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Visitor mode is to encapsulate some operations applied to a certain data structure. Once these operations need to be modified, the data structure that accepts this operation can be saved unchanged. Visitor mode is suitable for systems with relatively stable data structures. It reduces the coupling degree between data structures and operations acting on data structures, making the set of operations relatively free to change. The specific structure diagram is as follows:此事有功无过，戚将军信不过我，也该信得过严大人。L a good place to climb mountains and exercise, you can even find a tea shop on the way to have tea or eat snacks at Tianhu Lake after climbing the top.Time relays are divided into two types: on-time delay type and off-time delay type, and their symbols are shown in the following figure. For the power-on delay type time relay, when the coil is powered on, the power-on delay type contact acts after the delay time (the normally closed contact is opened and the normally open contact is closed), and the contact immediately returns to normal after the coil is powered off; For the power-off delay type time relay, when the coil is powered on, the power-off delay type contact acts immediately (the normally closed contact is opened and the normally open contact is closed). After the coil is powered off, the contact will not return to normal until the delay time is passed.免礼免礼。A brief introduction is as follows: