function OnApplicationFocus () : void
Description描述
Sent to all game objects when the player gets or looses focus.
当玩家获得或失去焦点时发送给所有游戏物体。
OnApplicationFocus can be a co-routine, simply use the yield statement in the function.
OnApplicationFocus 可以作为协同程序，在函数中使用yield语句即可。
