固定不动的ui放在一个layer, 并且设定该layer的Parallax=0. Parallax Change the rate at which the layer scrolls in the horizontal and vertical directions. A parallax rate of 100, 100 means ordinary scrolling, 0, 0 means it will never scroll (useful for UIs), 50, 50 means scrolling half as fast, etc. Also useful for multi-layer parallaxing backgrounds.