different approach to independent scrolling, involving ArdourCanvas::ScrollGroup

The idea now is that a scroll group item can be added to the canvas which will causes its children to scroll in either or both
directions (horizontal or vertical). There are few complications: the position() of the ScrollGroup is ambiguous depending
on whether you want it with scroll taken into account or not, so Item::canvas_position() was added, which defaults to
the same value as Item::position() but is overridden by ScrollGroup to return the position independent of scrolling. This
method is used when translating between item/canvas/window coordinate systems.

Note that the basic idea is that we MOVE the scroll group when a scroll happens. This mirrors what happens in the GnomeCanvas,
where Nick Mainsbridge came up with a great idea that allowed unification of the time bar and track canvases.

1 files changed, 63 insertions, 0 deletions

diff --git a/libs/canvas/scroll_group.cc b/libs/canvas/scroll_group.cc
new file mode 100644
index 0000000000..bda6911ca7
--- /dev/null
+++ b/libs/canvas/scroll_group.cc
@@ -0,0 +1,63 @@
+/*
+    Copyright (C) 2014 Paul Davis
+
+    This program is free software; you can redistribute it and/or modify
+    it under the terms of the GNU General Public License as published by
+    the Free Software Foundation; either version 2 of the License, or
+    (at your option) any later version.
+
+    This program is distributed in the hope that it will be useful,
+    but WITHOUT ANY WARRANTY; without even the implied warranty of
+    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+    GNU General Public License for more details.
+
+    You should have received a copy of the GNU General Public License
+    along with this program; if not, write to the Free Software
+    Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
+*/
+
+#include <iostream>
+#include "canvas/scroll_group.h"
+
+using namespace std;
+using namespace ArdourCanvas;
+
+ScrollGroup::ScrollGroup (Group* parent, ScrollSensitivity s)
+	: Group (parent)
+	, _scroll_sensitivity (s)	
+{
+}
+
+ScrollGroup::ScrollGroup (Group* parent, Duple position, ScrollSensitivity s)
+	: Group (parent, position)
+	, _scroll_sensitivity (s)
+{
+}
+
+void
+ScrollGroup::scroll_to (Duple const& d)
+{
+	Duple base_pos (position().translate (_scroll_offset));
+
+	if (_scroll_sensitivity & ScrollsHorizontally) {
+		base_pos.x -= d.x;
+		_scroll_offset.x = d.x;
+	} 
+
+	if (_scroll_sensitivity & ScrollsVertically) {
+		base_pos.y -= d.y;
+		_scroll_offset.y = d.y;
+	}
+
+	set_position (base_pos);
+}
+
+Duple
+ScrollGroup::canvas_position() const
+{
+	/* return the normal "base" position of this item
+	   rather its position as affected by any scroll
+	   offset.
+	*/
+	return _position.translate (_scroll_offset);
+}