【SpriteKit】
Sprite Kit provides a graphics rendering and animation infrastructure that you can use to animate arbitrary textured images, or sprites。
提供render和animation。
Sprite Kit uses a traditional rendering loop where the contents of each frame are processed before the frame is rendered.
先update后render。
Sprite Kit also provides other functionality that is useful for games, including basic sound playback support and physics simulation.
也提供basic sound play back和physics simulation。
SpriteKit一帧周期图：
很显示分为四个过程：
1、update：更新数据。
2、evaluates actions，dideEvaluateActions，执行动画。
3、simulate physics、didSimulate Physics，模拟物理效果。
4、render。
其中1)是需要我们个性的地方，SpriteKit提供2)、3）、4）机制来帮助我们做游戏。
Sprite Kit supports a rich rendering infrastructure and handles all of the low-level work to submit drawing commands to OpenGL, you can focus your efforts on solving higher-level design problems and creating great gameplay.
OpenGL细节的确很多，SpriteKit帮助我们摆脱这些东西。