Question

我一直在学习如何使用GDI，但我很难理解Invalidate（）和覆盖onPaint事件是如何工作的，而且似乎是围成一圈。

我有以下代码

private void DrawLine()
{
     System.Drawing.Pen myPen = new System.Drawing.Pen(System.Drawing.Color.Red);
     System.Drawing.Graphics formGraphics;
     formGraphics = this.CreateGraphics();
     formGraphics.SmoothingMode = System.Drawing.Drawing2D.SmoothingMode.AntiAlias;
     formGraphics.DrawLine(myPen, mouseDown.X, mouseDown.Y, mouseUp.X, mouseUp.Y);
     myPen.Dispose();
     formGraphics.Dispose();
}

protected override void OnPaint(PaintEventArgs e)
{
     DrawLine();
}

private void Form1_MouseDown(object sender, MouseEventArgs e)
{
     mouseDown = e.Location;

}

private void Form1_MouseUp(object sender, MouseEventArgs e)
{
     mouseUp = e.Location;           
}

然而，我能够获得我刚刚绘制的线的唯一方法是手动调用Invalidate（），当我这样做时会清除之前的任何行。有谁能告诉我，我在哪里出错？

Answer 1

将其更改为：

Bitmap bmp = new Bitmap(256, 256);

protected override void OnPaint(PaintEventArgs e)
{
  e.Graphics.DrawImage(bmp, new Point(0, 0));
}

private void Form1_MouseDown(object sender, MouseEventArgs e)
{
  mouseDown = e.Location;
}

private void Form1_MouseUp(object sender, MouseEventArgs e)
{
   mouseUp = e.Location;
   using (Graphics g = Graphics.FromImage(bmp))
   {
     g.SmoothingMode = System.Drawing.Drawing2D.SmoothingMode.AntiAlias;
     g.DrawLine(Pens.Red, mouseDown.X, mouseDown.Y, mouseUp.X, mouseUp.Y);
   }
   this.Invalidate();           
}

OnPaint方法提供了用于绘制的图形画布。事实上，你很少需要自己调用CreateGraphics。 MouseUp上的Invalidate（）调用告诉您的控件调用OnPaint事件。

Answer 2

让DrawLine采用Graphics参数：

public void DrawLine(Graphics g) { 
//...
}

在OnPaint中，使用e：

中的图形

protected override void OnPaint(PaintEventArgs e)
{
     DrawLine(e.Graphics);
}

在MouseUp中，再次使用形式的背景图像中的Graphics对象调用DrawLine：

private void Form1_MouseUp(object sender, MouseEventArgs e)
{
     mouseUp = e.Location;   
     DrawLine(Graphics.FromImage(BackgroundImage));  
     Invalidate();
}

添加MouseMove方法：

private void Form1_MouseMove(object sender, MouseEventArgs e)
{
     mouseUp = e.Location;   
     Invalidate();
}

可选，为清楚起见，将mouseDown重命名为StartPoint和mouseUp - EndPoint。

使用GDI绘制线条 - Invalidate（）/ onPaint问题

2 个答案: