org-latex-preview: Eldoc support for live previews

* lisp/org-latex-preview.el (org-latex-preview-live-display-type,
org-latex-preview-live--display-in-eldoc,
org-latex-preview-live--update-eldoc,
org-latex-preview-live--setup, org-latex-preview-live--teardown):
Add Eldoc support for live previews.  The user option
`org-latex-preview-live-display-type' now accepts the symbols
buffer and eldoc as values.  When it is set to eldoc and live
previews are enabled, display of the continuously updated previews is
handled by Eldoc.  Note that previewing larger images with Eldoc may
require `eldoc-echo-area-use-multiline-p' and `max-mini-window-height'
to be set appropriately.

lisp/org-latex-preview.el

@@ -34,6 +34,7 @@
 (declare-function org-persist-read "org-persist")
 (declare-function org-persist-register "org-persist")
 (declare-function org-persist-unregister "org-persist")
+(declare-function eldoc--invoke-strategy "eldoc")
 
 (defgroup org-latex-preview nil
   "Options for generation of LaTeX previews in Org mode."
@@ -853,7 +854,8 @@ them or vice-versa, customize the variable `org-latex-preview-auto-generate'."
 ;; previews are generated in the background with each change to the
 ;; LaTeX fragment being edited.  This continuously updated preview is
 ;; shown to the right of the LaTeX fragment, or under the LaTeX
-;; environment being edited.
+;; environment being edited.  Alternatively, it can be shown using
+;; Eldoc (see `org-latex-preview-live-display-type').
 ;;
 ;; The code works as follows (simplified description):
 
@@ -888,6 +890,27 @@ with LaTeX environments."
   :group 'org-latex-preview
   :type 'boolean)
 
+(defcustom org-latex-preview-live-display-type 'buffer
+  "How to display live-updating previews of LaTeX snippets.
+
+This option is meaningful when live previews are enabled, by
+setting `org-latex-preview-auto-generate' to `live' and enabling
+`org-latex-preview-auto-mode'.
+
+The currently supported options are the symbols
+
+- buffer: Display live previews next to or under the LaTeX
+  fragment in the Org buffer.
+
+- eldoc: Display live previews using Eldoc.  Requires
+  `eldoc-mode' to be turned on in the Org buffer.  Note that
+  Eldoc in turn offers various display functions, such as the
+  echo area, the dedicated `eldoc-doc-buffer' and more."
+  :group 'org-latex-preview
+  :type  '(choice
+           (const :tag "Display next to fragment" buffer)
+           (const :tag "Display in Eldoc" eldoc)))
+
 (defcustom org-latex-preview-debounce 1.0
   "Idle time before regenerating LaTeX previews.
 
@@ -1014,11 +1037,27 @@ BOX-FACE is the face to apply in addition."
   "Update the live LaTeX preview for overlay OV."
   (when (and (memq ov (overlays-at (point)))
              (overlay-get ov 'view-text))
-    (if (overlay-get ov 'after-string)
+    (if (or (overlay-get ov 'after-string)
+            (eq org-latex-preview-live-display-type
+                'eldoc))
         (org-latex-preview-live--update-props
          (overlay-get ov 'preview-image))
       (org-latex-preview-live--ensure-overlay ov))))
 
+;; Eldoc support for live previews
+(defun org-latex-preview-live--display-in-eldoc (callback)
+  "Eldoc documentation function for live LaTeX previews.
+
+CALLBACK is supplied by Eldoc, see
+`eldoc-documentation-functions'."
+  (when (and org-latex-preview-live--docstring
+             (get-char-property (point) 'org-overlay-type))
+    (funcall callback org-latex-preview-live--docstring)))
+
+(defun org-latex-preview-live--update-eldoc (_ov)
+  "Force eldoc to update when a preview is updated."
+  (eldoc--invoke-strategy nil))
+
 ;; Live preview setup and teardown.
 (defun org-latex-preview-live--setup ()
   "Set up hooks for live LaTeX previews.
@@ -1031,6 +1070,9 @@ See `org-latex-preview-auto-generate' for details."
                              org-latex-preview-throttle)
                             (org-latex-preview-live--debounce
                              org-latex-preview-debounce)))
+  (when (eq org-latex-preview-live-display-type 'eldoc)
+    (add-hook 'eldoc-documentation-functions #'org-latex-preview-live--display-in-eldoc nil t)
+    (add-hook 'org-latex-preview-update-overlay-functions #'org-latex-preview-live--update-eldoc nil 'local))
   (add-hook 'org-latex-preview-close-functions #'org-latex-preview-live--clearout nil 'local)
   (add-hook 'org-latex-preview-open-functions #'org-latex-preview-live--ensure-overlay nil 'local)
   (add-hook 'after-change-functions org-latex-preview-live--generator 90 'local)
@@ -1044,6 +1086,9 @@ See `org-latex-preview-auto-generate' for details."
               ((eq (car props) 'org-latex-overlay))
               (ov (cdr props)))
     (org-latex-preview-live--clearout ov))
+  (when (eq org-latex-preview-live-display-type 'eldoc)
+    (remove-hook 'eldoc-documentation-functions #'org-latex-preview-live--display-in-eldoc t)
+    (remove-hook 'org-latex-preview-update-overlay-functions #'org-latex-preview-live--update-eldoc 'local))
   (remove-hook 'org-latex-preview-close-functions #'org-latex-preview-live--clearout 'local)
   (remove-hook 'org-latex-preview-open-functions #'org-latex-preview-live--ensure-overlay 'local)
   (remove-hook 'after-change-functions org-latex-preview-live--generator 'local)