other 事件进入该触发器时调用 OnTriggerEnter。
This message is sent to the trigger Collider and the Rigidbody (if any) that the trigger Collider belongs
to, and to the Rigidbody (or the Collider if there is no Rigidbody) that touches the trigger.
Notes: Trigger events are only sent if one of the Colliders also has a Rigidbody attached. Trigger events will be sent to disabled MonoBehaviours, to allow enabling Behaviours in response to collisions.
OnTriggerEnter occurs on the FixedUpdate after a collision. The Colliders involved are not guaranteed to be at the point of initial contact.
Note: OnTriggerEnter is not technically part of Collision. It is a MonoBehaviour function.