android ijkplayer c层分析-初始化(续1 java层与c层衔接)

初始化的过程上一篇其实并未完全分析完，这回接着来。java层的initPlayer函数中，最后还有native_setup的调用，走的是c层的IjkMediaPlayer_native_setup。来看看他干了什么吧：

IjkMediaPlayer_native_setup(JNIEnv *env, jobject thiz, jobject weak_this)
{
    MPTRACE("%s\n", __func__);
    IjkMediaPlayer *mp = ijkmp_android_create(message_loop);
    JNI_CHECK_GOTO(mp, env, "java/lang/OutOfMemoryError", "mpjni: native_setup: ijkmp_create() failed", LABEL_RETURN);

    jni_set_media_player(env, thiz, mp);
    ijkmp_set_weak_thiz(mp, (*env)->NewGlobalRef(env, weak_this));
    ijkmp_set_inject_opaque(mp, ijkmp_get_weak_thiz(mp));
    ijkmp_android_set_mediacodec_select_callback(mp, mediacodec_select_callback, ijkmp_get_weak_thiz(mp));

LABEL_RETURN:
    ijkmp_dec_ref_p(&mp);
}

首先创建播放器IjkMediaPlayer，传入一个message_loop。继续看ijkmp_android_create：

IjkMediaPlayer *ijkmp_android_create(int(*msg_loop)(void*))
{
    IjkMediaPlayer *mp = ijkmp_create(msg_loop);
    if (!mp)
        goto fail;

    mp->ffplayer->vout = SDL_VoutAndroid_CreateForAndroidSurface();
    if (!mp->ffplayer->vout)
        goto fail;

    mp->ffplayer->pipeline = ffpipeline_create_from_android(mp->ffplayer);
    if (!mp->ffplayer->pipeline)
        goto fail;

    ffpipeline_set_vout(mp->ffplayer->pipeline, mp->ffplayer->vout);

    return mp;

fail:
    ijkmp_dec_ref_p(&mp);
    return NULL;
}

好吧，往下继续看ijkmp_create：

IjkMediaPlayer *ijkmp_create(int (*msg_loop)(void*))
{
    IjkMediaPlayer *mp = (IjkMediaPlayer *) mallocz(sizeof(IjkMediaPlayer));
    if (!mp)
        goto fail;

    mp->ffplayer = ffp_create();
    if (!mp->ffplayer)
        goto fail;

    mp->msg_loop = msg_loop;

    ijkmp_inc_ref(mp);
    pthread_mutex_init(&mp->mutex, NULL);

    return mp;

    fail:
    ijkmp_destroy_p(&mp);
    return NULL;
}

一上来为结构体IjkMediaPlayer分配空间，然后填充里面的内容，例如ffplayer和msg_loop。结构体如下：

struct IjkMediaPlayer {
    volatile int ref_count;
    pthread_mutex_t mutex;
    FFPlayer *ffplayer;

    int (*msg_loop)(void*);
    SDL_Thread *msg_thread;
    SDL_Thread _msg_thread;

    int mp_state;
    char *data_source;
    void *weak_thiz;

    int restart;
    int restart_from_beginning;
    int seek_req;
    long seek_msec;
};

ffmpeg的player和sdl，以及msg_loop，其余是状态及seek的内容。
回来到ijkmp_create。看这里将这个函数指针给了mp的msg_loop。然后是ijkmp_inc_ref，这里设置了mp的引用计数加1。好吧，先回来看下这个loop是什么东西，回到最初的IjkMediaPlayer_native_setup。

static int message_loop(void *arg)
{
    MPTRACE("%s\n", __func__);

    JNIEnv *env = NULL;
    (*g_jvm)->AttachCurrentThread(g_jvm, &env, NULL );

    IjkMediaPlayer *mp = (IjkMediaPlayer*) arg;
    JNI_CHECK_GOTO(mp, env, NULL, "mpjni: native_message_loop: null mp", LABEL_RETURN);

    message_loop_n(env, mp);

LABEL_RETURN:
    ijkmp_dec_ref_p(&mp);
    (*g_jvm)->DetachCurrentThread(g_jvm);

    MPTRACE("message_loop exit");
    return 0;
}

AttachCurrentThread为了获取JNIEnv，然后关键点是message_loop_n：

static void message_loop_n(JNIEnv *env, IjkMediaPlayer *mp)
{
    jobject weak_thiz = (jobject) ijkmp_get_weak_thiz(mp);
    JNI_CHECK_GOTO(weak_thiz, env, NULL, "mpjni: message_loop_n: null weak_thiz", LABEL_RETURN);

    while (1) {
        AVMessage msg;

        int retval = ijkmp_get_msg(mp, &msg, 1);
        if (retval < 0)
            break;

        // block-get should never return 0
        assert(retval > 0);

        switch (msg.what) {
        case FFP_MSG_FLUSH:
            MPTRACE("FFP_MSG_FLUSH:\n");
            post_event(env, weak_thiz, MEDIA_NOP, 0, 0);
            break;
        case FFP_MSG_ERROR:
            MPTRACE("FFP_MSG_ERROR: %d\n", msg.arg1);
            post_event(env, weak_thiz, MEDIA_ERROR, MEDIA_ERROR_IJK_PLAYER, msg.arg1);
            break;
        case FFP_MSG_PREPARED:
            MPTRACE("FFP_MSG_PREPARED:\n");
            post_event(env, weak_thiz, MEDIA_PREPARED, 0, 0);
            break;
        case FFP_MSG_COMPLETED:
            MPTRACE("FFP_MSG_COMPLETED:\n");
            post_event(env, weak_thiz, MEDIA_PLAYBACK_COMPLETE, 0, 0);
            break;
        case FFP_MSG_VIDEO_SIZE_CHANGED:
            MPTRACE("FFP_MSG_VIDEO_SIZE_CHANGED: %d, %d\n", msg.arg1, msg.arg2);
            post_event(env, weak_thiz, MEDIA_SET_VIDEO_SIZE, msg.arg1, msg.arg2);
            break;
        case FFP_MSG_SAR_CHANGED:
            MPTRACE("FFP_MSG_SAR_CHANGED: %d, %d\n", msg.arg1, msg.arg2);
            post_event(env, weak_thiz, MEDIA_SET_VIDEO_SAR, msg.arg1, msg.arg2);
            break;
        case FFP_MSG_VIDEO_RENDERING_START:
            MPTRACE("FFP_MSG_VIDEO_RENDERING_START:\n");
            post_event(env, weak_thiz, MEDIA_INFO, MEDIA_INFO_VIDEO_RENDERING_START, 0);
            break;
        case FFP_MSG_AUDIO_RENDERING_START:
            MPTRACE("FFP_MSG_AUDIO_RENDERING_START:\n");
            post_event(env, weak_thiz, MEDIA_INFO, MEDIA_INFO_AUDIO_RENDERING_START, 0);
            break;
        case FFP_MSG_VIDEO_ROTATION_CHANGED:
            MPTRACE("FFP_MSG_VIDEO_ROTATION_CHANGED: %d\n", msg.arg1);
            post_event(env, weak_thiz, MEDIA_INFO, MEDIA_INFO_VIDEO_ROTATION_CHANGED, msg.arg1);
            break;
        case FFP_MSG_BUFFERING_START:
            MPTRACE("FFP_MSG_BUFFERING_START:\n");
            post_event(env, weak_thiz, MEDIA_INFO, MEDIA_INFO_BUFFERING_START, 0);
            break;
        case FFP_MSG_BUFFERING_END:
            MPTRACE("FFP_MSG_BUFFERING_END:\n");
            post_event(env, weak_thiz, MEDIA_INFO, MEDIA_INFO_BUFFERING_END, 0);
            break;
        case FFP_MSG_BUFFERING_UPDATE:
            // MPTRACE("FFP_MSG_BUFFERING_UPDATE: %d, %d", msg.arg1, msg.arg2);
            post_event(env, weak_thiz, MEDIA_BUFFERING_UPDATE, msg.arg1, msg.arg2);
            break;
        case FFP_MSG_BUFFERING_BYTES_UPDATE:
            break;
        case FFP_MSG_BUFFERING_TIME_UPDATE:
            break;
        case FFP_MSG_SEEK_COMPLETE:
            MPTRACE("FFP_MSG_SEEK_COMPLETE:\n");
            post_event(env, weak_thiz, MEDIA_SEEK_COMPLETE, 0, 0);
            break;
        case FFP_MSG_PLAYBACK_STATE_CHANGED:
            break;
        case FFP_MSG_TIMED_TEXT:
            if (msg.obj) {
                jstring text = (*env)->NewStringUTF(env, (char *)msg.obj);
                post_event2(env, weak_thiz, MEDIA_TIMED_TEXT, 0, 0, text);
                J4A_DeleteLocalRef__p(env, &text);
            }
            else {
                post_event2(env, weak_thiz, MEDIA_TIMED_TEXT, 0, 0, NULL);
            }
            break;
        default:
            ALOGE("unknown FFP_MSG_xxx(%d)\n", msg.what);
            break;
        }
        msg_free_res(&msg);
    }

LABEL_RETURN:
    ;
}

这明显是个事件处理loop，关键是post_event，里面就一句话：J4AC_IjkMediaPlayer__postEventFromNative(env, weak_this, what, arg1, arg2, NULL);最后追到J4AC_tv_danmaku_ijk_media_player_IjkMediaPlayer__postEventFromNative，里面是(*env)->CallStaticVoidMethod(env, class_J4AC_tv_danmaku_ijk_media_player_IjkMediaPlayer.id, class_J4AC_tv_danmaku_ijk_media_player_IjkMediaPlayer.method_postEventFromNative, weakThiz, what, arg1, arg2, obj);调用java层的函数，再看下去：J4A_loadClass__J4AC_tv_danmaku_ijk_media_player_IjkMediaPlayer里面:

class_id = class_J4AC_tv_danmaku_ijk_media_player_IjkMediaPlayer.id;
    name     = "postEventFromNative";
    sign     = "(Ljava/lang/Object;IIILjava/lang/Object;)V";
    class_J4AC_tv_danmaku_ijk_media_player_IjkMediaPlayer.method_postEventFromNative = J4A_GetStaticMethodID__catchAll(env, class_id, name, sign);
    if (class_J4AC_tv_danmaku_ijk_media_player_IjkMediaPlayer.method_postEventFromNative == NULL)
        goto fail;

不用说了吧，走的是java层的函数postEventFromNative。

@CalledByNative
    private static void postEventFromNative(Object weakThiz, int what,
            int arg1, int arg2, Object obj) {
        if (weakThiz == null)
            return;

        @SuppressWarnings("rawtypes")
        IjkMediaPlayer mp = (IjkMediaPlayer) ((WeakReference) weakThiz).get();
        if (mp == null) {
            return;
        }

        if (what == MEDIA_INFO && arg1 == MEDIA_INFO_STARTED_AS_NEXT) {
            // this acquires the wakelock if needed, and sets the client side
            // state
            mp.start();
        }
        if (mp.mEventHandler != null) {
            Message m = mp.mEventHandler.obtainMessage(what, arg1, arg2, obj);
            mp.mEventHandler.sendMessage(m);
        }
    }

这里把弱引用的IjkMediaPlayer取出来了，然后调用了mp.mEventHandler.sendMessage(m);那么这个IjkMediaPlayer是哪里来的呢？答案就在IjkMediaPlayer_native_setup，也就是java层的native_setup函数中传递进去的，在最初的initPlayer函数中调用的。那么java层的postEventFromNative里面的mp.mEventHandler是什么呢？就是initPlayer里面创建的looper。这下子串起来了吧，java层建立的IjkMediaPlayer，并填充eventhandler,c层在触发特定的一些动作（例如打开直播等），会调用java层的函数向looper里面发送message，于是java层就收到了内容，可以进行相关处理了。整个过程是个异步的过程，并不阻塞。至于ffmpeg的东西，都是在c层创建并填充的。