裁剪（二）

private void Form1_Paint(object sender, PaintEventArgs e)
        {
            Graphics g = e.Graphics;
            g.FillRectangle(Brushes.White, this.ClientRectangle);
            //create essensial objects for painting text strings
            SizeF sizeF = g.MeasureString("Test", this.Font);
            StringFormat sf = new StringFormat();
            sf.LineAlignment = StringAlignment.Center;
            //set properties of the grid
            int cellHeight = (int)sizeF.Height + 4;
            int cellWidth = 80;
            int nbrColumns = 50;
            int nbrRows = 50;
            //output general info to console
            Console.WriteLine("-------------------------");
            Console.WriteLine("e.ClipRectangle = " + e.ClipRectangle);
            Console.WriteLine("The following cells need to be redrawn " + "(in whole or in part):");
            //draw the cells and the output to console
            for (int row = 0; row < nbrRows; ++row)
            {
                for (int col = 0; col < nbrColumns; ++col)
                {
                    Point cellLocation = new Point(col * cellWidth, row * cellHeight);
                    Rectangle cellRect = new Rectangle(cellLocation.X, cellLocation.Y, cellWidth, cellHeight);
                    if (cellRect.IntersectsWith(e.ClipRectangle))
                    {
                        Console.WriteLine("Row:{0} cel:{1}", row, col);
                        g.FillRectangle(Brushes.LightBlue, cellRect);
                        g.DrawRectangle(Pens.Black, cellRect);
                        string s = String.Format("{0},{1}", col, row);
                        g.DrawString(s, this.Font, Brushes.Black, cellRect, sf);
                    }
                    else
                    { 
                    }
                }



这段代码中的if语句指出，只有矩形与ClipRectangle的交集不为空，事件处理程序才将创建此矩形。

换句话说，如果操作系统不打算向绘图表面提交矩形的至少部分区域，它将不会创建矩形。

如果没有if。。else语句，事件处理程序将需要生成50行和50列，也就是2500个矩形，

但是if。。else语句将每一个矩形的预定位置与clipRectangle进行比较，所以代码生成的唯一矩形是实际绘制到可视的绘图表面的那些矩形。

可以通过Graphics。IsVisibleClipEmpty测试代替Rectangle。IntersectsWith测试来实现一个类似的效果，

即将if语句改为if(!g.IsVisibleClipEmpty)区别是IntersectsWith把每一个矩形与裁剪矩形比较，IsVisibleClipEmpty把每一个矩形与客户矩形比较。