Emacs triple click handler

Question

What handler is called when I make triple click ? For me, it select whole line. I want this behavior to be binded to C-l, how to do it ?

EDIT:

I tried to bind this to key, but not works, how to fix it?

(defun custom/mark-whole-line  ()
   (interactive)
   (mouse-start-end 0 (buffer-size) 2))

Answer 1

I guess you mean the key-binding <triple-mouse-1>.

You can sort-of answer your question by using C-h k followed by triple-clicking mouse-1 (the first/left mouse button).

This is what I see when I do that in the Emacs 26 pretest:

<mouse-1> (translated from <triple-mouse-1>) at that spot runs the command mouse-set-point (found in global-map), which is an interactive compiled Lisp function in mouse.el.

It is bound to mouse-1.

(mouse-set-point EVENT &optional PROMOTE-TO-REGION)

Move point to the position clicked on with the mouse.

This should be bound to a mouse click event type.

If PROMOTE-TO-REGION is non-nil and event is a multiple-click, select the corresponding element around point, with the resulting position of point determined by mouse-select-region-move-to-beginning.

In Emacs prior to Emacs 26 you also see the doc for the button-down event, <down-mouse-1>.

To figure out what's happening you need to check the code of mouse-set-point. You'll see that when called interactively its second arg, PROMOTE-TO-REGION, is non-nil (coming from the numeric prefix arg). So it calls mouse-set-region.

Function mouse-set-region then calls mouse-start-end, passing (1- click-count), where click-count is the number of clicks. For a triple-click it thus passes 2.

Finally, given the value 2 as its MODE argument, function mouse-start-end selects the whole line. Its doc string (C-h f mouse-start-end) tells you:

mouse-start-end is a compiled Lisp function in mouse.el.

(mouse-start-end START END MODE)

Return a list of region bounds based on START and END according to MODE.

If MODE is 0 then set point to (min START END), mark to (max START END).

If MODE is 1 then set point to start of word at (min START END), mark to end of word at (max START END).

If MODE is 2 then do the same for lines.

Whew! Not so easy, but Emacs finally tells you what you wanted to know. It is (ultimately) function mouse-start-end that selects the whole line. But it does that only because of the MODE parameter passed to it from mouse-set-region. And mouse-set-region only does the right thing because of code that is in mouse-set-point.