c# – 如何在刷新表单后保留绘制的形状？

我试图在Visual Studio 2015中创建一个小的绘图应用程序.我的项目属于 Windows窗体应用程序的类别.我有以下问题：

private void Form1_MouseMove(object sender,MouseEventArgs e)
    {
        if (a == 1)
        {
            if (r == 1 || el == 1)
            {
                int x = Math.Min(inX,e.X);
                int y = Math.Min(inY,e.Y);
                int width = Math.Max(inX,e.X) - Math.Min(inX,e.X);
                int height = Math.Max(inY,e.Y) - Math.Min(inY,e.Y);
                rect = new Rectangle(x,y,width,height);
                Refresh();
            }
            else if (l == 1)
            {
                ep = e.Location;
                Refresh();
            }
            else
            {
                ep = e.Location;
                g = this.CreateGraphics();
                g.DrawLine(p,sp,ep);
                sp = ep;
            }
        }
    }

我的代码的这一部分创建了一个Rectangular(第2个if),一个线段(第3个if)和一条线.它的工作原理与MS Paint几乎相同;在用户释放鼠标左键(鼠标向上)之前,矩形或线段不会完成.但是当我再次尝试创建另一个矩形时最终制作了一个矩形,表单刷新(Refresh();)并且我丢失了之前绘制的所有矩形或线条.我试过替换Refresh(); with Invalidate(rect);和Update();,但我没有得到我想要的结果.

相反,我明白了：

解决方法

您应该将所有绘图都放到一个单独的位图“缓冲区”中.将形状绘制到该位图,然后在实际需要更新屏幕时,将缓冲区绘制到屏幕上.

此外,无论何时调用Graphics.FromImage,您都需要记住dispose,否则它会像疯了一样泄漏资源.

令人难以置信的简单例子

using System;
using System.Drawing;
using System.Drawing.Imaging;
using System.Windows.Forms;

namespace DrawExample
{
    public partial class Form1 : Form
    {

        private Bitmap _canvas; //This is the offscreen drawing buffer
        private Point _anchor; //The start point for click-drag operations
        private Rectangle? _ghost;
        private Brush _ghostBrush;

        public Form1()
        {
            InitializeComponent();
        }

        private void Form1_Load(object sender,EventArgs e)
        {
            _ghostBrush = new SolidBrush(Color.FromArgb(200,200,255)); //This creates a slightly blue,transparent brush for the ghost preview
            ResizeCanvas();
        }

        private void Form1_Resize(object sender,EventArgs e)
        {
            ResizeCanvas();
        }

        /// <summary>
        /// Resizes the offscreen bitmap to match the current size of the window,it preserves what is currently in the bitmap.
        /// </summary>
        private void ResizeCanvas()
        {
            Bitmap tmp = new Bitmap(this.Width,this.Height,PixelFormat.Format32bppRgb);
            using (Graphics g = Graphics.FromImage(tmp))
            {
                g.Clear(Color.White);
                if (_canvas != null)
                {
                    g.DrawImage(_canvas,0);
                    _canvas.dispose();
                }
            }
            _canvas = tmp;
        }

        private void Form1_MouseDown(object sender,MouseEventArgs e)
        {
            if (e.Button == MouseButtons.Left)
            {
                _anchor = new Point(e.X,e.Y);
            }
        }
        private void Form1_MouseMove(object sender,MouseEventArgs e)
        {
            if (e.Button == MouseButtons.Left)
            {
                _ghost = new Rectangle(_anchor.X,_anchor.Y,e.X - _anchor.X,e.Y - _anchor.Y);
                this.Invalidate();
            }
        }

        private void Form1_MouseUp(object sender,MouseEventArgs e)
        {
            if (e.Button == MouseButtons.Left)
            {
                //Create a Graphics for the offscreen bitmap
                using (Graphics g = Graphics.FromImage(_canvas))
                {
                    Rectangle rect = new Rectangle(_anchor.X,e.Y - _anchor.Y);
                    g.FillRectangle(Brushes.White,rect);
                    g.DrawRectangle(Pens.Black,rect);
                }

                _ghost = null;

                //This queues up a redraw call for the form
                this.Invalidate();
            }
        }

        private void Form1_Paint(object sender,PaintEventArgs e)
        {

            if (_ghost.HasValue)
            {
                using (Bitmap tmp = new Bitmap(_canvas))
                {
                    using (Graphics g = Graphics.FromImage(tmp))
                    {
                        g.FillRectangle(_ghostBrush,_ghost.Value);
                        g.DrawRectangle(Pens.Black,_ghost.Value);

                        e.Graphics.DrawImage(tmp,0);
                    }
                }
            }
            else
            {
                e.Graphics.DrawImage(_canvas,0);
            }
        }


        //This stops the flickering
        protected override void OnPaintBackground(PaintEventArgs e)
        {
            //Do nothing
        }
    }
}