Do not write even a line of Objective - C code, every Swift app running in the Objective - C runtime, open dynamic task distributing and the runtime object associated with the world. More precisely, may run only when the library only use Swift Swift runtime. But the Objective - C runtime coexist with us so long time, we should also to play to the extreme.

-对象关联与方法互换基于运行时来达到我们想要的效果，通常用于有必要更改系统设置时。