PaintEvent不过度涂装

Question

我在C#中有一个Windows应用程序，带有绘图面板和按钮-用于绘制线条。

当您单击该按钮时，您可以为2个随机点绘制一条线。

Pen p = new Pen(Color.Black, 5);
//point for start 
Point ps = new Point();
//point for end 
Point pe = new Point();

private void drawPanel_MouseDown(object sender, MouseEventArgs e)
{
  ps.X = e.X;
  ps.Y = e.Y;
  pe = ps;
}

private void drawPanel_MouseMove(object sender, MouseEventArgs e)
{
  // when button is clicked for drawing draw = true;
  if (draw)
  {
    if (e.Button == MouseButtons.Left)
    { 
      pe = new Point(e.X, e.Y);
    }
  }
}

private void drawPanel_MouseUp(object sender, MouseEventArgs e)
{
  onMouseUpFlag = true;
}

private void drawPanel_Paint(object sender, PaintEventArgs e)
{
  Graphics g = drawPanel.CreateGraphics();
  if (onMouseUpFlag)
  {
    g.DrawLine(p, ps, pe); 
    g.Dispose(); 
  }
} 

这个程序有一些缺陷：

• 当你画一条线时，它会显示出来，只有当主窗口移到某个地方时(通常是当我隐藏它时)。
• 它只能画一条线。

对如何修复这些错误有什么建议吗？

编辑

我读过你的答案并做了一些修改：

Pen p = new Pen(Color.Black, 5); 
Point ps = new Point();
Point pe = new Point();

List<Point> linesStart= new List<Point>();
List<Point> linesEnd= new List<Point>();

private void drawPanel_MouseDown(object sender, MouseEventArgs e)
{
  ps.X = e.X;
  ps.Y = e.Y;

  linesStart.Add(ps);

  pe = ps;
}

private void drawPanel_MouseMove(object sender, MouseEventArgs e)
{
  if (e.Button == MouseButtons.Left)
  {
    pe = new Point(e.X, e.Y);

    //adding end point .. actually adds a lot of points 
    linesEnd.Add(pe);
  }
}

bool onMouseUpFlag = false;

private void drawPanel_MouseUp(object sender, MouseEventArgs e)
{
  onMouseUpFlag = true;
  drawPanel.Invalidate();
}

private void drawPanel_Paint(object sender, PaintEventArgs e)
{ 
  if (onMouseUpFlag)
  {
    for (int i = 0; i < linesStart.Count; i++)
    {
      e.Graphics.DrawLine(p, linesStart[i], linesEnd[i]);
    } 
  }
} 

现在，我正在尝试修复多行的DrawLine。fine事件可以执行多行操作，但只有起点才行。不知何故，终点并不是很正确。我可以在哪里准确地设置MouseMove事件的最后一点？

Answer 1

您必须调用面板上的失效方法：

private void drawPanel_MouseUp(object sender, MouseEventArgs e)
{
  onMouseUpFlag = true;
  drawPanel.Invalidate();
}

另外，使用PaintEvent中的图形对象：

private void drawPanel_Paint(object sender, PaintEventArgs e)
{
  if (onMouseUpFlag)
  {
    e.Graphics.DrawLine(p, ps, pe); 
  }
} 

对于多行，您必须保存集合对象中的点。

根据您更新的代码，下面是我认为您正在尝试的工作示例：

private class Line {
  public Point Starting { get; set; }
  public Point Ending { get; set; }

  public Line(Point starting, Point ending) {
    this.Starting = starting;
    this.Ending = ending;
  }
}
List<Line> lines = new List<Line>();

private Point downPoint = Point.Empty;
private Point movePoint = Point.Empty;
private bool movingLine = false;

public Form1() {
  InitializeComponent();

  panel1.Paint += panel1_Paint;
  panel1.MouseDown += panel1_MouseDown;
  panel1.MouseMove += panel1_MouseMove;
  panel1.MouseUp += panel1_MouseUp;
}

void panel1_MouseDown(object sender, MouseEventArgs e) {
  if (e.Button == MouseButtons.Left) {
    downPoint = e.Location;
  }
}

void panel1_MouseMove(object sender, MouseEventArgs e) {
  if (e.Button == MouseButtons.Left) {
    movingLine = true;
    movePoint = e.Location;
    panel1.Invalidate();
  }
}

void panel1_MouseUp(object sender, MouseEventArgs e) {
  if (e.Button == MouseButtons.Left) {
    movingLine = false;
    lines.Add(new Line(downPoint, e.Location));
    panel1.Invalidate();
  }
}

void panel1_Paint(object sender, PaintEventArgs e) {
  e.Graphics.Clear(Color.White);
  foreach (Line l in lines) {
    e.Graphics.DrawLine(Pens.Black, l.Starting, l.Ending);
  }

  if (movingLine) {
    e.Graphics.DrawLine(Pens.Black, downPoint, movePoint);
  }

}

使用inherited panel打开DoubleBuffer属性以避免闪烁。

Answer 2

无论何时您想要重绘(在代码中)，您都应该调用drawPanel.Invalidate()使其失效。

当你完成鼠标移动并释放按钮

private void drawPanel_MouseUp(object sender, MouseEventArgs e)
{
            onMouseUpFlag = true;
            drawPanel.Invalidate();
}

最后，我建议在您的drawPanel_Paint中使用此代码，而不是您的代码。您应该使用EventArgs提供的图形，而不是创建新的图形。

private void drawPanel_Paint(object sender, PaintEventArgs e)
{
            if (onMouseUpFlag)
            {
                  e.Graphics.DrawLine(p, ps, pe); 
            }
}

Answer 3

1. 在鼠标松开之后，您需要触发一个重新绘制事件。只需调用drawPanel.Invalidate()就可以重新绘制表单。
2. 要绘制多条线，您必须将每条线的信息存储在列表或其他内容中，并在重绘中绘制每条线。油漆方法基本上每次都以空白开始，因此它将只绘制当前设置它的最新方式。