ref: http://answers.unity3d.com/questions/1268607/ontriggerstay2d-do-not-called-every-fixedupdate-un.html#answer-1268627
unity, OnTriggerStay/OnTriggerStay2D not called every fixedUpdate frame
原文：http://www.cnblogs.com/wantnon/p/6044171.html
