lisp/scroll-lock.el

   1 ;;; scroll-lock.el --- Scroll lock scrolling.
   2
   3 ;; Copyright (C) 2005-2017 Free Software Foundation, Inc.
   4
   5 ;; Author: Ralf Angeli <angeli@iwi.uni-sb.de>
   6 ;; Maintainer: emacs-devel@gnu.org
   7 ;; Created: 2005-06-18
   8
   9 ;; This file is part of GNU Emacs.
  10
  11 ;; GNU Emacs is free software: you can redistribute it and/or modify
  12 ;; it under the terms of the GNU General Public License as published by
  13 ;; the Free Software Foundation, either version 3 of the License, or
  14 ;; (at your option) any later version.
  15
  16 ;; GNU Emacs is distributed in the hope that it will be useful,
  17 ;; but WITHOUT ANY WARRANTY; without even the implied warranty of
  18 ;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  19 ;; GNU General Public License for more details.
  20
  21 ;; You should have received a copy of the GNU General Public License
  22 ;; along with GNU Emacs.  If not, see <http://www.gnu.org/licenses/>.
  23
  24 ;;; Commentary:
  25
  26 ;; By activating Scroll Lock mode, keys for moving point by line or
  27 ;; paragraph will scroll the buffer by the respective amount of lines
  28 ;; instead.  Point will be kept vertically fixed relative to window
  29 ;; boundaries.
  30
  31 ;;; Code:
  32
  33 (defvar scroll-lock-mode-map
  34   (let ((map (make-sparse-keymap)))
  35     (define-key map [remap next-line] 'scroll-lock-next-line)
  36     (define-key map [remap previous-line] 'scroll-lock-previous-line)
  37     (define-key map [remap forward-paragraph] 'scroll-lock-forward-paragraph)
  38     (define-key map [remap backward-paragraph] 'scroll-lock-backward-paragraph)
  39     map)
  40   "Keymap for Scroll Lock mode.")
  41
  42 (defvar scroll-lock-preserve-screen-pos-save scroll-preserve-screen-position
  43   "Used for saving the state of `scroll-preserve-screen-position'.")
  44 (make-variable-buffer-local 'scroll-lock-preserve-screen-pos-save)
  45
  46 (defvar scroll-lock-temporary-goal-column 0
  47   "Like `temporary-goal-column' but for scroll-lock-* commands.")
  48
  49 ;;;###autoload
  50 (define-minor-mode scroll-lock-mode
  51   "Buffer-local minor mode for pager-like scrolling.
  52 With a prefix argument ARG, enable the mode if ARG is positive,
  53 and disable it otherwise.  If called from Lisp, enable the mode
  54 if ARG is omitted or nil.  When enabled, keys that normally move
  55 point by line or paragraph will scroll the buffer by the
  56 respective amount of lines instead and point will be kept
  57 vertically fixed relative to window boundaries during scrolling."
  58   :lighter " ScrLck"
  59   :keymap scroll-lock-mode-map
  60   (if scroll-lock-mode
  61       (progn
  62         (setq scroll-lock-preserve-screen-pos-save
  63               scroll-preserve-screen-position)
  64         (set (make-local-variable 'scroll-preserve-screen-position) 'always))
  65     (setq scroll-preserve-screen-position
  66           scroll-lock-preserve-screen-pos-save)))
  67
  68 (defun scroll-lock-update-goal-column ()
  69   "Update `scroll-lock-temporary-goal-column' if necessary."
  70   (unless (memq last-command '(scroll-lock-next-line
  71                                scroll-lock-previous-line
  72                                scroll-lock-forward-paragraph
  73                                scroll-lock-backward-paragraph))
  74     (setq scroll-lock-temporary-goal-column (current-column))))
  75
  76 (defun scroll-lock-move-to-column (column)
  77   "Like `move-to-column' but cater for wrapped lines."
  78   (if (or (bolp)
  79           ;; Start of a screen line.
  80           (not (zerop (mod (- (point) (line-beginning-position))
  81                            (window-width)))))
  82       (move-to-column column)
  83     (forward-char (min column (- (line-end-position) (point))))))
  84
  85 (defun scroll-lock-next-line (&optional arg)
  86   "Scroll up ARG lines keeping point fixed."
  87   (interactive "p")
  88   (or arg (setq arg 1))
  89   (scroll-lock-update-goal-column)
  90   (if (pos-visible-in-window-p (point-max))
  91       (forward-line arg)
  92     (scroll-up arg))
  93   (scroll-lock-move-to-column scroll-lock-temporary-goal-column))
  94
  95 (defun scroll-lock-previous-line (&optional arg)
  96   "Scroll up ARG lines keeping point fixed."
  97   (interactive "p")
  98   (or arg (setq arg 1))
  99   (scroll-lock-update-goal-column)
 100   (condition-case nil
 101       (scroll-down arg)
 102     (beginning-of-buffer (forward-line (- arg))))
 103   (scroll-lock-move-to-column scroll-lock-temporary-goal-column))
 104
 105 (defun scroll-lock-forward-paragraph (&optional arg)
 106   "Scroll down ARG paragraphs keeping point fixed."
 107   (interactive "p")
 108   (or arg (setq arg 1))
 109   (scroll-lock-update-goal-column)
 110   (scroll-up (count-screen-lines (point) (save-excursion
 111                                            (forward-paragraph arg)
 112                                            (point))))
 113   (scroll-lock-move-to-column scroll-lock-temporary-goal-column))
 114
 115 (defun scroll-lock-backward-paragraph (&optional arg)
 116   "Scroll up ARG paragraphs keeping point fixed."
 117   (interactive "p")
 118   (or arg (setq arg 1))
 119   (scroll-lock-update-goal-column)
 120   (let ((goal (save-excursion (backward-paragraph arg) (point))))
 121     (condition-case nil
 122         (scroll-down (count-screen-lines goal (point)))
 123       (beginning-of-buffer (goto-char goal))))
 124   (scroll-lock-move-to-column scroll-lock-temporary-goal-column))
 125
 126 (provide 'scroll-lock)
 127
 128 ;;; scroll-lock.el ends here