I figured out why I couldn't find the server control in the Ajax method.

The server control depends on pageload, and only the page submits their request server. If there is no pageload, they will not request the server, so Ajax does not go through pageload, and the server does not know what they are doing.
Therefore, JavaScript cannot use the response events of the back-end dependency and pageload controls. Finally, I thought about the methods of the original server controls and dependencies and servers, all of which depend on pageload, only pageload requests the server, and the server can know what the client is doing. This is ProgramThe initial (Client/Server) Mechanism
The so-called Ajax is a local mechanism for laziness, and it feels a little out of sync with the pageload mechanism.
After thinking so much, let alone anything else.
(Do not laugh at me and understand it yourself. If you have any misunderstanding, please point it out to me. Thank you)