void TextureCache::addImageAsync(const std::string &path, const std::function<void(Texture2D*)>& callback)
{
    Texture2D *texture = nullptr;

    std::string fullpath = FileUtils::getInstance()->fullPathForFilename(path);

    auto it = _textures.find(fullpath);
    if( it != _textures.end() )
        texture = it->second;

    if (texture != nullptr)
    {
        callback(texture);
        return;
    }

    // lazy init
    if (_asyncStructQueue == nullptr)
    {             
        _asyncStructQueue = new queue<AsyncStruct*>();
        _imageInfoQueue   = new deque<ImageInfo*>();        

        // create a new thread to load images
        _loadingThread = new std::thread(&TextureCache::loadImage, this);

        _needQuit = false;
    }

    if (0 == _asyncRefCount)
    {
        Director::getInstance()->getScheduler()->schedule(CC_SCHEDULE_SELECTOR(TextureCache::addImageAsyncCallBack), this, 0, false);
    }

    ++_asyncRefCount;

    // generate async struct
    AsyncStruct *data = new (std::nothrow) AsyncStruct(fullpath, callback);

    // add async struct into queue
    _asyncStructQueueMutex.lock();
    _asyncStructQueue->push(data);
    _asyncStructQueueMutex.unlock();

    _sleepCondition.notify_one();
}