Add minor mode to keep buffer content centered on window

1 files changed, 29 insertions, 15 deletions

diff --git a/.emacs b/.emacs
index 275518d..6cdd7b1 100644
--- a/.emacs
+++ b/.emacs
@@ -163,21 +163,35 @@
     (set-variable 'tab-width new-width)
     (message "changed from %s to %s" old-width new-width)))
 
-(defun my/center-window (text-width)
-  (interactive
-   (list (cond
-          ;; Explicit length.
-          ((integerp current-prefix-arg)
-           current-prefix-arg)
-          ;; C-u.
-          ((and (listp current-prefix-arg) current-prefix-arg)
-           nil)
-          ;; Default.
-           (80))))
-  (let ((margin-width (when text-width
-                        (/ (- (window-width) text-width) 2))))
+(defvar-local my/centered-width 'fill-column)
+
+(define-minor-mode my/centered-mode
+  "Update margins to keep content centered."
+  :init-value nil
+  (if my/centered-mode
+      (progn
+        (add-hook 'window-state-change-functions 'my/centered-set-margins nil t)
+        (dolist (win (get-buffer-window-list))
+          (my/centered-set-margins win)))
+    (remove-hook 'window-state-change-functions 'my/centered-set-margins t)
+    (dolist (win (get-buffer-window-list))
+      (set-window-margins win nil))))
+
+(defun my/centered-set-margins (window)
+  (let* ((target-body-width
+          (cond
+           ((symbolp my/centered-width)
+            (buffer-local-value my/centered-width (window-buffer window)))
+           ((integerp my/centered-width)
+            my/centered-width)))
+         (adjustable-width
+          (- (window-total-width window)
+             (+ (fringe-columns 'left) (fringe-columns 'right))))
+         (target-margin
+          (when (> adjustable-width target-body-width)
+            (/ (- adjustable-width target-body-width) 2))))
     ;; Only set left margin, so that long lines are not truncated.
-    (set-window-margins nil margin-width)))
+    (set-window-margins window target-margin)))
 
 (defun my/kill (stuff)
   (kill-new stuff)
@@ -232,7 +246,7 @@
 
 (my/define-prefix-command my/display-map
   "Keymap for display-related commands."
-  '(("c" my/center-window)
+  '(("c" my/centered-mode)
     ("t" toggle-truncate-lines)
     ("v" visual-line-mode)))