The right of defense to perform at the same time means that the parties bear debts to each other, and if there is no sequence of performance, it shall be performed at the same time  One party has the right to refuse its request for performance before the performance of the other party
One party shall have the right to refuse its corresponding request for performance if the other party's performance of the debt is not in conformity with the agreement
A borrowed the camera from B, and then B borrowed the mobile phone from A. A did not return the camera, and B did not return the phone.