canvas: add a drawing-request-freeze/thaw API

If queue_draw is "frozen", we simply accumulate drawing
requests in a (union) rectangle, and when finally "thawed"
the canvas submits a single redraw request for the entire
accumulated rect.

Although in theory this is all that GTK/GDK does for
draw requests, callgrind reveals significant costs
associated with the actual calltree for GtkWidget::queue_draw_area().

One potential cost is that GDK also maintains a list of
invalidated rectangles in addition to the union, and
for MIDI regions with thousands of notes, this can represent
real overhead. This approach dispenses with the rect list,
since our Canvas drawing model only uses the union rectangle
anyway.

libs/canvas/canvas.cc

@@ -55,6 +55,7 @@ uint32_t Canvas::tooltip_timeout_msecs = 750;
 /** Construct a new Canvas */
 Canvas::Canvas ()
 	: _root (this)
+	, _queue_draw_frozen (0)
 	, _bg_color (Gtkmm2ext::rgba_to_color (0, 1.0, 0.0, 1.0))
 	, _last_render_start_timestamp(0)
 	, _use_intermediate_surface (false)
@@ -230,6 +231,25 @@ Canvas::dump (ostream& o) const
 	_root.dump (o);
 }
 
+void
+Canvas::freeze_queue_draw ()
+{
+	_queue_draw_frozen++;
+}
+
+void
+Canvas::thaw_queue_draw ()
+{
+	if (_queue_draw_frozen) {
+		_queue_draw_frozen--;
+
+		if (_queue_draw_frozen == 0 && !frozen_area.empty()) {
+			request_redraw (frozen_area);
+			frozen_area = Rect();
+		}
+	}
+}
+
 /** Called when an item has been shown or hidden.
  *  @param item Item that has been shown or hidden.
  */
@@ -238,6 +258,11 @@ Canvas::item_shown_or_hidden (Item* item)
 {
 	Rect bbox = item->bounding_box ();
 	if (bbox) {
+		if (_queue_draw_frozen) {
+			frozen_area = frozen_area.extend (compute_draw_item_area (item, bbox));
+			return;
+		}
+
 		if (item->item_to_window (bbox).intersection (visible_area ())) {
 			queue_draw_item_area (item, bbox);
 		}
@@ -416,41 +441,48 @@ Canvas::item_moved (Item* item, Rect pre_change_parent_bounding_box)
 void
 Canvas::queue_draw_item_area (Item* item, Rect area)
 {
+	request_redraw (compute_draw_item_area (item, area));
+}
+
+Rect
+Canvas::compute_draw_item_area (Item* item, Rect area)
+{
+	Rect r;
+
 	if ((area.width()) > 1.0 && (area.height() > 1.0)) {
 		/* item has a rectangular bounding box, which may fall
 		 * on non-integer locations. Expand it appropriately.
 		 */
-		Rect r = item->item_to_window (area, false);
+		r = item->item_to_window (area, false);
 		r.x0 = floor (r.x0);
 		r.y0 = floor (r.y0);
 		r.x1 = ceil (r.x1);
 		r.y1 = ceil (r.y1);
 		//std::cerr << "redraw box, adjust from " << area << " to " << r << std::endl;
-		request_redraw (r);
-		return;
 	} else if (area.width() > 1.0 && area.height() == 1.0) {
 		/* horizontal line, which may fall on non-integer
 		 * coordinates.
 		 */
-		Rect r = item->item_to_window (area, false);
+		r = item->item_to_window (area, false);
 		r.y0 = floor (r.y0);
 		r.y1 = ceil (r.y1);
 		//std::cerr << "redraw HLine, adjust from " << area << " to " << r << std::endl;
-		request_redraw (r);
 	} else if (area.width() == 1.0 && area.height() > 1.0) {
 		/* vertical single pixel line, which may fall on non-integer
 		 * coordinates
 		 */
-		Rect r = item->item_to_window (area, false);
+		r = item->item_to_window (area, false);
 		r.x0 = floor (r.x0);
 		r.x1 = ceil (r.x1);
 		//std::cerr << "redraw VLine, adjust from " << area << " to " << r << std::endl;
-		request_redraw (r);
+
 	} else {
 		/* impossible? one of width or height must be zero ... */
 		//std::cerr << "redraw IMPOSSIBLE of " << area  << std::endl;
-		request_redraw (item->item_to_window (area, false));
+		r =  item->item_to_window (area, false);
 	}
+
+	return r;
 }
 
 void

libs/canvas/canvas/canvas.h

@@ -107,6 +107,9 @@ public:
 		return &_root;
 	}
 
+	void freeze_queue_draw ();
+	void thaw_queue_draw ();
+
 	void set_background_color (Gtkmm2ext::Color);
 	Gtkmm2ext::Color background_color() const { return _bg_color; }
 
@@ -185,6 +188,8 @@ public:
 
 protected:
 	Root             _root;
+	uint32_t         _queue_draw_frozen;
+	Rect              frozen_area;
 	Gtkmm2ext::Color _bg_color;
 
 	mutable gint64 _last_render_start_timestamp;
@@ -192,6 +197,7 @@ protected:
 	static uint32_t tooltip_timeout_msecs;
 
 	void queue_draw_item_area (Item *, Rect);
+	Rect compute_draw_item_area (Item *, Rect);
 
 	virtual void pick_current_item (int state) = 0;
 	virtual void pick_current_item (Duple const &, int state) = 0;