Android 11(R) MultiMedia（十五）MediaCodec同步模式实现一个简易播放器

这一节是实践，学习android原生SimplePlayer并自己码一遍，果然学到很多。

MyPlayer.h

#include <media/stagefright/foundation/AHandler.h>
#include <media/stagefright/foundation/AString.h>
#include <utils/KeyedVector.h>

namespace android {

class IGraphicBufferProducer;
struct MediaCodec;
class MediaCodecBuffer;
struct NuMediaExtractor;
class Surface;
struct ABuffer;
class AudioTrack;
struct ALooper;

class MyPlayer : public AHandler {

public:
    MyPlayer();

    status_t setDataSource(const char* path);
    status_t setSurface(const sp<IGraphicBufferProducer> &bufferProducer);
    status_t prepare();
    status_t start();
    status_t stop();

 protected:
    virtual ~MyPlayer();
    virtual void onMessageReceived(const sp<AMessage> &msg);

 private:

    enum {
        kWhatSetDataSource = 0,
        kWhatSetSurface,
        kWhatPrepare,
        kWhatStart,
        kWhatStop,
        kWhatDoMoreStuff,
    };

    enum State{
        UNINITIALIZED,
        UNPREPARED,
        STOPPED,
        STARTED,
    };

    struct BufferInfo{
        size_t mIndex;
        size_t mOffset;
        size_t mSize;
        int64_t mPresentationTimeUs;
        uint32_t mFlags;
    };

    struct CodecState {
        sp<MediaCodec> mCodec;
        Vector<sp<MediaCodecBuffer>> mBuffers[2];
        Vector<sp<ABuffer>> mCSD;
        List<size_t> mAvailInputBufferIndices;
        List<BufferInfo> mAvailOutputBufferInfos;
        sp<AudioTrack> mAudioTrack;
        uint32_t mNumFramesWritten;
    };

    status_t onPrepare();
    status_t onStart();
    status_t onStop();
    status_t onDoMoreStuff();
    status_t onOutputFormatChanged(CodecState *);
    status_t renderAudio(CodecState* state, BufferInfo* info, const sp<MediaCodecBuffer> &buffer);

    AString mPath;
    State mState;
    sp<Surface> mSurface;
    sp<NuMediaExtractor> mExtractor;
    sp<ALooper> mCodecLooper;
    KeyedVector<size_t, CodecState> mStateByTrackIndex;
    int32_t mDoMoreStuffGeneration;
    int64_t mStartTimeRealUs;
};

}

MyPlayer.cpp

//define LOG_NDEBUG 0
#define LOG_TAG "MyPlayer"

#include "MyPlayer.h"

#include <media/stagefright/foundation/ALooper.h>
#include <media/stagefright/foundation/AMessage.h>
#include <media/stagefright/foundation/ADebug.h>
#include <gui/Surface.h>
#include <media/stagefright/MediaCodec.h>
#include <media/stagefright/NuMediaExtractor.h>
#include <media/stagefright/foundation/ABuffer.h>
#include <media/MediaCodecBuffer.h>
#include <media/IMediaHTTPService.h>
#include <mediadrm/ICrypto.h>
#include <media/AudioTrack.h>

namespace android {

status_t PostAndAwaitResponse(const sp<AMessage> &msg, sp<AMessage> *response)
{
    status_t err = msg->postAndAwaitResponse(response);
    if(err != OK)
        return err;

    if(!(*response)->findInt32("err", &err))
    {
        err = OK;
    }
    return err;
}

MyPlayer::MyPlayer()
    : mState(UNINITIALIZED),
    mDoMoreStuffGeneration(0),
    mStartTimeRealUs(-1ll)
{

}

MyPlayer::~MyPlayer()
{

}

status_t MyPlayer::setDataSource(const char* path)
{
    sp<AMessage> msg = new AMessage(kWhatSetDataSource, this);
    msg->setString("path", path);
    sp<AMessage> response;
    return PostAndAwaitResponse(msg, &response);
}

status_t MyPlayer::setSurface(const sp<IGraphicBufferProducer> &BufferProducer)
{
    sp<AMessage> msg = new AMessage(kWhatSetSurface, this);

    sp<Surface> surface;
    if(BufferProducer != NULL)
        surface = new Surface(BufferProducer);

    msg->setObject("surface", surface);

    sp<AMessage> response;
    return PostAndAwaitResponse(msg, &response);
}

status_t MyPlayer::prepare()
{
    sp<AMessage> msg = new AMessage(kWhatPrepare, this);
    sp<AMessage> response;
    return PostAndAwaitResponse(msg, &response);
}

status_t MyPlayer::start()
{
    sp<AMessage> msg = new AMessage(kWhatStart, this);
    sp<AMessage> response;
    return PostAndAwaitResponse(msg, &response);
}

status_t MyPlayer::stop()
{
    sp<AMessage> msg = new AMessage(kWhatStop, this);
    sp<AMessage> response;
    return PostAndAwaitResponse(msg, &response);
}

void MyPlayer::onMessageReceived(const sp<AMessage> &msg)
{
    switch(msg->what())
    {
        case kWhatSetDataSource:
        {
            status_t err;
            if(mState != UNINITIALIZED)
                err = INVALID_OPERATION;
            else {
                CHECK(msg->findString("path", &mPath));
                mState = UNPREPARED;
            }
            sp<AReplyToken> replyID;
            CHECK(msg->senderAwaitsResponse(&replyID));
            sp<AMessage> response = new AMessage;
            response->setInt32("err", err);
            response->postReply(replyID);
            break;
        }
        case kWhatSetSurface:
        {
            status_t err;
            if(mState != UNPREPARED)
                err = INVALID_OPERATION;
            else{
                sp<RefBase> obj;
                CHECK(msg->findObject("surface", &obj));
                mSurface = static_cast<Surface*>(obj.get());
            }
            sp<AReplyToken> replyID;
            CHECK(msg->senderAwaitsResponse(&replyID));
            sp<AMessage> response = new AMessage;
            response->setInt32("err", err);
            response->postReply(replyID);
            break;
        }
        case kWhatPrepare:
        {
            status_t err;
            if(mState != UNPREPARED)
                err = INVALID_OPERATION;
            else{
                err = onPrepare();
                if(err == OK)
                    mState = STOPPED;
            }
            sp<AReplyToken> replyID;
            CHECK(msg->senderAwaitsResponse(&replyID));
            sp<AMessage> response = new AMessage;
            response->setInt32("err", err);
            response->postReply(replyID);
            break;
        }
        case kWhatStart:
        {
            status_t err = OK;
            if(mState == UNPREPARED){
                err = onPrepare();
                if(err == OK)
                    mState = STOPPED;
            }

            if(err == OK)
            {
                if(mState != STOPPED)
                    err = INVALID_OPERATION;
                else{
                    err = onStart();
                    if(err == OK)
                        mState = STARTED;
                }
            }
            sp<AReplyToken> replyID;
            CHECK(msg->senderAwaitsResponse(&replyID));
            sp<AMessage> response = new AMessage;
            response->setInt32("err", err);
            response->postReply(replyID);
            break;
        }
        case kWhatStop:
        {
            status_t err;
            if(mState != STARTED)
                err = INVALID_OPERATION;
            else{
                err = onStop();
                if(err == OK)
                    mState = STOPPED;
            }
            sp<AReplyToken> replyID;
            CHECK(msg->senderAwaitsResponse(&replyID));
            sp<AMessage> response = new AMessage;
            response->setInt32("err", err);
            response->postReply(replyID);
            break;
        }
        case kWhatDoMoreStuff:
        {
            int32_t generation;
            CHECK(msg->findInt32("generation", &generation));
            if(generation != mDoMoreStuffGeneration)
                break;

            status_t err = onDoMoreStuff();

            if(err == OK)
                msg->post(10000ll);

            break;
        }

        default:
            break;
    }
}

status_t MyPlayer::onPrepare()
{
    CHECK_EQ(mState, UNPREPARED);

    // 创建NuMediaExtractor
    mExtractor = new NuMediaExtractor();

    status_t err =  mExtractor->setDataSource(NULL, mPath.c_str());

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

    if(mCodecLooper == NULL)
    {
        mCodecLooper = new ALooper;
        mCodecLooper->start();
    }

    bool haveAudio = false;
    bool haveVideo = false;
    for(size_t i = 0; i < mExtractor->countTracks(); i++)
    {
        sp<AMessage> format;
        status_t err = mExtractor->getTrackFormat(i, &format);
        CHECK_EQ(err, (status_t)OK);

        AString mime;
        CHECK(format->findString("mime", &mime));
        bool isVideo = !strncasecmp(mime.c_str(), "video/", 6);
        if(!haveAudio && !strncasecmp(mime.c_str(), "audio/", 6))
            haveAudio = true;
        else if(!haveVideo && isVideo)
            haveVideo = true;
        else
            continue;

        // 获取IMediaSource
        err = mExtractor->selectTrack(i);
        CHECK_EQ(err, (status_t)OK);
        // 为audio 和 video track分别创建一个CodecState结构体，保存mediacodec相关内容
        // 这里涉及到KeyedVector的用法，add方法的第一个参数为key，第二个参数为value，返回添加元素的索引
        // editValueAt，查找指定索引对应的value
        // keyAt，找到指定索引对应的key值
        // editValueFor，查找指定key对应的value
        CodecState* state = &mStateByTrackIndex.editValueAt(mStateByTrackIndex.add(i, CodecState()));
        state->mCodec = MediaCodec::CreateByType(mCodecLooper, mime.c_str(), false);
        state->mNumFramesWritten = 0;   // 用于audio
        CHECK(state->mCodec != NULL);

        // configure mediacodec
        err = state->mCodec->configure(format, isVideo?mSurface:NULL, NULL, 0);

        CHECK_EQ(err, OK);

        size_t j = 0;
        sp<ABuffer> buffer;
        // 检查mediaformat中是否有csd buffer，可以使用AStringPrintf做字符串拼接
        while(format->findBuffer(AStringPrintf("csd-%d", j).c_str(), &buffer)){
            state->mCSD.push_back(buffer);
            j++;
        }
    }

    for(size_t i = 0; i < mStateByTrackIndex.size(); i++)
    {
        ALOGD("mStateByTrackIndex[%d].key = %d", i, mStateByTrackIndex.keyAt(i));

    }

    for(size_t i = 0; i < mStateByTrackIndex.size(); i++)
    {
        CodecState* state = &mStateByTrackIndex.editValueAt(i);
        // 先启动mediacodec，启动之后才会完成buffer的分配工作
        status_t err = state->mCodec->start();
        CHECK_EQ(err, (status_t)OK);

        // 将两个track中mediacodec的input和output buffer都保存到CodecState中
        err = state->mCodec->getInputBuffers(&state->mBuffers[0]);
        CHECK_EQ(err, (status_t)OK);

        err = state->mCodec->getOutputBuffers(&state->mBuffers[1]);
        CHECK_EQ(err, (status_t)OK);

        // 如果有csd buffer要先写给decoder
        for(size_t j = 0; j < state->mCSD.size(); ++j)
        {
            const sp<ABuffer> &srcBuffer = state->mCSD.itemAt(j);

            size_t index;
            // 阻塞等待获取一个inputbuffer index
            err = state->mCodec->dequeueInputBuffer(&index, -1ll);
            CHECK_EQ(err, (status_t)OK);

            const sp<MediaCodecBuffer> &dstBuffer = state->mBuffers[0].itemAt(index);

            // 检查dstbuffer的空间是否足够
            // ABuffer的size方法返回真实数据长度，capactiy返回最大容量，offset返回起始读取位置
            CHECK_LE(srcBuffer->size(), dstBuffer->capacity());
            dstBuffer->setRange(0, srcBuffer->size());
            memcpy(dstBuffer->data(), srcBuffer->data(), srcBuffer->size());

            // 将buffer送给decoder，由于是CSD buffer，所以flag需要添加BUFFER_FLAG_CODECCONFIG
            err = state->mCodec->queueInputBuffer(index, 0, dstBuffer->size(), 0ll, MediaCodec::BUFFER_FLAG_CODECCONFIG);

            CHECK_EQ(err, (status_t)OK);
        }

    }

    return OK;

}

status_t MyPlayer::onStart()
{
    CHECK_EQ(mState, STOPPED);

    // start之后会一直发送kWhatDoMoreStuff消息，不断执行onDoMoreStuff方法
    sp<AMessage> msg = new AMessage(kWhatDoMoreStuff, this);
    msg->setInt32("generation", ++mDoMoreStuffGeneration);
    msg->post();
    return OK;
}

status_t MyPlayer::onStop()
{
    return OK;
}

status_t MyPlayer::onDoMoreStuff()
{
    // 1、为每个track获取一个input 和一个output buffer
    for(size_t i = 0; i < mStateByTrackIndex.size(); i++)
    {
        CodecState *state = &mStateByTrackIndex.editValueAt(i);

        status_t err;

        // 这里会不阻塞轮询，直到获取到一个input buffer
        do{
            size_t index;
            err = state->mCodec->dequeueInputBuffer(&index);
            if(err == OK)
            {
                // 将index加入到列表当中
                state->mAvailInputBufferIndices.push_back(index);
            }
            else
            {

            }
        }while(err == OK);

        // 同样是不阻塞轮询，直到获取到一个ouput buffer
        do{
            BufferInfo info;
            // dequeueOutputBuffer拿到的不仅仅是index，还有buffer的offset、size，PTS，FLAGS等信息；
            // dequeueOutputBuffer的返回值带有一些特殊信息，所以传回的buffer并不一定是解码结果，也可能是decoder送回的信息
            err = state->mCodec->dequeueOutputBuffer(&info.mIndex, &info.mOffset, &info.mSize, &info.mPresentationTimeUs, &info.mFlags);

            if(err == OK)
            {
                // 如果是解码后的数据，将bufferInfo加入到列表等待处理
                state->mAvailOutputBufferInfos.push_back(info);
            }
            else if(err == INFO_FORMAT_CHANGED)
            {
                // 如果返回值为INFO_FORMAT_CHANGED，说明视频格式发生变化
                // 执行onOutPutFormatChanged方法中会创建AudioTrack
                // 我觉得 AudioTrack也可以在MediaExtractor->getTrackFormat拿到audio track时创建
                err = onOutputFormatChanged(state);
                CHECK_EQ(err, (status_t)OK);
            }
            else if(err == INFO_OUTPUT_BUFFERS_CHANGED)
            {
                // 如果是INFO_OUTPUT_BUFFERS_CHANGED，说明用于输出的buffer发生变化，需要重新获取一次output buffer列表
                err = state->mCodec->getOutputBuffers(&state->mBuffers[1]);
                CHECK_EQ(err, (status_t)OK);
            }
            else
            {

            }
        }while(err == OK || err == INFO_FORMAT_CHANGED || err == INFO_OUTPUT_BUFFERS_CHANGED);
    }

    // 2、处理input buffer，这里不会向上面遍历所有track，这里的方法是按顺序读取文件，如果有某个track没有input buffer了就退出循环
    for(;;)
    {
        size_t trackIndex;
        // 获取当前数据所属的track index，返回值不等于OK说明到达了EOS
        status_t err = mExtractor->getSampleTrackIndex(&trackIndex);
        if(err != OK){
            ALOGD("get input EOS");
            break;
        }
        else{
            // 拿到track对应的codecstate
            CodecState *state = &mStateByTrackIndex.editValueFor(trackIndex);
            // 如果当前CodecState中没有input buffer则直接退出循环
            if(state->mAvailInputBufferIndices.empty())
                break;

            // 取出一个index
            size_t index = *state->mAvailInputBufferIndices.begin();
            state->mAvailInputBufferIndices.erase(state->mAvailInputBufferIndices.begin());

            // input和output buffer都是MediaCodecBuffer，base方法可以获取到存储数据的指针
            const sp<MediaCodecBuffer> &dstBuffer = state->mBuffers[0].itemAt(index);
            sp<ABuffer> abuffer = new ABuffer(dstBuffer->base(), dstBuffer->capacity());

            // 将数据读到MediaCodecBuffer中
            err = mExtractor->readSampleData(abuffer);
            CHECK_EQ(err, (status_t)OK);
            // 设置可读取范围
            dstBuffer->setRange(abuffer->offset(), abuffer->size());

            int64_t timeUs;
            // 获取数据对应的PTS
            CHECK_EQ(mExtractor->getSampleTime(&timeUs), OK);

            // 将数据送给decoder，同时附带PTS信息，不带FLAG
            err = state->mCodec->queueInputBuffer(index, dstBuffer->offset(), dstBuffer->size(), timeUs, 0);
            CHECK_EQ(err, (status_t)OK);

            ALOGD("enqueue input data on track %u", trackIndex);

            // 将Extractor指向下一笔数据
            err = mExtractor->advance();
            CHECK_EQ(err, (status_t)OK);
        }
    }

    int64_t nowUs = ALooper::GetNowUs();
    // 初始化开始播放时间
    if(mStartTimeRealUs < 0ll)
    {
        mStartTimeRealUs = nowUs + 1000000ll;
    }

    // 3、处理output buffer并做av sync，遍历所有track
    for(size_t i = 0; i < mStateByTrackIndex.size(); i++)
    {
        CodecState *state = &mStateByTrackIndex.editValueAt(i);

        // 一次性处理所有的outputbuffer
        while(!state->mAvailOutputBufferInfos.empty()){
            BufferInfo *info = &*state->mAvailOutputBufferInfos.begin();

            // 计算真实播放时间
            int64_t whenRealUs = info->mPresentationTimeUs + mStartTimeRealUs;
            // 计算真实播放时间和当前系统时间的距离
            int64_t lateByUs = nowUs - whenRealUs;

            // 如果系统时间距离播放时间小于10ms，则去渲染
            if(lateByUs > -10000ll)
            {
                bool release = true;

                // 如果系统时间已经过了播放时间30ms则丢弃帧
                if(lateByUs > 30000ll)
                {
                    ALOGD("track %u, buffer late by %lld us, dropping", mStateByTrackIndex.keyAt(i),lateByUs);
                    // 直接调用release
                    state->mCodec->releaseOutputBuffer(info->mIndex);
                }
                else{
                    // 用于audio
                    if(state->mAudioTrack != NULL)
                    {
                        // 获取到output buffer
                        const sp<MediaCodecBuffer> &srcBuffer = state->mBuffers[1].itemAt(info->mIndex);
                        // 将数据写到audioTrack中，但是有可能一次写不完
                        renderAudio(state, info, srcBuffer);
                        // 根据mSize判断是否写完，写完了才回去release
                        if(info->mSize > 0)
                            release = false;

                    }
                    // render，由于是等到绘制时间才去release and render，所以不用指定PTS
                    if(release)
                        state->mCodec->renderOutputBufferAndRelease(info->mIndex);
                }
                // 从列表中删除
                if(release){
                    state->mAvailOutputBufferInfos.erase(state->mAvailOutputBufferInfos.begin());
                    info = NULL;
                }
                else
                    break;
            }else {
                // 如果系统时间早于播放时间10ms，则不去渲染
                ALOGD("track %u buffer early by %lld us", mStateByTrackIndex.keyAt(i), lateByUs);
                break;
            }
        }
    }

    return OK;
}

status_t MyPlayer::onOutputFormatChanged(CodecState *state)
{
    sp<AMessage> format;
    // getOutputFormat可以获取decoder返回的format
    status_t err = state->mCodec->getOutputFormat(&format);
    if(err != OK)
        return err;

    AString mime;
    CHECK(format->findString("mime", &mime));
    if(!strncasecmp(mime.c_str(), "audio/", 6))
    {
        int32_t channelCount;
        int32_t sampleRate = 0;
        // 获取通道数和采样率
        CHECK(format->findInt32("channel-count", &channelCount));
        CHECK(format->findInt32("sample-rate", &sampleRate));
        // 创建一个AudioTrack
        state->mAudioTrack = new AudioTrack(AUDIO_STREAM_MUSIC, sampleRate, AUDIO_FORMAT_PCM_16_BIT, audio_channel_out_mask_from_count(channelCount), 0);
    }

    return OK;
}

status_t MyPlayer::renderAudio(CodecState* state, BufferInfo* info, const sp<MediaCodecBuffer> &buffer)
{
    CHECK(state->mAudioTrack != NULL);
    // 开启audio render
    if(state->mAudioTrack->stopped())
        state->mAudioTrack->start();

    uint32_t numFramesPlayed;
    // 获取当前已经播放的帧数
    CHECK_EQ(state->mAudioTrack->getPosition(&numFramesPlayed), OK);

    // 获取可以写入的帧数
    uint32_t numFramesAvailableToWrite = state->mAudioTrack->frameCount() - (state->mNumFramesWritten - numFramesPlayed);

    // 计算可写入数据大小
    size_t numBytesAvailableToWrite = numFramesAvailableToWrite * state->mAudioTrack->frameSize();

    size_t copy = info->mSize;
    if(copy > numBytesAvailableToWrite)
        copy = numBytesAvailableToWrite;

    if(copy == 0)
        return OK;

    int64_t startTimeUs = ALooper::GetNowUs();

    // 写入数据，注意这里有偏移量offset（上次可能没写完）
    ssize_t nbytes = state->mAudioTrack->write(buffer->base() + info->mOffset, copy);

    CHECK_EQ(nbytes, (ssize_t)copy);

    int64_t delayUs = ALooper::GetNowUs() - startTimeUs;

    // 计算写入帧数
    uint32_t numFramesWritten = nbytes / state->mAudioTrack->frameSize();

    if(delayUs > 2000ll){
        ALOGD("AudioTrack write took %lld us", delayUs);
    }

    // 将offset和size记录并返回，用于判断数据是否全部写入
    info->mOffset += nbytes;
    info->mSize -= nbytes;

    // 记录已经写入帧数
    state->mNumFramesWritten += numFramesWritten;
    return OK;
}
}

TestPlayer.cpp

//#define LOG_NDEBUG 0
#define LOG_TAG "TestPlayer"

#include "MyPlayer.h"
#include <gui/SurfaceControl.h>
#include <gui/SurfaceComposerClient.h>
#include <gui/Surface.h>
#include <ui/DisplayConfig.h>
#include <media/stagefright/foundation/ALooper.h>
#include <binder/IBinder.h>
#include <media/stagefright/foundation/ADebug.h>

int main(int argc, char** argv)
{
    using namespace android;

    sp<MyPlayer> player = new MyPlayer();
    sp<android::ALooper> looper =  new android::ALooper();
    // 创建looper处理MyPlayer的消息
    looper->registerHandler(player);
    // 运行在新的线程上
    looper->start();

    // 创建Surface
    sp<SurfaceComposerClient> composerClient = new SurfaceComposerClient;
    CHECK_EQ(composerClient->initCheck(), (status_t)OK);

    const sp<IBinder> display = SurfaceComposerClient::getInternalDisplayToken();
    CHECK(display != NULL);

    DisplayConfig config;
    CHECK_EQ(SurfaceComposerClient::getActiveDisplayConfig(display, &config), NO_ERROR);

    const ui::Size &resolution = config.resolution;
    const ssize_t displayWidth = resolution.getWidth();
    const ssize_t displayHeight = resolution.getHeight();
    ALOGD("display is %d x %d\n", displayWidth, displayHeight);

    sp<SurfaceControl> control = composerClient->createSurface(String8("A Surface"), displayWidth/2, displayHeight/2, PIXEL_FORMAT_RGB_565, 0);
    CHECK(control != NULL);
    CHECK(control->isValid());

    SurfaceComposerClient::Transaction{}.setLayer(control, INT_MAX).show(control).apply();
    sp<Surface> surface = control->getSurface();
    CHECK(surface != NULL);

    // 开始播放
    player->setDataSource(argv[1]);
    player->setSurface(surface->getIGraphicBufferProducer());
    player->start();

    // sleep 60s，等待播放60s
    sleep(60);

    composerClient->dispose();

    looper->stop();

    return 0;

}

Android.bp

cc_binary {
    name: "MyPlayer",

    srcs: [
        "TestPlayer.cpp",
        "MyPlayer.cpp",
    ],

    local_include_dirs: [
        "include"
    ],

    shared_libs: [
        "libstagefright",
        "libmedia",
        "libstagefright_foundation",
        "libgui",
        "libaudioclient",
        "liblog",
        "libutils",
        "libcutils",
        "libmedia_omx",
        "libui",
    ],

    /*export_include_dirs: [
        "include"
    ],*/

    export_shared_lib_headers:[
        "libstagefright",
        "libmedia",
    ],

    header_libs: [
        "libmediametrics_headers",
        "libmediadrm_headers",
        "libaudioclient_headers"
    ],

}

记录一下编译过程中发现的问题：

1、local_include_dirs 和 export_include_dirs都是列表，所以要用 [] 将引用的内容框起来

2、cc_binary中不允许出现export_include_dirs，因为不需要导出头文件

3、main函数不能被包在 namespace android当中，不然会找不到main函数，可以在main函数中加上using namespace android，ALooper仍然需要命名空间、

4、还有以class形式替代头文件可能会出现的错误