25.1 Command Loop Overview

The command loop in SXEmacs is a standard event loop, reading events one at a time with next-event and handling them with dispatch-event. An event is typically a single user action, such as a keypress, mouse movement, or menu selection; but they can also be notifications from the window system, informing SXEmacs that (for example) part of its window was just uncovered and needs to be redrawn. See Events. Pending events are held in a first-in, first-out list called the event queue: events are read from the head of the list, and newly arriving events are added to the tail. In this way, events are always processed in the order in which they arrive.

dispatch-event does most of the work of handling user actions. The first thing it must do is put the events together into a key sequence, which is a sequence of events that translates into a command. It does this by consulting the active keymaps, which specify what the valid key sequences are and how to translate them into commands. See Key Lookup, for information on how this is done. The result of the translation should be a keyboard macro or an interactively callable function. If the key is M-x, then it reads the name of another command, which it then calls. This is done by the command execute-extended-command (see Interactive Call).

To execute a command requires first reading the arguments for it. This is done by calling command-execute (see Interactive Call). For commands written in Lisp, the interactive specification says how to read the arguments. This may use the prefix argument (see Prefix Command Arguments) or may read with prompting in the minibuffer (see Minibuffers).

For example, the command find-file has an interactive specification which says to read a file name using the minibuffer. The command’s function body does not use the minibuffer; if you call this command from Lisp code as a function, you must supply the file name string as an ordinary Lisp function argument.

If the command is a string or vector (i.e., a keyboard macro) then execute-kbd-macro is used to execute it. You can call this function yourself (see Keyboard Macros).

To terminate the execution of a running command, type C-g. This character causes quitting (see Quitting).

Variable: pre-command-hook

The editor command loop runs this normal hook before each command. At that time, this-command contains the command that is about to run, and last-command describes the previous command. See Hooks.

Variable: post-command-hook

The editor command loop runs this normal hook after each command. (In FSF Emacs, it is also run when the command loop is entered, or reentered after an error or quit.) At that time, this-command describes the command that just ran, and last-command describes the command before that. See Hooks.

Quitting is suppressed while running pre-command-hook and post-command-hook. If an error happens while executing one of these hooks, it terminates execution of the hook, but that is all it does.