MediaPlayer(七)--prepareAsync

frameworks/base/media/java/android/media/MediaPlayer.java

    /**
     * Prepares the player for playback, asynchronously.
     *
     * After setting the datasource and the display surface, you need to either
     * call prepare() or prepareAsync(). For streams, you should call prepareAsync(),
     * which returns immediately, rather than blocking until enough data has been
     * buffered.
     *
     * @throws IllegalStateException if it is called in an invalid state
     */
    public native void prepareAsync() throws IllegalStateException;

MediaPlayer.java 没有任何流程，直接调用了jni。从注释可以了解到，设置完datasource和 display surface就需要调用prepare或prepareAsync。prepare是阻塞的接口，prepareAsync是异步的接口。

/home/llm/project/rk/8.1/frameworks/base/media/jni/android_media_MediaPlayer.cpp

static void
android_media_MediaPlayer_prepareAsync(JNIEnv *env, jobject thiz)
{
    sp<MediaPlayer> mp = getMediaPlayer(env, thiz);
    if (mp == NULL ) {
        jniThrowException(env, "java/lang/IllegalStateException", NULL);
        return;
    }

    // Handle the case where the display surface was set before the mp was
    // initialized. We try again to make it stick.
    sp<IGraphicBufferProducer> st = getVideoSurfaceTexture(env, thiz);
    mp->setVideoSurfaceTexture(st);

    process_media_player_call( env, thiz, mp->prepareAsync(), "java/io/IOException", "Prepare Async failed." );
}

首先重新设置了display surface，从注释看setDisplay接口可以在setDataSource之前被调用。接着调用到mediaplayer.cpp的接口

status_t MediaPlayer::prepareAsync()
{
    ALOGV("prepareAsync");
    Mutex::Autolock _l(mLock);
    return prepareAsync_l();
}

// must call with lock held
status_t MediaPlayer::prepareAsync_l()
{
    if ( (mPlayer != 0) && ( mCurrentState & (MEDIA_PLAYER_INITIALIZED | MEDIA_PLAYER_STOPPED) ) ) {
        if (mAudioAttributesParcel != NULL) {
            mPlayer->setParameter(KEY_PARAMETER_AUDIO_ATTRIBUTES, *mAudioAttributesParcel);
        } else {
            mPlayer->setAudioStreamType(mStreamType);
        }
        mCurrentState = MEDIA_PLAYER_PREPARING;
        return mPlayer->prepareAsync();
    }
    ALOGE("prepareAsync called in state %d, mPlayer(%p)", mCurrentState, mPlayer.get());
    return INVALID_OPERATION;
}

首先是对audio的设置，这部分笔者也没研究。
其次是将播放器的状态置为MEDIA_PLAYER_PREPARING
最后调用服务端的prepareAsync

frameworks/av/media/libmediaplayerservice/MediaPlayerService.cpp

status_t MediaPlayerService::Client::prepareAsync()
{
    ALOGV("[%d] prepareAsync", mConnId);
    mMaybeVideoAlive = (mConnectedWindow.get() != NULL);
    sp<MediaPlayerBase> p = getPlayer();
    if (p == 0) return UNKNOWN_ERROR;
    status_t ret = p->prepareAsync();
#if CALLBACK_ANTAGONIZER
    ALOGD("start Antagonizer");
    if (ret == NO_ERROR) mAntagonizer->start();
#endif
    return ret;
}prepare

留下两个知识盲点：mConnectedWindow，CALLBACK_ANTAGONIZER
主要是直接调用到NuPlayer的prepareAsync.NuPlayer处理完后会通过notify回调函数通知java层，流程大概如下
NuPlayer -->MediaPlayerService::Client::notify--> MediaPlayer::notify--> JNIMediaPlayerListener::notify -->postEventFromNative（java）

根据MediaPlayer的状态图，MeidiaPlayer在执行往prepareAsync后会将状态置为prepared，这个操作是在MediaPlayer::notify处理的

void MediaPlayer::notify(int msg, int ext1, int ext2, const Parcel *obj)
{
    case MEDIA_PREPARED:
        ALOGV("MediaPlayer::notify() prepared");
        mCurrentState = MEDIA_PLAYER_PREPARED;
        if (mPrepareSync) {
            ALOGV("signal application thread");
            mPrepareSync = false;
            mPrepareStatus = NO_ERROR;
            mSignal.signal();
        }
        break;

}

mSignal.signal();是一种信号量的通讯方式，其他调用了mSignal.wait的地方会阻塞等待，当收到signal的信号后就可以继续往下执行。我们知道MediaPlayer prepare方法是阻塞的，实际上prepare就是调用mSignal.wait进行等待，其他地方跟prepareAsync的流程是差不多的

status_t MediaPlayer::prepare()
{
    ALOGV("prepare");
    Mutex::Autolock _l(mLock);
    mLockThreadId = getThreadId();
    if (mPrepareSync) {
        mLockThreadId = 0;
        return -EALREADY;
    }
    mPrepareSync = true;
    status_t ret = prepareAsync_l();
    if (ret != NO_ERROR) {
        mLockThreadId = 0;
        return ret;
    }

    if (mPrepareSync) {
        mSignal.wait(mLock);  // wait for prepare done
        mPrepareSync = false;
    }
    ALOGV("prepare complete - status=%d", mPrepareStatus);
    mLockThreadId = 0;
    return mPrepareStatus;
}