canvas: provide a means of blocking change notifications from Item propagating up the object tree

libs/canvas/canvas/item.h

@@ -197,6 +197,9 @@ public:
 	virtual void hide ();
 	virtual void show ();
 
+	void block_change_notifications ();
+	void unblock_change_notifications ();
+
 	/** @return true if this item is visible (ie it will be rendered),
 	 *  otherwise false
 	 */
@@ -374,10 +377,23 @@ private:
 
 	void find_scroll_parent ();
 	void propagate_show_hide ();
+
+	int change_blocked;
 };
 
 extern LIBCANVAS_API std::ostream& operator<< (std::ostream&, const ArdourCanvas::Item&);
 
+/* RAII wrapper for blocking item change notifications */
+
+class LIBCANVAS_API ItemChangeBlocker
+{
+  public:
+	ItemChangeBlocker (Item& i) : item (i) { item.block_change_notifications (); }
+	~ItemChangeBlocker() { item.block_change_notifications (); }
+  private:
+	Item& item;
+};
+
 }
 
 #endif

libs/canvas/item.cc

@@ -51,6 +51,7 @@ Item::Item (Canvas* canvas)
 	, _ignore_events (false)
 	, _scroll_translation (true)
 	, _bounding_box_dirty (true)
+	, change_blocked (0)
 {
 	DEBUG_TRACE (DEBUG::CanvasItems, string_compose ("new canvas item %1\n", this));
 }
@@ -71,6 +72,7 @@ Item::Item (Item* parent)
 	, _ignore_events (false)
 	, _scroll_translation (true)
 	, _bounding_box_dirty (true)
+	, change_blocked (0)
 {
 	DEBUG_TRACE (DEBUG::CanvasItems, string_compose ("new canvas item %1\n", this));
 
@@ -98,6 +100,7 @@ Item::Item (Item* parent, Duple const& p)
 	, _ignore_events (false)
 	, _scroll_translation (true)
 	, _bounding_box_dirty (true)
+	, change_blocked (0)
 {
 	DEBUG_TRACE (DEBUG::CanvasItems, string_compose ("new canvas item %1\n", this));
 
@@ -750,12 +753,18 @@ Item::redraw () const
 void
 Item::begin_change ()
 {
-	_pre_change_bounding_box = bounding_box ();
+	if (!change_blocked) {
+		_pre_change_bounding_box = bounding_box ();
+	}
 }
 
 void
 Item::end_change ()
 {
+	if (change_blocked) {
+		return;
+	}
+
 	if (visible()) {
 		_canvas->item_changed (this, _pre_change_bounding_box);
 
@@ -1205,7 +1214,7 @@ Item::child_changed (bool bbox_changed)
 		set_bbox_dirty ();
 	}
 
-	if (_parent) {
+	if (!change_blocked && _parent) {
 		_parent->child_changed (bbox_changed);
 	}
 }
@@ -1359,3 +1368,22 @@ Item::disable_scroll_translation ()
 {
 	_scroll_translation = false;
 }
+
+void
+Item::block_change_notifications ()
+{
+	if (!change_blocked) {
+		begin_change ();
+	}
+	change_blocked++;
+}
+
+void
+Item::unblock_change_notifications ()
+{
+	if (change_blocked) {
+		if (--change_blocked == 0) {
+			end_change ();
+		}
+	}
+}