwesomePlayer::prepareAsync_l() {
    if (mFlags & PREPARING) {
        return UNKNOWN_ERROR;  // async prepare already pending
    }

    if (!mQueueStarted) {
        mQueue.start();
        mQueueStarted = true;
    }

    mFlags |= PREPARING;
    mAsyncPrepareEvent = new AwesomeEvent(
            this, &AwesomePlayer::onPrepareAsyncEvent);

    mQueue.postEvent(mAsyncPrepareEvent);

    return OK;
}

3.onprepareAsyncEvent被触发，根据传来文件的header来创建相应的解析器，并初始化音视频解码器：

void AwesomePlayer::onPrepareAsyncEvent() {
    sp<Prefetcher> prefetcher;

    {
        Mutex::Autolock autoLock(mLock);

        if (mFlags & PREPARE_CANCELLED) {
            LOGI("prepare was cancelled before doing anything");
            abortPrepare(UNKNOWN_ERROR);
            return;
        }

        if (mUri.size() > 0) {
           //在这个方法中创建解析器
           <strong>status_t err = finishSetDataSource_l();</strong>

            if (err != OK) {
                abortPrepare(err);
                return;
            }
        }

        if (mVideoTrack != NULL && mVideoSource == NULL) {
            //初始化视频解码器
            <strong>status_t err = initVideoDecoder();</strong>

            if (err != OK) {
                abortPrepare(err);
                return;
            }
        }

        if (mAudioTrack != NULL && mAudioSource == NULL) {
            //初始化音频解码器
            <strong>status_t err = initAudioDecoder();
</strong>
            if (err != OK) {
                abortPrepare(err);
                return;
            }
        }

        prefetcher = mPrefetcher;
    }
 if (prefetcher != NULL) {
        {
            Mutex::Autolock autoLock(mLock);
            if (mFlags & PREPARE_CANCELLED) {
                LOGI("prepare was cancelled before preparing the prefetcher");

                prefetcher.clear();
                abortPrepare(UNKNOWN_ERROR);
                return;
            }
        }

        LOGI("calling prefetcher->prepare()");
        status_t result =
            prefetcher->prepare(&AwesomePlayer::ContinuePreparation, this);

        prefetcher.clear();

        if (result == OK) {
            LOGI("prefetcher is done preparing");
        } else {
            Mutex::Autolock autoLock(mLock);

            CHECK_EQ(result, -EINTR);

            LOGI("prefetcher->prepare() was cancelled early.");
            abortPrepare(UNKNOWN_ERROR);
            return;
        }
    }

 Mutex::Autolock autoLock(mLock);

    if (mIsAsyncPrepare) {
        if (mVideoWidth < 0 || mVideoHeight < 0) {
            notifyListener_l(MEDIA_SET_VIDEO_SIZE, 0, 0);
        } else {
            notifyListener_l(MEDIA_SET_VIDEO_SIZE, mVideoWidth, mVideoHeight);
        }

        notifyListener_l(MEDIA_PREPARED);
    }

    mPrepareResult = OK;
    mFlags &= ~(PREPARING|PREPARE_CANCELLED);
    mFlags |= PREPARED;
    mAsyncPrepareEvent = NULL;
    mPreparedCondition.broadcast();

    postBufferingEvent_l();
}

status_t AwesomePlayer::finishSetDataSource_l() {
    sp<DataSource> dataSource;

    if (!strncasecmp("http://", mUri.string(), 7)) {
        mConnectingDataSource = new HTTPDataSource(mUri, &mUriHeaders);

        mLock.unlock();
        status_t err = mConnectingDataSource->connect();
        mLock.lock();

        if (err != OK) {
            mConnectingDataSource.clear();

            LOGI("mConnectingDataSource->connect() returned %d", err);
            return err;
        }

        dataSource = new CachingDataSource(
                mConnectingDataSource, 64 * 1024, 10);

        mConnectingDataSource.clear();
    } else {
        dataSource = DataSource::CreateFromURI(mUri.string(), &mUriHeaders);
    }

    if (dataSource == NULL) {
        return UNKNOWN_ERROR;
    }

    <strong>sp<MediaExtractor> extractor = MediaExtractor::Create(dataSource);
</strong>
    if (extractor == NULL) {
        return UNKNOWN_ERROR;
    }

    dataSource->getDrmInfo(&mDecryptHandle, &mDrmManagerClient);
    if (mDecryptHandle != NULL
            && RightsStatus::RIGHTS_VALID != mDecryptHandle->status) {
        notifyListener_l(MEDIA_ERROR, MEDIA_ERROR_UNKNOWN, ERROR_NO_LICENSE);
    }

    if (dataSource->flags() & DataSource::kWantsPrefetching) {
        mPrefetcher = new Prefetcher;
    }

   <strong> return setDataSource_l(extractor)</strong>;
}

status_t AwesomePlayer::setDataSource_l(const sp<MediaExtractor> &extractor) {
    bool haveAudio = false;
    bool haveVideo = false;
    for (size_t i = 0; i < extractor->countTracks(); ++i) {
        sp<MetaData> meta = extractor->getTrackMetaData(i);

        const char *mime;
        CHECK(meta->findCString(kKeyMIMEType, &mime));

        if (!haveVideo && !strncasecmp(mime, "video/", 6)) {
            setVideoSource(extractor->getTrack(i));
            haveVideo = true;
        } else if (!haveAudio && !strncasecmp(mime, "audio/", 6)) {
            setAudioSource(extractor->getTrack(i));
            haveAudio = true;
        }

        if (haveAudio && haveVideo) {
            break;
        }
    }

    if (!haveAudio && !haveVideo) {
        return UNKNOWN_ERROR;
    }

    mExtractorFlags = extractor->flags();

    return OK;
}

5.将解析后的音视频数据分别交给VideoTrack和AudioTrack:

void AwesomePlayer::setVideoSource(sp<MediaSource> source) {
    CHECK(source != NULL);

    if (mPrefetcher != NULL) {
        source = mPrefetcher->addSource(source);
    }

    mVideoTrack = source;
}

void AwesomePlayer::setAudioSource(sp<MediaSource> source) {
    CHECK(source != NULL);

    if (mPrefetcher != NULL) {
        source = mPrefetcher->addSource(source);
    }

    mAudioTrack = source;
}

6.根据mVideoTrck中的编码类型来选择 video decoder 同理根据mAudioTrack中的编码类型来选择 audio decoder:

status_t AwesomePlayer::initVideoDecoder() {
    mVideoSource = OMXCodec::Create(
            mClient.interface(), mVideoTrack->getFormat(),
            false, // createEncoder
            mVideoTrack);

    if (mVideoSource != NULL) {
        int64_t durationUs;
        if (mVideoTrack->getFormat()->findInt64(kKeyDuration, &durationUs)) {
            Mutex::Autolock autoLock(mMiscStateLock);
            if (mDurationUs < 0 || durationUs > mDurationUs) {
                mDurationUs = durationUs;
            }
        }

        CHECK(mVideoTrack->getFormat()->findInt32(kKeyWidth, &mVideoWidth));
        CHECK(mVideoTrack->getFormat()->findInt32(kKeyHeight, &mVideoHeight));

        status_t err = mVideoSource->start();

        if (err != OK) {
            mVideoSource.clear();
            return err;
        }
    }

    return mVideoSource != NULL ? OK : UNKNOWN_ERROR;
}

7.将mVideoEvent放入mQueue中，开始解码播放，并交由mvideoRenderer来画出   audio的数据则交由audioplayer来管理，它最终将解码的数据交给audioTrack并由audioTrack和audioFlinger进行交互，最终将数据交给audio hal层，这个我们以后会做讲解：

status_t AwesomePlayer::play() {
    Mutex::Autolock autoLock(mLock);
    return play_l();
}
status_t AwesomePlayer::play_l() {
    if (mFlags & PLAYING) {
        return OK;
    }

    if (!(mFlags & PREPARED)) {
        status_t err = prepare_l();

        if (err != OK) {
            return err;
        }
    }

    mFlags |= PLAYING;
    mFlags |= FIRST_FRAME;

    bool deferredAudioSeek = false;

    if (mAudioSource != NULL) {
        if (mAudioPlayer == NULL) {
            if (mAudioSink != NULL) {
                //音频数据由audioplayer进行管理
                mAudioPlayer = new AudioPlayer(mAudioSink);
                mAudioPlayer->setSource(mAudioSource);

                // We've already started the MediaSource in order to enable
                // the prefetcher to read its data.
                  //调用audioPlayer的start方法则是调用audioSource对数据进行解码
                   //并将解码似得数据最终交给audioTrack,并调用audioTrack的start方法与audioFlinger进行交互
                   status_t err = mAudioPlayer->start(
                        true /* sourceAlreadyStarted */);

                if (err != OK) {
                    delete mAudioPlayer;
                    mAudioPlayer = NULL;

                    mFlags &= ~(PLAYING | FIRST_FRAME);

                    return err;
                }

                delete mTimeSource;
                mTimeSource = mAudioPlayer;

                deferredAudioSeek = true;

                mWatchForAudioSeekComplete = false;
                mWatchForAudioEOS = true;
            }
        } else {
            mAudioPlayer->resume();
        }

        postCheckAudioStatusEvent_l();
    }

    if (mTimeSource == NULL && mAudioPlayer == NULL) {
        mTimeSource = new SystemTimeSource;
    }

    if (mVideoSource != NULL) {
        // Kick off video playback
        //将mVideoEvent放入queue中
          postVideoEvent_l();
    }

    if (deferredAudioSeek) {
        // If there was a seek request while we were paused
        // and we're just starting up again, honor the request now.
        seekAudioIfNecessary_l();
    }

    if (mFlags & AT_EOS) {
        // Legacy behaviour, if a stream finishes playing and then
        // is started again, we play from the start...
        seekTo_l(0);
    }

    if (mDecryptHandle != NULL) {
        int64_t position;
        getPosition(&position);
        mDrmManagerClient->setPlaybackStatus(mDecryptHandle,
                Playback::START, position / 1000);
    }

    return OK;
}

 void AwesomePlayer::postVideoEvent_l(int64_t delayUs) {
    if (mVideoEventPending) {
        return;
    }

    mVideoEventPending = true;
    mQueue.postEventWithDelay(mVideoEvent, delayUs < 0 ? 10000 : delayUs);
}

void AwesomePlayer::onVideoEvent()
{
  mVideoSource->read(&mVideoBuffer, &options);
  mVideoRenderer->render(mVideoBuffer);

  postVideoEvent_l();
}