Samples: add CustomCellRenderer, adopted from https://github.com/mono/gtk-sharp/blob/master/sample/CustomCellRenderer.cs

Source/Samples/Sections/Widgets/CellRendererSection.cs

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+using Gtk;
+using System;
+namespace Samples
+{
+
+	[Section (ContentType = typeof(CustomCellRenderer), Category = Category.Widgets)]
+	class CellRendererSection : ListSection
+	{
+		public CellRendererSection ()
+		{
+			AddItem (CreateCellRenderer ());
+         
+		}
+		ListStore liststore;
+
+		public (string, Widget) CreateCellRenderer ()
+		{
+			liststore = new ListStore (typeof (float), typeof (string));
+			liststore.AppendValues (0.5f, "50%");
+
+			TreeView view = new TreeView (liststore);
+
+			view.AppendColumn ("Progress", new CellRendererText (), "text", 1);
+			view.AppendColumn ("Progress", new CustomCellRenderer (), "percent", 0);
+			GLib.Timeout.Add (50, new GLib.TimeoutHandler (update_percent));
+
+			return (nameof(CustomCellRenderer), view);
+		}
+		
+		bool increasing = true;
+		bool update_percent ()
+		{
+			TreeIter iter;
+			liststore.GetIterFirst (out iter);
+			float perc = (float) liststore.GetValue (iter, 0);
+
+			if ((perc > 0.99) || (perc < 0.01 && perc > 0)) {
+				increasing = !increasing;
+			}
+
+			if (increasing)
+				perc += 0.01f;
+			else
+				perc -= 0.01f;
+
+			liststore.SetValue (iter, 0, perc);
+			liststore.SetValue (iter, 1, Convert.ToInt32 (perc * 100) + "%");
+
+			return true;
+		}
+	}
+
+}

Source/Samples/Sections/Widgets/CustomWidgets/CustomCellRenderer.cs

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+// CustomCellRenderer.cs : C# implementation of an example custom cellrenderer
+// from http://scentric.net/tutorial/sec-custom-cell-renderers.html
+//
+// Author: Todd Berman <tberman@sevenl.net>
+//
+// (c) 2004 Todd Berman
+
+// adopted from: https://github.com/mono/gtk-sharp/commits/2.99.3/sample/CustomCellRenderer.cs
+
+using System;
+using Gtk;
+using Gdk;
+using GLib;
+
+namespace Samples
+{
+
+	public class CustomCellRenderer : CellRenderer
+	{
+
+		private float percent;
+
+		[GLib.Property ("percent")]
+		public float Percentage {
+			get { return percent; }
+			set { percent = value; }
+		}
+
+		protected override void OnGetSize (Widget widget, ref Rectangle cell_area, out int x_offset, out int y_offset, out int width, out int height)
+		{
+			base.OnGetSize (widget, ref cell_area, out x_offset, out y_offset, out width, out height);
+
+			int calc_width = (int) this.Xpad * 2 + 100;
+			int calc_height = (int) this.Ypad * 2 + 10;
+
+			width = calc_width;
+			height = calc_height;
+
+			x_offset = 0;
+			y_offset = 0;
+			if (!cell_area.Equals (Rectangle.Zero)) {
+				x_offset = (int) (this.Xalign * (cell_area.Width - calc_width));
+				x_offset = Math.Max (x_offset, 0);
+
+				y_offset = (int) (this.Yalign * (cell_area.Height - calc_height));
+				y_offset = Math.Max (y_offset, 0);
+			}
+		}
+
+		protected override void OnRender (Cairo.Context cr, Widget widget, Rectangle background_area, Rectangle cell_area, CellRendererState flags)
+		{
+			int x = (int) (cell_area.X + this.Xpad);
+			int y = (int) (cell_area.Y + this.Ypad);
+			int width = (int) (cell_area.Width - this.Xpad * 2);
+			int height = (int) (cell_area.Height - this.Ypad * 2);
+
+			widget.StyleContext.Save ();
+			widget.StyleContext.AddClass ("trough");
+			widget.StyleContext.RenderBackground (cr, x, y, width, height);
+			widget.StyleContext.RenderFrame (cr, x, y, width, height);
+
+			Border padding = widget.StyleContext.GetPadding (StateFlags.Normal);
+			x += padding.Left;
+			y += padding.Top;
+			width -= padding.Left + padding.Right;
+			height -= padding.Top + padding.Bottom;
+
+			widget.StyleContext.Restore ();
+
+			widget.StyleContext.Save ();
+			widget.StyleContext.AddClass ("progressbar");
+			widget.StyleContext.RenderActivity (cr, x, y, (int) (width * Percentage), height);
+			widget.StyleContext.Restore ();
+		}
+	}
+
+}