关节另一端附加到的 Rigidbody2D 对象（即没有关节组件的对象）。
如果此属性设置为 null，则关节会附加到空间中的固定点而不是另一个 Rigidbody2D。
Unity does not support connecting a joint to a Rigidbody2D in a different Scene that is using a local physics Scene. If a joint is connected to a Rigidbody2D, and then that Rigidbody2D is moved to a Scene that uses LocalPhysicsMode.Physics2D, then the joint is automatically disconnected from the Rigidbody2D.
See Also: Scene, LocalPhysicsMode, Rigidbody2D and collideConnected.