"本文转载自：[VNanyesheshou]的Android MultiMedia框架——OMXPlugin"

1.概述

  上一篇《Android多媒体框架--19：OMX服务启动》中提到OMXMaster添加两个Plugin，一个是SoftOMXPlugin，另一个是供应商实现的硬件Plugin，其父类是OMXPluginBase。

2.OMXPluginBase

  OMXPluginBase主要包含四个虚拟方法。硬件厂商接入自己的编解码器，需要继承OMXPluginBase 类，并实现抽象方法。

• frameworks/native/headers/media_plugin/media/hardware/OMXPluginBase.h

struct OMXPluginBase {
    OMXPluginBase() {}
    virtual ~OMXPluginBase() {}

    virtual OMX_ERRORTYPE makeComponentInstance(
            const char *name,
            const OMX_CALLBACKTYPE *callbacks,
            OMX_PTR appData,
            OMX_COMPONENTTYPE **component) = 0;

    virtual OMX_ERRORTYPE destroyComponentInstance(
            OMX_COMPONENTTYPE *component) = 0;

    virtual OMX_ERRORTYPE enumerateComponents(
            OMX_STRING name,
            size_t size,
            OMX_U32 index) = 0;

    virtual OMX_ERRORTYPE getRolesOfComponent(
            const char *name,
            Vector<String8> *roles) = 0;

private:
    OMXPluginBase(const OMXPluginBase &);
    OMXPluginBase &operator=(const OMXPluginBase &);
};

下面先看下硬件厂商实现的Plugin。

3.QComOMXPlugin

 &emspQComOMXPlugin继承自OMXPluginBase，是高通实现的硬编解码。

• hardware/qcom/media/**/libstagefrighthw/QComOMXPlugin.cpp

OMXPluginBase *createOMXPlugin() {
    //（1）创建QComOMXPlugin
    return new QComOMXPlugin;
}
---------------
QComOMXPlugin::QComOMXPlugin()
    //（2）加载libOmxCore.so
    : mLibHandle(dlopen("libOmxCore.so", RTLD_NOW)),
      mInit(NULL),
      mDeinit(NULL),
      mComponentNameEnum(NULL),
      mGetHandle(NULL),
      mFreeHandle(NULL),
      mGetRolesOfComponentHandle(NULL) {
    if (mLibHandle != NULL) {
        //（3）通过dlsym查找OMX_Init、OMX_Deinit等符号值
        mInit = (InitFunc)dlsym(mLibHandle, "OMX_Init");
        mDeinit = (DeinitFunc)dlsym(mLibHandle, "OMX_Deinit");

        mComponentNameEnum =
            (ComponentNameEnumFunc)dlsym(mLibHandle, "OMX_ComponentNameEnum");

        mGetHandle = (GetHandleFunc)dlsym(mLibHandle, "OMX_GetHandle");
        mFreeHandle = (FreeHandleFunc)dlsym(mLibHandle, "OMX_FreeHandle");

        mGetRolesOfComponentHandle =
            (GetRolesOfComponentFunc)dlsym(
                    mLibHandle, "OMX_GetRolesOfComponent");

        if (!mInit || !mDeinit || !mComponentNameEnum || !mGetHandle ||
            !mFreeHandle || !mGetRolesOfComponentHandle) {
            dlclose(mLibHandle);
            mLibHandle = NULL;
        } else
            (*mInit)();
    }
}

（1）OMXMaster通过createOMXPlugin创建QComOMXPlugin对象；
（2）QComOMXPlugin构造函数中加载libOmxCore.so；
（3）通过dlsym查找OMX_Init、OMX_Deinit等符号值；
（4）这些方法对应OMXPluginBase中的抽象方法。
libOmxCore.so中为厂商高通实现的具体硬件编解码功能，这里先不分析了。

4.SoftOMXPlugin

  SoftOMXPlugin是google提供的原生编解码插件，它并不负责实现编解码，而是负责管理加载编解码器。如下是其支持的音视频编解码格式，这些都是软件编解码。

• frameworks/av/media/libstagefright/omx/SoftOMXPlugin.cpp

static const struct {
    const char *mName;
    const char *mLibNameSuffix;
    const char *mRole;

} kComponents[] = {
    // two choices for aac decoding.
    // configurable in media/libstagefright/data/media_codecs_google_audio.xml
    // default implementation
    { "OMX.google.aac.decoder", "aacdec", "audio_decoder.aac" },
    // alternate implementation
    { "OMX.google.xaac.decoder", "xaacdec", "audio_decoder.aac" },
    { "OMX.google.aac.encoder", "aacenc", "audio_encoder.aac" },
    { "OMX.google.amrnb.decoder", "amrdec", "audio_decoder.amrnb" },
    { "OMX.google.amrnb.encoder", "amrnbenc", "audio_encoder.amrnb" },
    { "OMX.google.amrwb.decoder", "amrdec", "audio_decoder.amrwb" },
    { "OMX.google.amrwb.encoder", "amrwbenc", "audio_encoder.amrwb" },
    { "OMX.google.h264.decoder", "avcdec", "video_decoder.avc" },
    { "OMX.google.h264.encoder", "avcenc", "video_encoder.avc" },
    { "OMX.google.hevc.decoder", "hevcdec", "video_decoder.hevc" },
    { "OMX.google.g711.alaw.decoder", "g711dec", "audio_decoder.g711alaw" },
    { "OMX.google.g711.mlaw.decoder", "g711dec", "audio_decoder.g711mlaw" },
    { "OMX.google.mpeg2.decoder", "mpeg2dec", "video_decoder.mpeg2" },
    { "OMX.google.h263.decoder", "mpeg4dec", "video_decoder.h263" },
    { "OMX.google.h263.encoder", "mpeg4enc", "video_encoder.h263" },
    { "OMX.google.mpeg4.decoder", "mpeg4dec", "video_decoder.mpeg4" },
    { "OMX.google.mpeg4.encoder", "mpeg4enc", "video_encoder.mpeg4" },
    { "OMX.google.mp3.decoder", "mp3dec", "audio_decoder.mp3" },
    { "OMX.google.vorbis.decoder", "vorbisdec", "audio_decoder.vorbis" },
    { "OMX.google.opus.decoder", "opusdec", "audio_decoder.opus" },
    { "OMX.google.vp8.decoder", "vpxdec", "video_decoder.vp8" },
    { "OMX.google.vp9.decoder", "vpxdec", "video_decoder.vp9" },
    { "OMX.google.vp8.encoder", "vpxenc", "video_encoder.vp8" },
    { "OMX.google.vp9.encoder", "vpxenc", "video_encoder.vp9" },
    { "OMX.google.raw.decoder", "rawdec", "audio_decoder.raw" },
    { "OMX.google.flac.decoder", "flacdec", "audio_decoder.flac" },
    { "OMX.google.flac.encoder", "flacenc", "audio_encoder.flac" },
    { "OMX.google.gsm.decoder", "gsmdec", "audio_decoder.gsm" },
};

makeComponentInstance函数：

OMX_ERRORTYPE SoftOMXPlugin::makeComponentInstance(
        const char *name,
        const OMX_CALLBACKTYPE *callbacks,
        OMX_PTR appData,
        OMX_COMPONENTTYPE **component) {
    ALOGV("makeComponentInstance '%s'", name);

    for (size_t i = 0; i < kNumComponents; ++i) {
        if (strcmp(name, kComponents[i].mName)) {
            continue;
        }

        AString libName = "libstagefright_soft_";
        libName.append(kComponents[i].mLibNameSuffix);
        libName.append(".so");

        // RTLD_NODELETE means we keep the shared library around forever.
        // this eliminates thrashing during sequences like loading soundpools.
        // It also leaves the rest of the logic around the dlopen()/dlclose()
        // calls in this file unchanged.
        //
        // Implications of the change:
        // -- the codec process (where this happens) will have a slightly larger
        //    long-term memory footprint as it accumulates the loaded shared libraries.
        //    This is expected to be a small amount of memory.
        // -- plugin codecs can no longer (and never should have) depend on a
        //    free reset of any static data as the library would have crossed
        //    a dlclose/dlopen cycle.
        //

        //（1）动态加载对应的编解码器
        void *libHandle = dlopen(libName.c_str(), RTLD_NOW|RTLD_NODELETE);

        if (libHandle == NULL) {
            ALOGE("unable to dlopen %s: %s", libName.c_str(), dlerror());

            return OMX_ErrorComponentNotFound;
        }

        typedef SoftOMXComponent *(*CreateSoftOMXComponentFunc)(
                const char *, const OMX_CALLBACKTYPE *,
                OMX_PTR, OMX_COMPONENTTYPE **);
        //（2）dlsym查找createSoftOMXComponent符号值
        CreateSoftOMXComponentFunc createSoftOMXComponent =
            (CreateSoftOMXComponentFunc)dlsym(
                    libHandle,
                    "_Z22createSoftOMXComponentPKcPK16OMX_CALLBACKTYPE"
                    "PvPP17OMX_COMPONENTTYPE");

        if (createSoftOMXComponent == NULL) {
            dlclose(libHandle);
            libHandle = NULL;

            return OMX_ErrorComponentNotFound;
        }

        // （3）创建编解码器
        sp<SoftOMXComponent> codec =
            (*createSoftOMXComponent)(name, callbacks, appData, component);

        if (codec == NULL) {
            dlclose(libHandle);
            libHandle = NULL;

            return OMX_ErrorInsufficientResources;
        }

        OMX_ERRORTYPE err = codec->initCheck();
        if (err != OMX_ErrorNone) {
            dlclose(libHandle);
            libHandle = NULL;

            return err;
        }

        codec->incStrong(this);
        codec->setLibHandle(libHandle);

        return OMX_ErrorNone;
    }

    return OMX_ErrorInvalidComponentName;
}

（1）该函数通过ACodec传递的name参数，动态加载对应的编解码器，如 libstagefright_soft_aacdec.so。其包含的so如下：

01.png

（2）dlsym 查找createSoftOMXComponent符号值；
（3）创建编解码器；

android::SoftOMXComponent *createSoftOMXComponent(
        const char *name, const OMX_CALLBACKTYPE *callbacks,
        OMX_PTR appData, OMX_COMPONENTTYPE **component) {
    return new android::SoftAACEncoder2(name, callbacks, appData, component);
}

SoftAACEncoder2为aac编码器对应的实现类。其继承关系如下：

02.png

5.总结

  OMXPlugin分为软硬编解码库，分别加载不同的so，实现其对应的编解码。