void DisplayLinkDirector::mainLoop()
{
    if (_purgeDirectorInNextLoop)
    {
        _purgeDirectorInNextLoop = false;
        purgeDirector();
    }
    else if (! _invalid)
    {
        drawScene();
     
        // release the objects
        PoolManager::sharedPoolManager()->pop();        
    }
}

// Draw the Scene
void Director::drawScene()
{
    //计算了每帧的时间差。
    calculateDeltaTime();

    if (_openGLView)
    {
        _openGLView->pollInputEvents();
    }

    //没有暂停的情况下，引用定时器
    if (! _paused)
    {
        _scheduler->update(_deltaTime);
        _eventDispatcher->dispatchEvent(_eventAfterUpdate);
    }

    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

    /* to avoid flickr, nextScene MUST be here: after tick and before draw.
     XXX: Which bug is this one. It seems that it can‘t be reproduced with v0.9 */
    if (_nextScene)
    {
        setNextScene();
    }

    kmGLPushMatrix();
    
    //construct the frustum
    {
        kmMat4 view;
        kmMat4 projection;
        kmGLGetMatrix(KM_GL_PROJECTION, &projection);
        kmGLGetMatrix(KM_GL_MODELVIEW, &view);
        
        _cullingFrustum->setupFromMatrix(view, projection);
    }
    
    // 绘制当前的场景
    if (_runningScene)
    {
        _runningScene->visit();
        _eventDispatcher->dispatchEvent(_eventAfterVisit);
    }

    // draw the notifications node
    if (_notificationNode)
    {
        _notificationNode->visit();
    }
    
     //是否显示帧数等一些信息
    if (_displayStats)
    {
        showStats();
    }

    _renderer->render();
    _eventDispatcher->dispatchEvent(_eventAfterDraw);

    kmGLPopMatrix();

    _totalFrames++;

    // swap buffers
    if (_openGLView)
    {
        _openGLView->swapBuffers();
    }
    
    if (_displayStats)
    {
        calculateMPF();
    }
}