OMXCodec與OMX事件處理流程

學習瞭解Mutilmedia Framework有一段時間了，今天閒下來稍微整理整理。OMXCodec.cpp類屬於libstagefright，在整個MM PF 相當OMX的適配層，供awesomeplayer調用，而OMX.cpp，OMXNoteInstance.cpp等相當於OpenMax中的OpenMax IL,首先講下OMXCodec與OMX callback事件的處理流程。先看整個流程的時序圖吧：

從時序圖看，首先我們要建立個OMXCodecObserver,該類是OMXCodec的內部類，在create函數中被創建，並把對應的OMXCodec加入都自己的觀察範圍內，具體代碼如下：

framework/base/media/libstagefright/OMXCodec.cpp

sp<MediaSource> OMXCodec::Create(
        const sp<IOMX> &omx,
        const sp<MetaData> &meta, bool createEncoder,
        const sp<MediaSource> &source,
        const char *matchComponentName,
        uint32_t flags) {

.....

sp<OMXCodecObserver> observer = new OMXCodecObserver;

...............

observer->setCodec(codec);

...................

}

其次初始化它的callback事件和事件的派發處理函數

OMX主要的callback事件有哪些呢？在framework/base/media/libstagefright/omx/OMXNodeInstance.cpp中的kCallbacks函數有如下定義：

// static
OMX_CALLBACKTYPE OMXNodeInstance::kCallbacks = {
&OnEvent, &OnEmptyBufferDone, &OnFillBufferDone
};

callback在哪定義呢？看framework/base/media/libstagefright/omx/OMX.cpp中的

status_t OMX::allocateNode(

.......................

OMXNodeInstance *instance = new OMXNodeInstance(this, observer);

    OMX_COMPONENTTYPE *handle;
    OMX_ERRORTYPE err = mMaster->makeComponentInstance(
            name, &OMXNodeInstance::kCallbacks,
            instance, &handle);

..............

mDispatchers.add(*node, new CallbackDispatcher(instance));

...................

}

即每個component對應一組callback事件。

這些callback由哪些函數返回呢？具體的定義在framework/base/media/libstagefright/openmax/OMX_Core.h

callback EventHandler()

#define OMX_SendCommand(                                    \
         hComponent,                                        \
         Cmd,                                               \
         nParam,                                            \
         pCmdData)                                          \
     ((OMX_COMPONENTTYPE*)hComponent)->SendCommand(         \
         hComponent,                                        \
         Cmd,                                               \
         nParam,                                            \
         pCmdData)

EmptyBufferDone call back.

#define OMX_EmptyThisBuffer(                                \
        hComponent,                                         \
        pBuffer)                                            \
    ((OMX_COMPONENTTYPE*)hComponent)->EmptyThisBuffer(      \
        hComponent,                                         \
        pBuffer)                        /* Macro End */

FillBufferDone call back

#define OMX_FillThisBuffer(                                 \
        hComponent,                                         \
        pBuffer)                                            \
    ((OMX_COMPONENTTYPE*)hComponent)->FillThisBuffer(       \
        hComponent,                                         \
        pBuffer)                        /* Macro End */

有了callback事件，如何dispatch呢？其實我們在allocateNote函數已經定義好了我們的dispatch函數

mDispatchers.add(*node, new CallbackDispatcher(instance));

有了oberser, callback event , callbackdispatcher，那麼一個callback event 如何從OMX傳到OMXCodec呢？下面我們以emptybuffer流程來具體看下，時序圖如下：