Every major mode command should finish by running the mode-independent normal hook change-major-mode-after-body-hook, its mode hook, and the normal hook after-change-major-mode-hook. It does this by calling run-mode-hooks. If the major mode is a derived mode, that is if it calls another major mode (the parent mode) in its body, it should do this inside delay-mode-hooks so that the parent won’t run these hooks itself. Instead, the derived mode’s call to run-mode-hooks runs the parent’s mode hook too. See Major Mode Conventions.
Emacs versions before Emacs 22 did not have delay-mode-hooks. Versions before 24 did not have change-major-mode-after-body-hook. When user-implemented major modes do not use run-mode-hooks and have not been updated to use these newer features, they won’t entirely follow these conventions: they may run the parent’s mode hook too early, or fail to run after-change-major-mode-hook. If you encounter such a major mode, please correct it to follow these conventions.
When you define a major mode using define-derived-mode, it automatically makes sure these conventions are followed. If you define a major mode “by hand”, not using define-derived-mode, use the following functions to handle these conventions automatically.
Major modes should run their mode hook using this function. It is similar to run-hooks (see Hooks), but it also runs change-major-mode-after-body-hook, hack-local-variables (when the buffer is visiting a file) (see File Local Variables), and after-change-major-mode-hook. The last thing it does is to evaluate any :after-hook forms declared by parent modes (see Derived Modes).
When this function is called during the execution of a delay-mode-hooks form, it does not run the hooks or hack-local-variables or evaluate the forms immediately. Instead, it arranges for the next call to run-mode-hooks to run them.
When one major mode command calls another, it should do so inside of delay-mode-hooks.
This macro executes body, but tells all run-mode-hooks calls during the execution of body to delay running their hooks. The hooks will actually run during the next call to run-mode-hooks after the end of the delay-mode-hooks construct.
This is a normal hook run by run-mode-hooks. It is run before the mode hooks.
This is a normal hook run by run-mode-hooks. It is run at the very end of every properly-written major mode command.
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https://www.gnu.org/software/emacs/manual/html_node/elisp/Mode-Hooks.html