[PATCH v4 5/5] emacs: show: implement lazy hidden part handling

[Mark Walters]

This adds the actual code to do the lazy insertion of hidden parts.

We use a memory inefficient but simple method: when we come to insert
the part if it is hidden we just store all of the arguments to the
part insertion function as a button property. This means when we want
to show the part we can just resume where we left off.

One thing is that we can't tell if a lazy part will produce text until
we try to render it so when unhiding a part we check to see if it
rendered; if not we invoke the default part handler (e.g. an external
viewer).
---
 emacs/notmuch-show.el |   52 ++++++++++++++++++++++++++++++++++++++++++-------
 1 file changed, 45 insertions(+), 7 deletions(-)

diff --git a/emacs/notmuch-show.el b/emacs/notmuch-show.el
index 49f85ba..92f481b 100644
--- a/emacs/notmuch-show.el
+++ b/emacs/notmuch-show.el
@@ -488,12 +488,14 @@ message at DEPTH in the current thread."
     ;; return button
     button))
 
-;; This is taken from notmuch-wash: maybe it should be unified?
 (defun notmuch-show-toggle-part-invisibility (&optional button)
   (interactive)
   (let* ((button (or button (button-at (point))))
-	 (overlay (button-get button 'overlay)))
-    (when overlay
+	 (overlay (button-get button 'overlay))
+	 (lazy-part (button-get button :notmuch-lazy-part)))
+    ;; We have a part to toggle if there is an overlay or if there is a lazy part.
+    ;; If neither is present we cannot toggle the part so we just return nil.
+    (when (or overlay lazy-part)
       (let* ((show (button-get button :notmuch-part-hidden))
 	     (new-start (button-start button))
 	     (button-label (button-get button :base-label))
@@ -509,7 +511,15 @@ message at DEPTH in the current thread."
 	  (move-overlay button new-start (point))
 	  (delete-region (point) old-end))
 	(goto-char (min old-point (1- (button-end button))))
-	(overlay-put overlay 'invisible (not show))))))
+	;; Return nil if there is a lazy-part, it is empty, and we are
+	;; trying to show it.  In all other cases return t.
+	(if lazy-part
+	    (when show
+	      (button-put button :notmuch-lazy-part nil)
+	      (notmuch-show-lazy-part lazy-part button))
+	  ;; else there must be an overlay.
+	  (overlay-put overlay 'invisible (not show))
+	  t)))))
 
 ;; MIME part renderers
 
@@ -807,6 +817,28 @@ message at DEPTH in the current thread."
     ;; Return true if we created an overlay.
     t))
 
+(defun notmuch-show-lazy-part (part-args button)
+  ;; Insert the lazy part after the button for the part.
+  (save-excursion
+    (goto-char (1+ (button-end button)))
+    (let* ((inhibit-read-only t)
+	   ;; We need to use markers for the start and end of the part
+	   ;; because the part insertion functions do not guarantee
+	   ;; to leave point at the end of the part.
+	   (part-beg (copy-marker (point) nil))
+	   (part-end (copy-marker (point) t))
+	   ;; We have to save the depth as we can't find the depth
+	   ;; when narrowed.
+	   (depth (notmuch-show-get-depth)))
+      (save-restriction
+	(narrow-to-region part-beg part-end)
+	(delete-region part-beg part-end)
+	(apply #'notmuch-show-insert-bodypart-internal part-args)
+	(indent-rigidly part-beg part-end depth))
+      ;; Create the overlay. If the lazy-part turned out to be empty/not
+      ;; showable this returns nil.
+      (notmuch-show-create-part-overlays button part-beg part-end))))
+
 (defun notmuch-show-insert-bodypart (msg part depth &optional hide)
   "Insert the body part PART at depth DEPTH in the current thread.
 
@@ -825,7 +857,11 @@ If HIDE is non-nil then initially hide this part."
 		   (notmuch-show-insert-part-header nth mime-type content-type (plist-get part :filename))))
 	 (content-beg (point)))
 
-    (notmuch-show-insert-bodypart-internal msg part mime-type nth depth button)
+    (if (not hide)
+        (notmuch-show-insert-bodypart-internal msg part mime-type nth depth button)
+      (button-put button :notmuch-lazy-part
+                  (list msg part mime-type nth depth button)))
+
     ;; Some of the body part handlers leave point somewhere up in the
     ;; part, so we make sure that we're down at the end.
     (goto-char (point-max))
@@ -2009,8 +2045,10 @@ is destroyed when FN returns."
 (defun notmuch-show-part-button-default (&optional button)
   (interactive)
   (let ((button (or button (button-at (point)))))
-    (if (button-get button 'overlay)
-	(notmuch-show-toggle-part-invisibility button)
+    ;; Try to toggle the part, if that fails then call the default
+    ;; action. The toggle fails if the part has no emacs renderable
+    ;; content.
+    (unless (notmuch-show-toggle-part-invisibility button)
       (call-interactively notmuch-show-part-button-default-action))))
 
 (defun notmuch-show-save-part ()
-- 
1.7.10.4