[longomatch] Fix blackboard clearing

[Andoni Morales Alastruey <amorales src gnome org>]

commit 2f648d19159447d00ba589ed812de6925b567735
Author: Andoni Morales Alastruey <ylatuya gmail com>
Date:   Mon Jun 30 18:48:47 2014 +0200

    Fix blackboard clearing

 LongoMatch.Drawing/Widgets/Blackboard.cs |   57 +++++++++++++++++++----------
 1 files changed, 37 insertions(+), 20 deletions(-)
---
diff --git a/LongoMatch.Drawing/Widgets/Blackboard.cs b/LongoMatch.Drawing/Widgets/Blackboard.cs
index 67a194b..2347e2b 100644
--- a/LongoMatch.Drawing/Widgets/Blackboard.cs
+++ b/LongoMatch.Drawing/Widgets/Blackboard.cs
@@ -53,6 +53,7 @@ namespace LongoMatch.Drawing.Widgets
                
                public FrameDrawing Drawing {
                        set {
+                               Clear (false);
                                drawing = value;
                                foreach (IBlackboardObject d in value.Drawables) {
                                        Add (d);
@@ -115,12 +116,19 @@ namespace LongoMatch.Drawing.Widgets
                        widget.ReDraw ();
                }
                
-               public void Clear () {
+               public void Clear (bool resetDrawing = true) {
                        ClearSelection ();
-                       drawing.Drawables.Clear ();
                        Objects.Clear ();
-                       backbuffer.Dispose ();
-                       backbuffer = tk.CreateSurface (Background.Width, Background.Height);
+                       if (drawing != null && resetDrawing) {
+                               drawing.Drawables.Clear ();
+                       }
+                       if (backbuffer != null) {
+                               using (IContext c = backbuffer.Context) {
+                                       tk.Context = c;
+                                       tk.Clear (new Color (0, 0, 0, 0));
+                                       tk.Context = null;
+                               };
+                       }
                        widget.ReDraw ();
                }
                
@@ -250,10 +258,12 @@ namespace LongoMatch.Drawing.Widgets
                
                protected override void SelectionChanged (System.Collections.Generic.List<Selection> sel)
                {
-                       if (sel != null && sel.Count > 0 && DrawableChangedEvent != null) {
-                               DrawableChangedEvent ((sel[0].Drawable as 
ICanvasDrawableObject).IDrawableObject);
-                       } else {
-                               DrawableChangedEvent (null);
+                       if (DrawableChangedEvent != null) {
+                               if (sel != null && sel.Count > 0) {
+                                       DrawableChangedEvent ((sel[0].Drawable as 
ICanvasDrawableObject).IDrawableObject);
+                               } else {
+                                       DrawableChangedEvent (null);
+                               }
                        }
                }
 
@@ -272,25 +282,32 @@ namespace LongoMatch.Drawing.Widgets
                protected override void CursorMoved (Point coords)
                {
                        if (handdrawing) {
-                               tk.Context = backbuffer.Context;
-                               tk.Begin ();
-                               tk.LineStyle = LineStyle.Normal;
-                               tk.LineWidth = LineWidth;
-                               if (tool == DrawTool.Eraser) {
-                                       tk.StrokeColor = tk.FillColor = new Color (0, 0, 0, 255);
-                                       tk.LineWidth = LineWidth * 4;
-                                       tk.Clear = true;
-                               } else {
-                                       tk.StrokeColor = tk.FillColor = Color;
+                               using (IContext c = backbuffer.Context) {
+                                       tk.Context = c;
+                                       tk.Begin ();
+                                       tk.LineStyle = LineStyle.Normal;
+                                       tk.LineWidth = LineWidth;
+                                       if (tool == DrawTool.Eraser) {
+                                               tk.StrokeColor = tk.FillColor = new Color (0, 0, 0, 255);
+                                               tk.LineWidth = LineWidth * 4;
+                                               tk.ClearOperation = true;
+                                       } else {
+                                               tk.StrokeColor = tk.FillColor = Color;
+                                       }
+                                       tk.DrawLine (start, coords);
+                                       tk.End ();
                                }
-                               tk.DrawLine (start, coords);
-                               tk.End ();
                                widget.ReDraw();
                        }
                }
                
                public override void Draw (IContext context, Area area)
                {
+                       tk.Context = context;
+                       tk.Begin ();
+                       tk.Clear (Color.Black);
+                       tk.End ();
+                       
                        base.Draw (context, area);
                        if (backbuffer != null) {
                                tk.Context = context;