How can I define a key to cancel selection only when I selecting something?

Question

How can I define a key to cancel selection only when I'm selecting something? Because I use this key to do other thing in global-mode-map.. Is there any specific mode for mark and selecting something? I just tried something like this but it didn't help.

(add-hook 'transient-mark-mode-hook
      (lambda()
        (local-set-key "a" 'deactivate-mark);;keyboard-quit also cant work
        (message "hello")
        )
      )

Answer 1

The transient-mark-mode-hook is called when you enter/leave the Transient Mark Mode, which is enabled by default starting from Emacs 23. Only when it's enabled, setting the mark will activate it and highlight the region.

You could try the following code.

(defconst my-alist
  `((mark-active
     ,@(let ((m (make-sparse-keymap)))
         (define-key m (kbd "h") (lambda() (interactive) (message "hello")))
         (define-key m (kbd "q") (lambda() (interactive) (deactivate-mark)))
         (define-key m (kbd "s") (lambda() (interactive) (message (buffer-substring (point) (mark)))))
         m))))

(add-to-list 'emulation-mode-map-alists 'my-alist)

Answer 2

You can test whether there is an active selection of text using function region-active-p. C-h f region-active-p tells you:

region-active-p is a compiled Lisp function in simple.el.

(region-active-p)

Return non-nil if Transient Mark mode is enabled and the mark is active.

Some commands act specially on the region when Transient Mark mode is enabled. Usually, such commands should use use-region-p instead of this function, because use-region-p also checks the value of use-empty-active-region.

And C-h f use-region-p tells you:

use-region-p is a compiled Lisp function in simple.el.

(use-region-p)

Return t if the region is active and it is appropriate to act on it. This is used by commands that act specially on the region under Transient Mark mode.

The return value is t if Transient Mark mode is enabled and the mark is active; furthermore, if ‘use-empty-active-region’ is nil, the region must not be empty. Otherwise, the return value is nil.

For some commands, it may be appropriate to ignore the value of use-empty-active-region; in that case, use region-active-p.

So you would use one of these functions to test whether there is an active selection. If so then you can call quit (or whatever you want), and if not you can do what you would otherwise normally do.

BTW, it is a good idea to use a named command in a hook, instead of using an anonymous function (aka lambda form). (For one thing, it makes it easier to remove the function from the hook.)