在一开始看Render里面代码的时候对render方法有点疑惑:

void Renderer::render()
{
    _isRendering = true;   
    if (_glViewAssigned)
    {
        //Process render commands
        //1. Sort render commands based on ID
        for (auto &renderqueue : _renderGroups)
        {
            renderqueue.sort();
        }
        visitRenderQueue(_renderGroups[0]);
    }
    clean();
    _isRendering = false;
}

带着这个疑问我们进入今天的正题。

首先_renderGroups中的每个元素都是一个渲染队列RenderQueue，在Render构造出来的时候会有一个默认的渲染队列放入到_renderGroups中。渲染队列中保存数据的是一个数组_commands，根据放入到这个渲染队列中的命令属性(globalZorder,is3D,isTransparent)决定到底把这个命令放入到_commands中的那个位置中。这个其实在将渲染机制的时候就已经提到过了。很简单，不用赘述。

在访问一个队列的时候，无非就是把其中的每个位置中所有的命令拿出来处理，processRenderCommand 就是对命令的具体处理方式：

void Renderer::processRenderCommand(RenderCommand* command)
{
    auto commandType = command->getType();
    if( RenderCommand::Type::TRIANGLES_COMMAND == commandType)
    {
        //Draw if we have batched other commands which are not triangle command
        flush3D();
        flushQuads();
        
        //Process triangle command
        auto cmd = static_cast<TrianglesCommand*>(command);
        
        //Draw batched Triangles if necessary
        if(cmd->isSkipBatching() || _filledVertex + cmd->getVertexCount() > VBO_SIZE || _filledIndex + cmd->getIndexCount() > INDEX_VBO_SIZE)
        {
            CCASSERT(cmd->getVertexCount()>= 0 && cmd->getVertexCount() < VBO_SIZE,"VBO for vertex is not big enough,please break the data down or use customized render command");
            CCASSERT(cmd->getIndexCount()>= 0 && cmd->getIndexCount() < INDEX_VBO_SIZE,"VBO for index is not big enough,please break the data down or use customized render command");
            //Draw batched Triangles if VBO is full
            drawBatchedTriangles();
        }
        
        //Batch Triangles
        _batchedCommands.push_back(cmd);
        
        fillVerticesAndIndices(cmd);
        
        if(cmd->isSkipBatching())
        {
            drawBatchedTriangles();
        }
        
    }
    else if ( RenderCommand::Type::QUAD_COMMAND == commandType )
    {
        //Draw if we have batched other commands which are not quad command
        flush3D();
        flushTriangles();
        
        //Process quad command
        auto cmd = static_cast<QuadCommand*>(command);
        
        //Draw batched quads if necessary
        if(cmd->isSkipBatching()|| (_numberQuads + cmd->getQuadCount()) * 4 > VBO_SIZE )
        {
            CCASSERT(cmd->getQuadCount()>= 0 && cmd->getQuadCount() * 4 < VBO_SIZE,please break the data down or use customized render command");
            //Draw batched quads if VBO is full
            drawBatchedQuads();
        }
        
        //Batch Quads
        _batchQuadCommands.push_back(cmd);
        
        fillQuads(cmd);
        
        if(cmd->isSkipBatching())
        {
            drawBatchedQuads();
        }
    }
    else if (RenderCommand::Type::MESH_COMMAND == commandType)
    {
        flush2D();
        auto cmd = static_cast<MeshCommand*>(command);
        
        if (cmd->isSkipBatching() || _lastBatchedMeshCommand == nullptr || _lastBatchedMeshCommand->getMaterialID() != cmd->getMaterialID())
        {
            flush3D();
            
            if(cmd->isSkipBatching())
            {
                cmd->execute();
            }
            else
            {
                cmd->preBatchDraw();
                cmd->batchDraw();
                _lastBatchedMeshCommand = cmd;
            }
        }
        else
        {
            cmd->batchDraw();
        }
    }
    else if(RenderCommand::Type::GROUP_COMMAND == commandType)
    {
        flush();
        int renderQueueID = ((GroupCommand*) command)->getRenderQueueID();
        visitRenderQueue(_renderGroups[renderQueueID]);
    }
    else if(RenderCommand::Type::CUSTOM_COMMAND == commandType)
    {
        flush();
        auto cmd = static_cast<CustomCommand*>(command);
        cmd->execute();
    }
    else if(RenderCommand::Type::BATCH_COMMAND == commandType)
    {
        flush();
        auto cmd = static_cast<BatchCommand*>(command);
        cmd->execute();
    }
    else if(RenderCommand::Type::PRIMITIVE_COMMAND == commandType)
    {
        flush();
        auto cmd = static_cast<PrimitiveCommand*>(command);
        cmd->execute();
    }
    else
    {
        CCLOGERROR("Unknown commands in renderQueue");
    }
}

当看到RenderCommand::Type::GROUP_COMMAND类型的命令时，会触发去访问其它渲染队列。具体是哪个渲染队列要看这个groupCommand的renderQueueID。

int renderQueueID = ((GroupCommand*) command)->getRenderQueueID();
 visitRenderQueue(_renderGroups[renderQueueID]);

在创建一个groupCommand的时候，会去向groupcommandmanager申请一个id,如果没有id，那么会创建一个新的id，并且Render会创建一个渲染队列。初始化完成之后，放加入到Render的_renderGroups中去，

void Renderer::addCommand(RenderCommand* command)
{
    int renderQueue =_commandGroupStack.top();
    addCommand(command,renderQueue);
}

void Renderer::addCommand(RenderCommand* command,int renderQueue)
{
    CCASSERT(!_isRendering,"Cannot add command while rendering");
    CCASSERT(renderQueue >=0,"Invalid render queue");
    CCASSERT(command->getType() != RenderCommand::Type::UNKNOWN_COMMAND,"Invalid Command Type");

    _renderGroups[renderQueue].push_back(command);
}

组命令加入之后，才会把它的id放入到_commandGroupStack中，这意味着所有后来加入的命令会放到下一个渲染队列中去。当执行到这个组命令的时候，会根据它的id去执行id对应的渲染队列。

总结：每个渲染队列的末尾总会是一个组命令(如果有的话)，这个组命令执行的时候触发了下一个渲染队列的执行。

OVER！！！