[PATCH 4/4] 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.

The only slight subtlety/hack is that to simplify the handling of the
invisibility overlay (for the hiding unhiding later) we do insert some
dummy text which we remove when we show the part.
---
 emacs/notmuch-show.el |   32 ++++++++++++++++++++++++++++++--
 1 files changed, 30 insertions(+), 2 deletions(-)

diff --git a/emacs/notmuch-show.el b/emacs/notmuch-show.el
index 2c48b24..a14a135 100644
--- a/emacs/notmuch-show.el
+++ b/emacs/notmuch-show.el
@@ -559,6 +559,7 @@ message at DEPTH in the current thread."
 	 (overlay (button-get button 'overlay)))
     (when overlay
       (let* ((show (overlay-get overlay 'invisible))
+	     (lazy-part (button-get button :notmuch-lazy-part))
 	     (new-start (button-start button))
 	     (button-label (button-get button :base-label))
 	     (old-point (point))
@@ -569,7 +570,11 @@ message at DEPTH in the current thread."
 	(let ((old-end (button-end button)))
 	  (move-overlay button new-start (point))
 	  (delete-region (point) old-end))
-	(goto-char (min old-point (1- (button-end button))))))))
+	(goto-char (min old-point (1- (button-end button))))
+	(when (and show lazy-part)
+	  (save-excursion
+	    (button-put button :notmuch-lazy-part nil)
+	    (notmuch-show-lazy-part lazy-part button)))))))
 
 (defun notmuch-show-multipart/*-to-list (part)
   (mapcar (lambda (inner-part) (plist-get inner-part :content-type))
@@ -857,6 +862,24 @@ message at DEPTH in the current thread."
       (setq handlers (cdr handlers))))
   t)
 
+(defun notmuch-show-lazy-part (part-args button)
+  (interactive)
+  ;; We have to save the depth as we can't find the depth when narrowed
+  (let ((inhibit-read-only t)
+	(overlay (button-get button 'overlay))
+	(depth (notmuch-show-get-depth)))
+    (save-restriction
+      (narrow-to-region (overlay-start overlay) (1- (overlay-end overlay)))
+      (delete-region (overlay-start overlay) (1- (overlay-end overlay)))
+      (goto-char (overlay-start overlay))
+      (apply #'notmuch-show-insert-bodypart-internal (nconc part-args (list button)))
+      (indent-rigidly (overlay-start overlay)
+		      (1- (overlay-end overlay))
+		      depth))
+    ;; We deferred deleting this character to simplify handling of the
+    ;; overlay: all of the above takes place inside the overlay.
+    (delete-region (1- (overlay-end overlay)) (overlay-end overlay))))
+
 (defun notmuch-show-create-part-overlays (msg beg end hide)
   "Add an overlay to the part between BEG and END"
   (let* ((button (button-at beg))
@@ -891,7 +914,12 @@ If HIDE is non-nil then initially hide this part."
 	 (button (unless (string= mime-type "text/plain")
 		   (notmuch-show-insert-part-header nth mime-type content-type (plist-get part :filename)))))
 
-    (notmuch-show-insert-bodypart-internal msg part mime-type nth depth content-type button)
+    (if (not hide)
+        (notmuch-show-insert-bodypart-internal msg part mime-type nth depth content-type button)
+      (insert "lazy part")
+      (button-put button :notmuch-lazy-part
+                  (list msg part mime-type nth depth content-type)))
+
     ;; 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))
-- 
1.7.9.1