获取在该接触点处碰撞体之间的距离。
Unity 的物理引擎跟踪所有可能重叠的碰撞体之间的距离。对于给定的一对碰撞体，一旦发生重叠（碰撞）后，立即产生接触并发送 OnCollisionEnter2D 事件。尽管该碰撞体对仍处于活动状态，但当这些碰撞体仍为分开、接触或重叠时，它们之间的距离分别可大于零、等于零，甚至小于零。
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