avoid recursing through the entire canvas when scrolling - only scroll explicitly identified ScrollGroups

gtk2_ardour/editor_canvas.cc

@@ -66,12 +66,14 @@ Editor::initialize_canvas ()
 {
 	_track_canvas_viewport = new ArdourCanvas::GtkCanvasViewport (horizontal_adjustment, vertical_adjustment);
 	_track_canvas = _track_canvas_viewport->canvas ();
-	_track_canvas->set_global_scroll (false);
 
-	hv_scroll_group = new ArdourCanvas::ScrollGroup (_track_canvas->root(), 
+	ArdourCanvas::ScrollGroup* hsg; 
-							 ArdourCanvas::ScrollGroup::ScrollSensitivity (ArdourCanvas::ScrollGroup::ScrollsVertically|
+
-												       ArdourCanvas::ScrollGroup::ScrollsHorizontally));
+	hv_scroll_group = hsg = new ArdourCanvas::ScrollGroup (_track_canvas->root(), 
+							       ArdourCanvas::ScrollGroup::ScrollSensitivity (ArdourCanvas::ScrollGroup::ScrollsVertically|
+													     ArdourCanvas::ScrollGroup::ScrollsHorizontally));
 	CANVAS_DEBUG_NAME (hv_scroll_group, "canvas hv scroll");
+	_track_canvas->add_scroller (*hsg);
 
 	_verbose_cursor = new VerboseCursor (this);
 

libs/canvas/canvas.cc

@@ -33,6 +33,7 @@
 #include "canvas/canvas.h"
 #include "canvas/debug.h"
 #include "canvas/line.h"
+#include "canvas/scroll_group.h"
 
 using namespace std;
 using namespace ArdourCanvas;
@@ -40,7 +41,6 @@ using namespace ArdourCanvas;
 /** Construct a new Canvas */
 Canvas::Canvas ()
 	: _root (this)
-	, _global_scroll (true)
 {
 	set_epoch ();
 }
@@ -48,19 +48,29 @@ Canvas::Canvas ()
 void
 Canvas::scroll_to (Coord x, Coord y)
 {
-	Duple d (x, y);
+	_scroll_offset = Duple (x, y);
 
-	_scroll_offset = d;
+	/* We do things this way because we do not want to recurse through
+	   the canvas for every scroll. In the presence of large MIDI
+	   tracks this means traversing item lists that include
+	   thousands of items (notes).
 
-	_root.scroll_to (d);
+	   This design limits us to moving only those items (groups, typically)
+	   that should move in certain ways as we scroll. In other terms, it
+	   becomes O(1) rather than O(N).
+	*/
+
+	for (list<ScrollGroup*>::iterator i = scrollers.begin(); i != scrollers.end(); ++i) {
+		(*i)->scroll_to (_scroll_offset);
+	}
 
 	pick_current_item (0); // no current mouse position 
 }
 
 void
-Canvas::set_global_scroll (bool yn)
+Canvas::add_scroller (ScrollGroup& i)
 {
-	_global_scroll = yn;
+	scrollers.push_back (&i);
 }
 
 void
@@ -602,6 +612,11 @@ GtkCanvas::item_going_away (Item* item, boost::optional<Rect> bounding_box)
 		_focused_item = 0;
 	}
 
+	ScrollGroup* sg = dynamic_cast<ScrollGroup*>(item);
+	if (sg) {
+		scrollers.remove (sg);
+	}
+
 	if (_current_item == item) {
 		/* no need to send a leave event to this item, since it is going away 
 		 */

libs/canvas/canvas/canvas.h

@@ -43,6 +43,7 @@ namespace ArdourCanvas
 
 class Rect;
 class Group;	
+class ScrollGroup;
 
 /** The base class for our different types of canvas.
  *
@@ -108,7 +109,7 @@ public:
         }
 
         void scroll_to (Coord x, Coord y);
-        void set_global_scroll (bool);
+	void add_scroller (ScrollGroup& i);
         
         virtual Rect visible_area () const = 0;
 
@@ -125,10 +126,11 @@ protected:
 	RootGroup _root;
 
         Duple _scroll_offset;
-        bool _global_scroll;
 
         virtual void pick_current_item (int state) = 0;
         virtual void pick_current_item (Duple const &, int state) = 0;
+
+	std::list<ScrollGroup*> scrollers;
 };
 
 /** A canvas which renders onto a GTK EventBox */