c# – NControl绘制方法没有触发

我遇到的问题是draw方法永远不会被调用,而且我不太确定我哪里出错了.我的代码在下面(我已经删除了所有不必要的位),非常感谢任何帮助！

public class CustomGrid : NControlView
    {
        public CustomGrid ()
        {
            base.Invalidate (); //Invalidating the control doesn't redraw the control
            Content = new Label {BackgroundColor = Xamarin.Forms.Color.Transparent};
            BackgroundColor = Xamarin.Forms.Color.Blue;
        }


        public override void Draw (NGraphics.ICanvas canvas,NGraphics.Rect rect)
        {
            base.Draw(canvas,rect);
            //foreach column draw the grid line on the right
            foreach (CustomColumn c in ColumnCollection) 
            {
                canvas.DrawLine (c.CoOrds.startX,c.CoOrds.startY,c.CoOrds.EndX,c.CoOrds.EndY,Colors.White);
            }

            //for each row draw the bottom grid line
            foreach (CustomRow r in RowCollection) 
            {
                canvas.DrawLine (r.CoOrds.startX,r.CoOrds.startY,r.CoOrds.EndX,r.CoOrds.EndY,Colors.White);
            }
        }

        public List<CustomColumn> ColumnCollection { get; set; }
        public List<CustomRow> RowCollection { get; set; }


    }

为了确保我不会发疯,我复制并粘贴了NControl GitHub Repo上的示例,但似乎也没有用.

先感谢您！