刷新表单后如何保持绘制的形状？

Question

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：

Answer 1

你应该把你所有的绘图都做到你保留的单独的Bitmap“缓冲区”。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，任何时候你打电话给Graphics.FromImage你需要记住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, 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, 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, _anchor.Y, e.X - _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, 0);
                    }
                }
            }
            else
            {
                e.Graphics.DrawImage(_canvas, 0, 0);
            }
        }


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

Answer 2

您正在直接绘制到表单的绘图表面上。那个表面并不持久。它会持续到下一个油漆循环。

相反，你应该：

1. 绘制到屏幕外的位图。
2. 将该位图绘制到例如图片框控件上。或者在Paint事件中将其直接绘制到表单的绘图表面上。