例如
ParticleSystem* particleSystem = ParticleSnow::create();
particleSystem->setTexture(Director::getInstance()->getTextureCache()->addImage("snow.png"));
particleSystem->setSpeed(200.0f);
ly->addChild(particleSystem);
怎么让他只显示在上半个屏幕里,现在不管ly大小多少,怎么设置都是全屏显示