draw方法中_customCommand.func一直显示为空

Cocos2d-x
#1

void TowerP::draw(Renderer *renderer, const kmMat4 &transform, bool transformUpdated){
if (m_isDebug){

    _customCommand.init(_globalZOrder);

    _customCommand.func = CC_CALLBACK_0(TowerP::onDraw, this, transform, transformUpdated);

    renderer->addCommand(&_customCommand);

}

}

void TowerP::onDraw(const kmMat4 &transform, bool transformUpdated){

kmGLPushMatrix();
kmGLLoadMatrix(&transform);

glLineWidth(5.0f);

Point srcPos = Point(m_pos.x - R, m_pos.y + R);

Point destPos = Point(m_pos.x+R, m_pos.y - R);

DrawPrimitives::drawRect(srcPos, destPos);

glLineWidth(1);
kmGLPopMatrix();

}

初始化TowerP，_customCommand.func一直显示为空。
求大神解答！