WikiTodo: Add basic advising help.

When to Use Advice?

See AdviceVsHooks for a discussion of when to use advice. Summary:

advice is good to have, but not necessarily good to follow all the time ;-).

Advised hello world

    (defun foo () (interactive) (insert "hello"))
    (defadvice foo (after foo-after activate) (insert " world!\n"))
    (foo) ;=> hello world!
   ;; now you can unadvise the advised function
   (ad-unadvise 'foo)

Simple Advising

To advise a function, you use `advice-add`. It takes the function you want to advise, quoted. An advice combinator (around, before, after, etc). And it takes the function you wrote to be run.

A simple piece of before advice looks like this:

    (defun my-message (&optional ARG PRED) 
      (message "hello!"))
    (advice-add 'save-some-buffers :before 'my-message)

Something to note is the function signatures. Functions used with `:before` and `:after` will receive the same arguments as the functions they advise.

Interactive functions

Maintaining interactive-p inside an advised function can be a pain. For details on this, read up the comments etc. in advice.el. Here’s an example from Le Wang on how to maintain interactive-p inside the function definition of an advised function.

    (defun bar ()
      (message "bar interactive: %s" (interactive-p))
      (sit-for 1))
    (defadvice bar (around advice-bar activate)
      (if (interactive-p)
	    (message "advice-bar interactive: %s" (interactive-p))
	    (sit-for 1)
	    (call-interactively (ad-get-orig-definition 'bar)))