如何在表单刷新后保留绘制的形状？

Question

我正在尝试在Visual Studio 2015中创建一个小的绘画应用程序。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; 
      } 
     } 
    } 

我的代码这部分创建一个矩形（第二如果），线段（如果），只是一条线。它和MS Paint非常相似;直到用户释放鼠标左键（鼠标向上），矩形或线段才会完成。但是当一个矩形最终被创建时，当我再次尝试创建另一个时，表单刷新（Refresh（）;），并且我丢失了所有先前绘制的矩形或线条。我尝试更换Refresh（）;与无效（rect）;和Update（）;，但我没有得到我想要的结果。

相反，我得到这样的：

Answer 1

你应该做你的绘图到一个单独的位图buffer你保持周围。将您的形状绘制到该位图上，然后在实际需要更新屏幕时，将缓冲区绘制到屏幕上。

此外，您随时拨打CreateGraphics您需要记住Dispose，否则它会像疯了一样泄露资源。

令人难以置信的是简单的例子

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

您直接绘制到窗体的绘图表面。该表面不是持久的。它持续到下一个绘画周期。

相反，你应该：

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